A review of the potential to re-establish macrophyte beds in Te Waihora (Lake Ellesmere)

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2 A review of the potential to re-establish macrophyte beds in Te Waihora (Lake Ellesmere) Report No. R09/38 ISBN Prepared for Environment Canterbury by Don Jellyman Jeremy Walsh Mary de Winton Donna Sutherland NIWA May 2009

3 Report R09/38 ISBN Kilmore Street PO Box 345 Christchurch 8140 Phone (03) Fax (03) Church Street PO Box 550 Timaru 7940 Phone (03) Fax (03) Website: Customer Services Phone

4 A review of the potential to re-establish macrophyte beds in Te Waihora (Lake Ellesmere) Don Jellyman Jeremy Walsh Mary de Winton Donna Sutherland Prepared for Environment Canterbury NIWA Client Report: CHC May 2009 NIWA Project: ENC08525 National Institute of Water & Atmospheric Research Ltd 10 Kyle Street, Riccarton, Christchurch 8011 P O Box 8602, Christchurch 8440, New Zealand Phone , Fax All rights reserved. This publication may not be reproduced or copied in any form without the permission of the client. Such permission is to be given only in accordance with the terms of the client's contract with NIWA. This copyright extends to all forms of copying and any storage of material in any kind of information retrieval system.

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6 Contents Executive Summary i 1. Introduction 1 2. Assessment of benefits, or otherwise, of macrophytes for the lake ecosystem Characteristics of flipped lakes Fish communities New Zealand case studies Lakes Waahi and Whangape Te Waihora history, invertebrate communities and fishery yields Overall summary of New Zealand flipped lakes The benefits and concerns (pros and cons) associated with possible restoration of macrophytes in Te Waihora Benefits Concerns Risks of side-effects A review of national and international macrophyte restoration projects and their applicability to Te Waihora Barriers to macrophyte re-colonisation Review of macrophyte restoration experiences International experiences New Zealand experiences Application of restoration techniques to Te Waihora To assess the conditions required for macrophyte rehabilitation in the lake, focusing especially on defining suitable windows for successful establishment and canopy development Wave modelling Wave exposure A possible scenario for re-establishing macrophytes Identify any critical gaps in information that are needed to determine suitable restoration conditions Concluding comments Acknowledgements References 41

7 Appendix I: Results of wave length modelling at the same wind scenarios used to generate wave height information. Reviewed by: Approved for release by: Rohan Wells Marty Bonnett

8 Executive Summary Historically, Te Waihora (Lake Ellesmere) had an extensive margin of macrophytes. Although these had disappeared periodically since 1904, the Wahine Storm of 1968 devastated the beds which have never recovered. The present report is in response to a request from Environment Canterbury to review the benefits and probability of success of endeavouring to re-establish macrophytes in the lake. A review of other flipped (regime - changed) lakes in New Zealand, showed that they are characterised by a high proportion of cyprinids (tench, goldfish, carp etc); a comparison of the fish production in such lakes before and after flipping indicated that, with the exception of some reduction in more sensitive species like smelt and brown trout, the abundance of other species appears unaffected. Fishery yields from Te Waihora are comparable with those of northern hemisphere temperate lakes, and while diets of eels and flounders (the main customary in commercial species) have changed as a result of the lake flipping, all indicators of well-being of these stocks show no change or some improvement. However, the virtual loss of brown trout and longfin eels from the lake may be partly associated with loss of macrophytes. The report also reviews the benefits and disadvantages of macrophyte restoration in the lake. While a seed bank still exists within the substrates of the lake, the viability of this seed appears to be low. Results from a wave exposure model indicated that the western shorelines were the areas least exposed to high wave energy, especially the vicinity of Harts Creek to Taumutu. Likewise the shoreline between the L II and Selwyn River mouths is relatively sheltered. Any attempt at macrophyte restoration should focus on such areas. Such efforts would require the use of wave baffles or berms to reduce wave fetch and the likelihood of macrophytes being uprooted. Restoration should involve planting of robust propagules or whole macrophytes to supplement any natural germination from existing seeds; some control over black swans would be required to prevent over-grazing of plants and uprooting of rhizomes. While there is some potential to re-establish the macrophytes within selected reaches of the lake, and there are a number of benefits that would arise from this, there are also a number of negative effects. Ultimately a decision will need to be made balancing the perceived benefits against the costs of attempting to establish a viable, self-sustaining area of vegetation and the risk that another extreme weather event could nullify the effort and expenditure involved. A review of the potential to re-establish macrophyte beds in Te Waihora (Lake Ellesmere) i

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10 1. Introduction Historically, Te Waihora (Lake Ellesmere) had an extensive fringing margin of aquatic weeds, macrophytes (Figure 1). There appear to have been cycles of abundance and scarcity over the years (Hughes et al. 1974) - for example the plants were so scarce that in the 1940 s the North Canterbury Acclimatisation Society requested research on the subject. The subsequent report (Mason 1946) indicated that macrophyte beds had disappeared periodically since When the macrophyte beds were mapped in 1960 (Figure 2), the largest areas lay between the mouths of the Selwyn and Halswell Rivers. In 1969, a continuous belt of macrophytes extended south from the mouth of the Selwyn River, along the west shore to Marshall Island, but macrophytes were absent from Marshall Island to Taumutu (Hughes et al. 1974). These beds were described as being so thick you could almost walk on them, with individual plants metres long - these formed such dense beds that water within the beds was calm and still even in strong winds (Hughes et al. 1974). Figure 1: Macrophyte beds off Timberyard Point, 1958 (photo reproduced from Taylor 1996, courtesy of W.K. Browne). The lane through the beds was cut by commercial fishers to gain access to the main body of the lake. A review of the potential to re-establish macrophyte beds in Te Waihora (Lake Ellesmere) 1

11 Figure 2: The distribution of aquatic macrophytes in Te Waihora, 1960 (from Hughes et al. 1974). Dominant macrophyte species were Ruppia megacarpa and Potamogeton pectinatus (Hughes et al. 1974), although Ruppia polycarpa was also present. After the decline in macrophytes beds in the 1940 s, their recovery during the 1950 s was spectacular and luxuriant growth persisted until the devastating effect of the Wahine Storm in 1968 (Gerbeaux 1993). Although there were scattered patches of plants seen in the early 1970 s and early 1980 s (Taylor 1996), the beds have never regained their former extent. Some Ruppia still germinates each year along the east of Greenpark but plants do not survive (Colin Arps, commercial fisher pers. comm.). Since the loss of the macrophyte beds, there has been periodic interest in the desirability and feasibility of re-establishing them in the lake. Ngai Tahu have expressed a desire to see restoration of macrophytes (Te Runanga o Ngai Tahu and Department of Conservation 2005) but acknowledge that this is likely to be at a substantial cost and the possible short and medium-term effects on health of Te Waihora are not well known. Under Methods to be used to improve management of the lake, this report lists; 4.3e encourage or undertake the regeneration of indigenous macrophyte vegetation within Te Waihora. Restoration of macrophytes was also one of the criteria suggested for investigation by Ngai Tahu in A review of the potential to re-establish macrophyte beds in Te Waihora (Lake Ellesmere) 2

12 their submission to ECan on the lake opening consent renewal process. Ngai Tahu have reported that in the years prior to loss of macrophytes, their people commonly fished for eels amongst the macrophytes something you could only dream of doing nowadays (O Connell 1996). In their Community Strategy, the Lake Ellesmere Issues Group has developed an action (1.4) to Re-establish the macrophyte beds in the lake, which includes an investigation of the feasibility of re-establishment, and identification of areas and species that would be the most appropriate or successful. The Te Waihora Eel Management Plan (1996) mentions that The committee believes that the re-establishment of these weed beds will improve the water clarity and quality, reduce shore erosion, and increase the productivity of the lake through the provision of habitat. The committee recommends the Regional Council investigate methods to reestablish the natural weed beds in Te Waihora. In response to the above concerns, Environment Canterbury, ECan, commissioned the present study to: Review of national and international experiences with macrophyte restoration projects and their applicability to Lake Ellesmere/Te Waihora Assess the conditions required for macrophyte rehabilitation in the lake, focusing especially on defining suitable windows for successful establishment and canopy development Identify any critical gaps in information that are needed to determine suitable restoration conditions Assess the benefits, or otherwise, of macrophytes for the lake ecosystem Assess the risks of side-effects such as increases in marginal phytoplankton populations during re-establishment, effects on lake users, nuisance bird populations etc. These objectives form the sections of the present report (although not in the same order). A review of the potential to re-establish macrophyte beds in Te Waihora (Lake Ellesmere) 3

13 2. Assessment of benefits, or otherwise, of macrophytes for the lake ecosystem 2.1. Characteristics of flipped lakes Lakes that have undergone a regime change from a macrophyte (aquatic weed) dominated phase to a phytoplankton dominated phase are colloquially termed a flipped lake. A decrease in water clarity usually accompanies this regime change, and is associated with both phytoplankton growth, and increased turbidity from winddriven resuspension of sediments Fish communities Fish communities are largely a reflection of the trophic status of a lake thus salmonids (trout, charrs, salmon) are typical of oligotrophic waters, while cyprinids (carps, goldfish, tench etc) thrive in eutrophic conditions. A list comprised of both flipped and non-flipped lakes of comparable size and location was compiled by Brian Sorrell (NIWA, Christchurch), and the fish species present per lake was added from the New Zealand Freshwater Fish Database. Table 1 gives averages of broad species groups for a list of both non-flipped and flipped lakes. The table shows that nonflipped lakes are dominated by native species (75% of all species recorded), whereas while natives are still the largest group in flipped lakes, the increased proportions of cyprinids ( coarse fish ) are clearly evident. However, it is not appropriate to ascribe cause and effect relationships to these data, such that the introduction of coarse fish species has caused lakes to flip, as in some instances, coarse fish have been introduced to flipped lakes (often illegally) as proponents of fishing for coarse fish have realised that such lakes would be suitable habitat for these fish species. Table 1: The fish species composition (%) of a series of New Zealand flipped and nonflipped lakes. Lake status No. of lakes Introduced species Native fish species Salmonids Cyprinids Perch Catfish Mosquito fish Non-flipped Flipped In a review of factors associated with regime shifts in New Zealand lakes (i.e. flipping lakes) Schallenberg and Sorrell (in press) found such shifts were positively related to the percent of the catchment in pasture, and negatively related to the percent of the catchment in forest. Other significant relationships were the occurrence of the exotic A review of the potential to re-establish macrophyte beds in Te Waihora (Lake Ellesmere) 4

14 macrophyte Egeria densa, and the presence of catfish and coarse fish (goldfish, rudd, carp, and tench). They commented that the data suggested that such regime shifts in lakes were unlikely to have been common in New Zealand prior to its anthropogenic alteration of both terrestrial and aquatic environments. Several coarse fish species are found in Te Waihora i.e. goldfish, tench and rudd (Jellyman and Smith in press); perch are also present. Although catfish were reputedly found in the lake in 1997 (Anon 1997), there have been no further sightings and the original records must be regarded with some scepticism. So, while the lake contains several species of coarse fish, these are only present in small numbers (Glova and Sagar, 2000) and at present levels present no significant threat to re-establishment of macrophytes. The lake still has high species diversity (15 native freshwater species, and 5 introduced species; Jellyman and Smith, in press), and high productivity (see 5.1.3) New Zealand case studies Lakes Waahi and Whangape In a comparative study of the diets of fish in Lakes Waahi (flipped, turbid and no submerged macrophytes) and Whangape (not flipped, clearer and dominated by submerged macrophytes), Hayes and Rutledge (1991) concluded that mysids can be key elements of the food chain of turbid shallow lakes in the lower Waikato; this has implications for management. In the past 15 years several shallow lakes in the lower Waikato have become turbid and devoid of submerged macrophytes. These changes should not necessarily be seen as catastrophic for fish populations, because if mysids are present they appear to be more than able to compensate for loss of food sources associated with macrophytes. Given the lack of research on the longer-term effects of such changes in diet, they noted that there was uncertainty about whether reliance by fish populations upon mysids is ecologically stable. A comparison of the fish communities of Lakes Waahi and Whangape (Hayes et al. 1992) recorded similar species richness, except that the lacustrine form of the common smelt disappeared from Lake Waahi when it became turbid in the late 1970 s. Measures of well-being of fish stocks (size, condition, and catch-per-unit-effort, CPUE) were generally similar between the 2 lakes of relevance to Waihora was the abundance of shortfin eels (measured as CPUE) showed no overall differences between the 2 lakes, but common bullies were more abundant in Lake Waahi (turbid). A follow-up survey of Lake Waahi (Chisnall et al. 1992; Table 2) found some significant changes had occurred. The mysid biomass had declined markedly (since 1990) and remained low; the biomass of shortfin eels had also declined, although it A review of the potential to re-establish macrophyte beds in Te Waihora (Lake Ellesmere) 5

15 was uncertain whether this was associated with the status of the lake, or restricted recruitment of juvenile eels. No systematic surveys of the lake have been carried out since 1992, but the lake still supports important commercial and customary eel fisheries (Beentjes and Chisnall 1998). Table 2: Mean catch per unit effort (CPUE; ± SE) for five species of fish and mysid shrimps caught in fine mesh traps set in Lake Waahi, February 1987, 1990 and Data from Chisnall et al CPUE (g. net. h -1 ) Species Mysids 14 ± ± ± 0.4 Shortfin eels 1886 ± ± ± 206 Common bullies 82 ± ± ± 16 Common smelt 4.2 ± ± ± 6.6 Inanga 1.1 ± ± ± 0.6 Mosquitofish 9.4 ± ± ± Te Waihora history, invertebrate communities and fishery yields History of flipping As mentioned previously, Te Waihora has a history of periodic declines in macrophytes since 1904 (Hughes et al. 1974) thus a decline in the macrophyte beds was recorded in the 1920 s, especially along the shoreline north from Taumutu. The beds disappeared in the 1940 s although a significant recovery occurred in the 1950 s and luxuriant growth continued until the 1968 Wahine storm which decimated the beds. Although scattered patches of plants were still seen in the 1970 s and early 1980 s, the beds have never recovered (Taylor 1996). Benthic invertebrates A study of changes in trophic linkages within Te Waihora (Kelly and Jellyman 2007) showed that historically the benthos was dominated by phytophyllic species (associated with plants) such as the snail Potamopyrgus antipodarum which comprised over 90 % of total invertebrate biomass during the 1960 s. In contrast, the benthos is now almost entirely comprised of substrate-dwelling species such as midge larvae (Chironomus zealandicus), and oligochaetes (aquatic worms), which together A review of the potential to re-establish macrophyte beds in Te Waihora (Lake Ellesmere) 6

16 comprise 82 % of the total invertebrate biomass. This shift in benthic communities has resulted in similar changes in the size-specific diet of juvenile shortfin eels (< 400 mm) in the mid-1970s, snails were the dominant food item (>30 % of total food biomass) whereas today chironomid larvae comprise 65 % of the diet of juvenile eels. A recent M Sc study has focussed on the spatial and temporal variability in the benthic community, and the foodweb structure and function of the lake (Wood 2008). Changes in the invertebrate composition that followed the final flipping episode of Te Waihora in 1968 have resulted in changes in the food available to resident fish. However, given that the most abundant species in the lake (eels, bullies, flatfish) are generalist feeders and eat what is available, there is no evidence that changes to the invertebrate community have resulted in reduced feeding and growing opportunities for fish. Fishery yields Eel fishery The eel fishery commenced in the early 1970 s and rapidly rose to be the largest single fishery in 1976 (Figure 3), when it comprised almost half of the total New Zealand eel catch. Because of concerns over declining catches, the lake was declared a controlled fishery in December 1978, with the initial total allowable catch (TAC) set at 256 t (which did not include migratory eels) and allocated to 17 fishers. The TAC (Total Allowable Catch) was reduced to t in 1986 and distributed among 11 fishers. Although the lake had an initial size limit of 150 g (1994), this was progressively increased at 10 g/year to reach the national minimum size of 220 g. With the entry of South Island eels into the Quota Management System (QMS) in 2000, the TAC was reviewed and allocations made for customary and recreational use (customary = t, recreational = 3.13 t). There are presently 5 commercial eel fishers on the lake, and the TACC ( t) is almost invariably caught (Jellyman and Smith in press). Unfortunately, there are few historic data on species composition to know whether longfins eels have been more prevalent than at present. In common with other lowland lakes, shortfin eels were always the dominant species in Te Waihora (Trevor Gould, former commercial fisher and processor, pers. comm.). As longfins prefer flowing water there will always have been substantial numbers of longfins in the lower reaches of tributaries and associated lake margins, and these remain today as the tributaries are exempt from commercial fishing. During initial research surveys (NIWA unpubl. data), a higher proportion of longfins was recorded than in subsequent years i.e , N = 6961, longfins = 4.3%; , N = 1242, longfins = 0.5% (Beentjes 1999). A review of the potential to re-establish macrophyte beds in Te Waihora (Lake Ellesmere) 7

17 Figure 3: Total Catch (tonnes) Year/fishing year / / / / /06 Trends in catches from the commercial eels fishery in Te Waihora This reduction will be largely associated with onset of commercial fishing (longfins are more vulnerable to fyke net capture than shortfins; Jellyman et al. 1995; Jellyman and Graynoth 2005), but possibly also associated with the loss of aquatic macrophytes. Although there has been little change in the average size of non-migratory shortfins, there has been a marked increase in growth rates. In 1974, shortfins (feeding eels) grew an average of 24 mm. y -1, but by 2007, the rate had increased by 63 % to 39 mm/year. The growth rate of migrating males does not appear to have changed much over the years (Table 3), but the growth rate and average size (Table 3, Figure 4) of (migratory) females have both increased substantially between the 1970 s and Presumably, conditions for growth of larger eels have improved over time, possibly as a result of a reduction in overall numbers. Female eels show accelerated growth rates with increasing size, associated with their change in diet from invertebrates (Kelly and Jellyman 2007) to fish (Jellyman 2001). However, the growth rate of juvenile eels is relatively slow (Graynoth and Jellyman 2002) and this shows in the smaller annual growth increment for migrating males relative to females (Table 3) as they are too small to eat fish. Catch-per-unit-effort (CPUE, measured as an index of kg net -1 night -1 ) analyses of Te Waihora commercial catches over the 16 years for which consistent data are available, show a dramatic increase after 2000 for shortfins (Beentjes and Dunn 2008; Figure 5). The All eel index is very similar to that of shortfins as this species comprises almost all (99%) of the total catch. While the longfin catch is minor, it does show a stable pattern over the past 14 years. A review of the potential to re-establish macrophyte beds in Te Waihora (Lake Ellesmere) 8

18 Table 3: Mean annual growth increment (mm/year) for feeding eels from Timberyard Point, and migrating eels from Taumutu, for various years. * = Beentjes and Chisnall 1998; ** = sample from Harts Creek reserve. Species Status Year Number aged Mean increment (mm y -1 ) Shortfin Feeding /97* Migrant males Migrant females Longfin Feeding ** SE Length (mm) Females Males Year Figure 4: Changes in the average length of migratory male and female shortfin eels from Te Waihora (bars are ± 1 SE) A review of the potential to re-establish macrophyte beds in Te Waihora (Lake Ellesmere) 9

19 (a) All eel catch index (b) SFE index 3 3 Index 2 Index Year Year 6 (c) LFE index 5 4 Index Year Figure 5: The catch-per-unit-effort (CPUE) trends for Te Waihora eels. Note that the All eel catch index (top left) is almost identical to that for SFE index (shortfin eels), indicating that longfins make a negligible contribution to the fishery (data from Beentjes and Dunn 2008). Thus a summary of the commercial eel fishery indicates: Pre Wahine Storm, diets of small eels were mainly snails, whereas now diets are mainly midge larvae The commercial fishery will always have been dominated by shortfins, although prior to the Wahine Storm there were more longfins in the lake. Reduction in this species will have been largely attributable to their capture in the commercial fishery, although loss of macrophyte beds is likely to be a contributing factor also. A review of the potential to re-establish macrophyte beds in Te Waihora (Lake Ellesmere) 10

20 While initial commercial harvest reduced the average size of shortfin eels, there has been no significant change over the last 30 years. Present growth rates are faster than historic rates, and CPUE has increased markedly over recent years. Collectively, these data indicate a healthy stock of shortfin eels. In general, the indicators of fishery well-being have improved as the fishery has become managed at a lower harvest level. The change from a macrophyte -dominated lake may have been somewhat detrimental for longfins, but the shortfin population is in a healthy state A different perception may be held by customary fishers, as eels are not as accessible to customary harvest methods as formerly. Historically, eels could be clubbed or speared when macrophyte was rolled back (O Connell 1996), but with the disappearance of the macrophyte beds, use of this technique is no longer possible. Flounder fishery The flounder fishery has a history of extreme variability (Figure 6), with catches in adjacent years varying up to 10 fold. While this variability is thought to be largely due to the timing and duration of lake openings (Jellyman 1992; Taylor 1996), it also reflects the variability of recruitment of juvenile flatfish generally (Annala and Sullivan 1997). In 1959, there were 39 vessels and 55 fishers engaged in flounder fishing on the lake. Flounders were declared a quota species in 1986, and currently only 8 fishers engage in commercial fishing. There are no specific Total Allowable Commercial Catches (TACC s) set for the lake as it is part of Fishstock FLA 3 (east coast and southern South Island). Also, for fishery management purposes, all species of flatfish are combined into a generic species code. The catch is dominated by three main species: black flounder, yellowbelly flounder, and sand flounder. Occasionally small quantities of greenback flounder are recorded. The species proportions vary considerably from year to year, but blacks provide the bulk of the catch (58% over past 23 years), followed by sands (22%) and yellowbellies (20%). The availability of flounders reflects lake opening regimes, and the success of New Zealand wide spawning of the three main species (blacks, sands, yellowbellies). As the fishery is based on 2 and 3 year old fish (Gorman 1960a; Jellyman in prep.), loss of a year s recruitment will have a profound effect, unlike eels where fish are typically > 10 years old at entry into the commercial fishery. Mature adult flounders A review of the potential to re-establish macrophyte beds in Te Waihora (Lake Ellesmere) 11

21 emigrate from the lake to spawn at sea in winter and spring. Thus the well-being of the flounder fishery is largely a function of the extent of recruitment 2 and 3 years previously Catch (tonnes) Year Figure 6: Commercial flounder catches from Te Waihora:, The diet of flounders will also have changed as a result of the loss of the macrophyte beds. In an earlier study of flatfish in the lake, Gorman (1960a) commented no serious attempt has been made to determine the feeding habits of each species. Between September and December, some stomachs were examined. The principal food consists of lamellibranchs (bivalve mollusc), copepods and polychaetes. The yellowbelly appears to favour the mollusc. In the black flounder, there seems to be some preference for what apparently is organic detritus. The only lamellibranch, recorded from the lake appears to be Cyclomatra ovata (Hughes et al. 1974); the mollusc probably refers to the snail Potamopyrgus. In contrast, the present-day diet of flounders is almost solely midge larvae (Don Jellyman, NIWA, pers. obs.), although larger yellow bellies are known to eat small bullies (Clem Smith, commercial fisher, pers. comm.). In summary, the flounder fishery is typified by high inter-annual variability in species proportions and overall catches. This variability will in turn reflect the strength of recruitment 2-3 years previously. While diets have changed with loss of macrophytes, growth rates remain high. While there are occasional reports of flatfish dying in nets, usually associated with extended calm periods and, presumably, localised de- A review of the potential to re-establish macrophyte beds in Te Waihora (Lake Ellesmere) 12

22 oxygenation of the water, there is no evidence that the loss of macrophytes has lead to any significant changes in production of flounders in Te Waihora. Overall biomass and productivity of Te Waihora A comprehensive survey of the lake using fyke nets (Glova and Sagar 2000) recorded that common bullies were the numerically dominant species (92.4% of all fish caught). Biomass estimates have been made using the length-frequencies provided by that study, and length/weight relationship data (NIWA unpublished data). The results, Figure 7 show that bullies also dominated the overall biomass (44.4 %; 90% by number), followed by shortfin eels (28.4 % biomass, 1.3 % by number) and black flounders (15.3 % biomass, 0.4 % by number). As these data are not related to a given area, it is not possible to use them to predict total biomass within the lake. However, the net weight of commercially-landed fish can be used as a comparative index of yield for comparison with other temperate freshwater lake fisheries Number (%) Biomass (%) Bullies Smelt Inanga Shortfin eel Black flounder Yellowbelly flounder Sand flounder Figure 7: The numbers and biomass of fish caught during a survey of the lake (data modified from Glova and Sagar 2000). For eels, the present TACC of 122 t and a customary catch of 5 t, equates to an annual yield of 6.3 kg/ha. As indicated, flatfish catches vary considerably between years; the average catch for the years was 79 t, but the maximum catch during this period was 201 t. Using an intermediate figure of 100 t of flatfish gives a total annual fisheries yield (eels and flatfish) of 227 t. Lake area varies with lake level, but at 1m ASL, the lake is ha (Taylor 1996). Thus at this level, the fisheries yield is 11.2 kg/ha; assuming the maximum catch of 200 t of flounders were sustainable, this would increase the yield to 16.1 kg/ha. A review of the potential to re-establish macrophyte beds in Te Waihora (Lake Ellesmere) 13

23 A comparison of fishery yields between Te Waihora and other temperate lakes (Table 4), indicates that yields of eels from Te Waihora are comparable to those of some of the well recognised eel producing waters in Europe like the Commachio Lagoon and the Ijsselmeer. The All fish yield is also within a similar range to the other lakes listed (except for eutrophic lakes in Germany). While there are no pre-flipped data from Te Waihora, these data indicate that the present yields from the lake are relatively high. Table 4: The fishery yields (kg/ha) from Te Waihora and a selection of temperate lakes (data from Tesch 2003). (a) = eutrophic lakes, (b) = oligotrophic lakes Lake Eel yield (kg/ha) All fish yield (kg/ha) Te Waihora Lough Neagh, Ireland 17 Lake Constance 3-6 Small German lakes (a) Small German lakes (b) Large German stocked lakes 2.6 Polish lakes 5.2 Commachio Lagoon, Italy 5-7 Ijsselmeer, Holland 10 Coastal Baltic lakes 3 15 Central Baltic lakes Brown trout fishery Historically, Te Waihora and the Selwyn River maintained a world-class fishery for brown trout. To quote from the North Canterbury Acclimatisation Society annual Report of 1936 (cited in McDowall 1994): for many years the lower Selwyn has borne the reputation of being the best three miles of brown trout fishing in the Dominion, or probably in the world, taking into consideration the numbers taken from the lower waters each season, and their large average sizes. The demise of the fishery is well known (e.g. Hardy 1989; Singleton 2007), although the reasons are less well understood. Hardy (1989) suggested a combination of issues including: increasing eutrophication of the lake disappearance of the macrophyte beds extensive bycatch, especially from flounder gill nets A review of the potential to re-establish macrophyte beds in Te Waihora (Lake Ellesmere) 14

24 habitat degradation, especially land drainage, channelisation, macrophyte clearing, and removal of streamside vegetation frequency and timing of lake openings maintaining the lake at low levels inadequate food supply insufficient recruitment of juvenile trout associated with low flows preventing access of adult fish to spawning grounds, losses of fish through stranding, and the failure of juvenile trout to survive in the lake partly because of the lack of macrophyte beds which formerly provided food and cover from predators. Certainly, the lake still has the potential to grow large trout as Glova (1996) found that recaptured hatchery-reared brown trout were all in excellent condition, and growth had been rapid. A conclusion from this study was that recruitment was limited, and that a major input of trout into the system could help to reverse the decline of the fishery ; the positive role of the former macrophyte beds in the lake was suggested but unproven Overall summary of New Zealand flipped lakes Invertebrate composition changes, but is still able to sustain similar levels of fish yield to pre-flipped condition Flipped lakes are characterised by a higher proportion of cyprinids ( coarse fish ) which are better adapted to eutrophic conditions than other species The few data on fish production before and after flipping, indicate that while there are small changes in species composition (with reductions in more sensitive species like smelt), the abundance of other species appears unaffected Fishery yields from Te Waihora are comparable to those of northern hemisphere temperate lakes. While diets of the main commercial species, eels and flounders, have changed as a result of the regime shift of the lake, all indicators of the well-being of the commercial fisheries for these species are positive A review of the potential to re-establish macrophyte beds in Te Waihora (Lake Ellesmere) 15

25 2.2. The benefits and concerns (pros and cons) associated with possible restoration of macrophytes in Te Waihora Benefits o restoration to as it was o macrophytes act as a sediment trap and nutrient sinks (increased inshore water clarity, and reduction of areas where sediment becomes re-suspended and nutrients mobilised) o reduce shoreline erosion o increased dissolved oxygen o produce shading and water temperature gradients o greater fish and bird habitat diversity Concerns o aesthetics (shoreline rotting macrophytes and reduced dissolved oxygen) o fisher access and net fouling o other recreational users e.g. power boats and wind surfers etc o overall stability-could the lake flip again? o viability of existing seed bank o salinity changes and macrophyte species o local de-oxygenation at night o the risks of side-effects (phytoplankton blooms, especially blue-green algae, possible nuisance numbers of swans) o practicality and cost To deal with these in more detail Benefits Restoration to as it was A number of stakeholder and interest groups have expressed the desire that the macrophyte beds in the lake be re-established (e.g. Ngai Tahu, the Lake Ellesmere Issues Group). Motivations vary, but a common theme is the knowledge that the lake A review of the potential to re-establish macrophyte beds in Te Waihora (Lake Ellesmere) 16

26 was clearer, at least around the margins inshore of macrophyte beds; there is also an assumption that macrophyte beds would provide biological benefits through such aspects as providing nutrient sinks, cover, and greater food producing areas, while the beds would also reduce shoreline erosion, a process assumed to exacerbate reduced water clarity through provision of additional suspended sediment. Sediment trap and nutrient sinks Historically, trout angling was carried out inshore of macrophyte beds where anglers could spot individual cruising fish, indicating that the inshore areas were much clearer than at present. Sediment resuspension is the main cause of the limited light penetration in Te Waihora (Gerbeaux and Ward 1991), and fluctuations in turbidity were related to wind velocity as a consequence, the resulting strong light attenuation is a major factor limiting macrophyte growth. A model of resuspension (Sagar et al. 2004) confirmed the importance of wind, and noted that even at high lake levels (relative to the present opening regime), wind action was sufficient to agitate sediments over much of the lake with resultant highly turbid water; only by maintaining the lake well above its current high levels would clearer water be possible. Gerbeaux (1989) noted that if nutrients were tied up in macrophyte biomass during the growing season, little nutrient would be available for phytoplankton growth, and the water in the littoral zone may become clearer than water in deeper zones. Further, extensive biomass growth could remove significant quantities of phosphorus from the sediment. No nutrient budget for the lake has been developed to see how significant such uptake could potentially be, but is considered unlikely that sufficient nutrient would become locked up in macrophytes to reduce the likelihood of significant phytoplankton and blue-green algal growth (partly due to nutrient cycling associated with macrophyte decay, but also due to ongoing nutrient input from catchment developments). Of course, the utilisation of nutrients by macrophytes does not remove nutrients from the catchment, and these nutrients become available again upon the death and decay of the macrophytes. To reduce nutrients, macrophytes would need to be harvested, something that currently occurs on a small scale with the macrophyte gathering that takes place associated with the macrophyte cutting in the Halswell Canal. Reduce shoreline erosion There is little doubt that the extensive macrophyte beds reduced wave action and hence shoreline erosion. Wavelap erosion is greatest on the north-east and south-west A review of the potential to re-establish macrophyte beds in Te Waihora (Lake Ellesmere) 17

27 shorelines as a result of the prevailing north easterly and south-westerly winds (Taylor 1996). In an assessment of the loss of swan breeding areas along the north-eastern shore of Kaitorete Spit over 20 years ( ), Maindonald (1990) estimated the shoreline had retreated up to 42 m he attributed this accelerated erosion to the loss of the macrophyte beds following the Wahine Storm of Similar concerns about shoreline erosion in the vicinity of Kaituna Lagoon were recently expressed by Ngai Tahu during their annual swan egg harvest (Jason Arnold, Ngai Tahu pers. comm.). Shoreline erosion is also a source of additional sediment and nutrient to the lake (Taylor et al. 1996). Increased dissolved oxygen. Gerbeaux (1989) measured daily dissolved oxygen changes as large as 8 mg/l in dense macrophyte beds; during daylight hours, water could become supersaturated with oxygen, but at night respiration depleted oxygen in macrophyte beds where there was little water circulation. Produce shading and water temperature gradients Again, Gerbeaux (1989) noted that dense macrophytes formed a heavy shading canopy that significantly reduced photosynthetic activity under the stands. The shading resulted in vertical temperature gradients as much as 10 o C over 1 m of depth. Greater fish and bird habitat diversity Eels are a very light intolerant species, and the biomass of large eels has been related to the availability of suitable cover (Burnet 1952). As indicated previously, a traditional means of capturing eels was using a club (patu), gaff or spear, and eels could be found sheltering under the macrophyte beds or the hull of a boat (O Connell 1996) indicating that macrophyte beds provided suitable daytime cover for large eels. In addition to cover, the macrophyte beds provided a ready source of food for eels, especially snails which comprised most of the diet (> 30%) of small eels (Ryan 1986). Reduction in the numbers of longfin eels in the lake may have been partly associated with loss of macrophytes (Jellyman and Smith 2008). The loss of aquatic macrophytes has been suggested as one of the contributing factors in the loss of the lake and Selwyn River trout fishery (Hardy 1989, Glova 1996). One hypothesis was that with the lack of juvenile rearing habitat in the Selwyn River, juvenile fish enter the lake at a small size and are subject to extensive predation as there is little cover from predators (eels and birds), whereas previously the extensive A review of the potential to re-establish macrophyte beds in Te Waihora (Lake Ellesmere) 18

28 macrophyte beds would have provided such cover. Certainly an experiment that released juvenile trout as yearlings (~ 50 g) produced encouraging returns of trout at a comparatively large size between 1 and 3 years later (Glova 1996). So, these results were consistent with the theory that a constraint to recovery of the trout fishery was the high juvenile mortality in the lake, with lack of macrophytes that provided both food and cover a possible reason. Macrophyte beds were dense enough for swans to nest on them. The beds provided a direct source of food for wading birds, but especially for black swans which feed by up-ending (Sagar et al. 2004). This feeding habit means that they can graze to a depth of 1 m below the surface (Howard-Williams and Davies 1988). In Te Waihora their preferred food plant was Ruppia (Sagar et al. 1995), and with the present relatively low lake level regime, swans would have the capacity of removing whole rhizomes as well as the growing stalks Concerns Aesthetics Rotting weed produced anaerobic conditions, as well as production of hydrogen sulphide (Hughes et al. 1974). Such conditions may have contributed to the disappearance of macrophyte beds in the past by rotting macrophyte bases (Hughes et al. 1974). No doubt, banks of rotting macrophyte that would have accumulated on windy shorelines were both unsightly and smelly. Macrophyte cutting and lake access Historically, wind caused loose macrophytes to drift into and clog nets to the extent they would not fish successfully (Gorman 1960a); clogging by macrophyte and slime was also recorded in light winds, and fish avoided such nets. The presence of macrophyte beds meant that drag netting was not popular in the lake (Gorman 1960b), while fishers had to cut lanes in the fringing beds to gain access to the outer lake (Fig. 1). Commercial fishers reported that much of the lake off Greenpark and towards Kaituna was virtually non-negotiable to boats because of the dense macrophyte growth throughout this area (Colin Arps, commercial fisher, pers. comm.). Several years ago, macrophyte cut from the Halswell canal and allowed to drift into the lake produced localised areas of poor water quality in such areas, fish were absent, and rotting macrophytes fouled nets (Clem Smith commercial fisher, pers. A review of the potential to re-establish macrophyte beds in Te Waihora (Lake Ellesmere) 19

29 comm.). Complaints by commercial fishers resulted in a macrophyte boom being installed in the lower Halswell canal to stop floating macrophyte entering the lake. Negative impacts for aquatic recreational users Although not documented, the macrophyte beds would have provided some constraints for the extensive power-boating and sailing regattas that were held up until the 1980 s (Singleton 2007). Hughes et al. (1974) commented that boat clubs had requested a high summer lake level, although this would have been more a response to the lack of depth as shallow water causes problems for outboard motor and boats with keels (Taylor 1996). Overall stability - could the lake flip again? The lake has had a record of significant reductions of macrophyte in the past (Hughes et al. 1974; Gerbeaux and Ward 1991; Taylor 1996). However, the coincidence of turbid water and a shallower lake mean that natural flipping to a macrophyte dominated state is highly unlikely in future. Were macrophytes able to be re-established, the question remains whether they might again disappear due to some cyclic events. Given the history of such events, the possibility of such a collapse cannot be discounted. Such a collapse could be precipitated by such features as: prolonged strong winds (with effects exacerbated by shallow water) low lake levels resulting in a greater effect of wave action and access to plant rhizomes by swans a proliferation of algal production, especially blue-green algae like Nodularia or Anabaena, which would again reduce light penetration and result in deoxygenated waters during its senescent phase. Over-grazing by waterfowl Viability of existing seed bank Enclosure trials by Webb (1982) indicated that viable seed was present in the lake at that time, but germination experiments of Ruppia megacarpa fruit by Gerbeaux (1993) found only 4% were viable. More recently, Jellyman (2007) reported on the finding and successful germination of a Ruppia seedball, although again the percentage of A review of the potential to re-establish macrophyte beds in Te Waihora (Lake Ellesmere) 20

30 seeds that germinated from this ball was low. In February 2007, Ruppia covered about a third of the boat harbour at Timberyard Point (about 1 strand per m 2 ), but this had disappeared by early April, presumably eaten by waterfowl (Don Jellyman, NIWA, pers. obs.). Ruppia is reported to germinate in spring each year off the mouth of the Halswell Canal, in Knuts Bay and along Grays Spit, but disappears in early summer (Colin Arps, commercial fisher, pers. comm.), presumably as it cannot withstand the combined effects of wind, dewatering and waterfowl browsing. Therefore, although viable seed still exists 40 years after the loss of the extensive macrophyte beds, the viability of this seed appears to be low. Proliferation of swans Blacks swans have been present at large numbers on the lake since at least 1867 (Hughes et al. 1974). As a result of persistent reports of damage to surrounding farmlands, in 1915 the North Canterbury Acclimatisation Society were given authority to control numbers by collecting eggs - the objective of this regime was to control the number of cygnets hatching to per year. Swans also posed persistent problems for electricity authorities as birds flying into Te Waihora at dusk often became entangled in power lines, cutting power to consumers (McDowall 1994). In 1960, the estimated population was 80,000. The 1968 Wahine storm not only destroyed the macrophyte beds that swans relied on for food and nesting sites, but an estimated 5000 birds were killed. Since then, many fewer birds have been present. For example, the estimated peak population size during the 1980 s was swans, plus a mute swans (Sagar et al. 2004). Given the current lack of macrophyte beds, there is little chance of swan populations being restored to their former numbers. Further, should macrophyte beds become established, some form of swan control (or swan exclusion) may be necessary to stop excessive browsing by swans until beds achieved a critical mass. Control of swan numbers can be achieved through shooting, or egg collection. In 2007, Ngai Tahu were able to carry out a limited harvest of swan eggs as a means of re-establishing a customary source of mahinga kai. Canada geese feed primarily on terrestrial vegetation such as grass, clover, lucerne and brassicas (Sagar et al. 2004), and as such, the birds cause considerable damage to pasture crops as well as fouling the pasture itself. As a result, Acclimatisation Societies formulated policies to control geese numbers, something which is maintained today. The current maximum number of geese allowed on Te Waihora is , and this number is achieved through culls of birds during the moult phase. A review of the potential to re-establish macrophyte beds in Te Waihora (Lake Ellesmere) 21

31 Thus, should swan numbers increase with establishment of macrophytes, the precedent for control is established, as well as the mechanisms (egg collections, bird culls, and shooting). Salinity changes and plant species Low salinities promote germination of Ruppia (Gerbeaux 1993), a reason why Johnson (2006) considered that the Harts Creek Wildlife Reserve Area would be the preferred site to attempt re-vegetation of Ruppia. High light levels promote more extensive rhizome development (Gerbeaux 1989), which improves the plants anchoring capacity and ability to withstand wave action. Again, the freshwater inflow at Harts Creek would be advantageous. Low light levels promote vertical growth. To promote redevelopment of Ruppia beds, Gerbeaux (1989) suggested that salinity should be low (close to fresh water) in spring to encourage germination, and lake levels should be low in spring to increase light penetration for the growth of seedlings. Of course, low lake levels would also make growing plants more vulnerable to being completely removed by swans, and the potential for uprooting by wave action would also be high. A survey of shoreline vegetation identified 54 different vegetation types, or plant communities, around the lake (Clark and Partridge 1984). Many of these plants are tolerant of varying salinities and inundation, and some areas of saltmarsh vegetation are considered to be of national botanical importance (Taylor 1996). Significant changes in salinity resulting from a different lake opening regime, would have implications for the well-being of some communities. Deoxygenation of lake waters Significant variations in dissolved oxygen can be both diel (day/night), and seasonal. During daylight hours, lake water becomes supersaturated with oxygen, previously from macrophyte photosynthesis, but nowadays from phytoplankton photosynthesis. However, at night, levels of dissolved oxygen reduce to a low at 0800 h (Taylor 1996) due to plant and microbial respiration. Seasonally, the coincidence of low phytoplankton activity and calm conditions make autumn the time when low oxygen concentrations might occur. Commercial fishers occasionally report dead flounders in nets, apparently as a result of low oxygen near the lake bed; such occurrences are most common during calm and misty conditions (Taylor 1996). Historically, extensive areas of the lake suffered from poor water quality because of rotting macrophytes (Colin Arps, commercial fisher, pers. comm.), especially in spring when dead flounders were common in nets set overnight. A review of the potential to re-establish macrophyte beds in Te Waihora (Lake Ellesmere) 22

32 Rotting of accumulated dead macrophytes can cause localised de-oxygenation. Although this is of little consequence to the lake ecosystem as a whole, it would produce an equivalent reduction of fish in the immediate area, plus be unsightly and a nuisance to lake users, especially fishers as it clogs nets Risks of side-effects Were extensive regeneration of macrophytes achieved, the likelihood of extensive blooms of nuisance algal species (especially blue-greens) would reduce slightly as nutrients would become locked-up in macrophytes. However, in the calmer inshore areas, conditions for blooms of blue-greens would be enhanced, and there would be greater likelihood of more frequent and persistent blooms of these nuisance species. Te Waihora is unlikely to be invaded by invasive freshwater macrophyte species such as Ceratophyllum demersum, Egeria densa or Lagarosiphon major (all previously recorded from Canterbury Region) because of the absence of suitable habitat. If habitat for macrophytes is restored in the lake, levels of salinity are still likely to restrict invasive species to limited areas of freshwater inflows. The bird species most likely to benefit from macrophyte regeneration would be black swans, as these nested on the old Ruppia beds, and used them for food. Given the present commitment to control numbers of swans, plus the advent of customary egg harvest, it is highly unlikely that swan numbers would proliferate to the extent they did historically when they were a major nuisance species. As most wading species and other waterfowl species use shallower water than would be colonised by Ruppia, they should be relatively unaffected (except for the presence of windblown macrophyte piles etc). 3. A review of national and international macrophyte restoration projects and their applicability to Te Waihora 3.1. Barriers to macrophyte re-colonisation Figure 8 briefly reviews some of the major barriers to macrophyte re-colonisation that may operate in Te Waihora. Available light is a major constraint to macrophyte growth in Te Waihora, with light attenuation by scattering from high levels of inorganic suspensoids (Gerbeaux and Ward 1991), in turn linked to bed disturbance by wind-generated waves. A high A review of the potential to re-establish macrophyte beds in Te Waihora (Lake Ellesmere) 23

33 phytoplankton biomass is also supported in the lake (Gerbeaux and Ward 1991). The mean light attenuation co-efficient (Kd) of 8.9 reported by Gerbeaux and Ward (1991) corresponded to a maximum depth for macrophyte development (Z c ) of 0.48 m according to predictions developed by Vant et al. (1986). Light barrier? Bathymetry Water depth Attenuation (turbidity, algae) Propagule reserves Wave or biotic disturbance? Wind speeds and fetch Bathymetry Water depth Benthivorous fish abundance Waterfowl populations Plant propagules absent? Seed bank Remnant beds Drift Germination and growth conditions Salinity Water level Desiccation Figure 8:. Potential barriers to macrophyte recolonisation in Te Waihora This restriction of macrophytes to shallow lake margins increases potential for direct disturbance from waves or waterfowl grazing. Wave energy sets the minimum depth for macrophyte growth and may preclude their establishment even in shallow areas where light conditions are suitable. Chambers (1987) predicted minimum plant depth (z min ) within four North American lakes from the depth of surface wave mixing (z wm ) as z wm = 1.10 z min 9.31 A review of the potential to re-establish macrophyte beds in Te Waihora (Lake Ellesmere) 24

34 where the depth of surface wave mixing (z wm ) was assumed to be equal to half the significant wavelength. Lack of macrophyte recovery can be expected where z min is equivalent to or exceeds Z c (Reeves et al. 2002). Although Te Waihora may differ in sediment or wave character from the North American lakes used in this predictive relationship, the equation may be useful to gauge the magnitude of wave restriction for macrophyte development in Te Waihora. Using an estimated wave length of 400 cm (wave model output, Section 4.1) from a sheltered site (Harts Creek) gives an approximate z wm of 200 cm and an estimated z min of cm. This indicates that, even at a sheltered site, the minimum depth that macrophytes could colonise greatly exceeds the depth (Z c ) of 48 cm where light would be adequate for macrophyte growth. Therefore, without substantial intervention to reduce the wave effects, the probability of significant macrophyte regeneration occurring must be considered remote. Waterfowl such as black swan graze macrophytes, although their role in preventing macrophyte recovery is debated (Noordhuis et al. 2002). As swan populations at Te Waihora can still number in the 1000 s (Sagar et al. 2004) they may exert a significant impact on recovering macrophytes. In contrast, coarse fish species (rudd, catfish, tench) that are known to disrupt macrophyte recovery elsewhere are not a large component of Te Waihora fish populations and probably have a minor impact. Scope for macrophyte recovery, speed and extent of re-colonisation may be limited by propagule availability. Sparse remnant stands of macrophytes have been observed over recent years in Te Waihora but are apparently short-lived (Colin Arps, commercial fisher, pers. comm.). Although viable seed of macrophytes is known, the distribution, densities and viability of seed banks are likely to have decreased over the extended period of time the lake has been de-vegetated (de Winton and Clayton 1996). Seed germination and seedling establishment will be particularly sensitive to light and disturbance regimes. Freshwater inflows to Te Waihora are likely to introduce macrophyte fragments, but their establishment would depend on transport to a suitable rooting site, and species specific tolerances for light and salinity. Apart from a requirement for shallow habitat with sufficient light and minimum disturbance, macrophyte recovery may be hampered by rapid drops in water level and desiccation during artificial lake openings, drought or localised wind lash. Fluctuations in salinity in Te Waihora of mostly 5-10 ppt (range ppt, Gerbeaux 1989) are associated with artificial lake openings, evaporation, and freshwater inflows. Different macrophyte species have different salinity optima and these may not be met at critical times of the plants life-history (e.g. germination criteria). A review of the potential to re-establish macrophyte beds in Te Waihora (Lake Ellesmere) 25

35 3.2. Review of macrophyte restoration experiences Submerged macrophyte restoration is relatively new and lags behind terrestrial and wetland restoration (Cooke et al. 2005). In this section we review the international literature for techniques used for in-lake restoration of submerged vegetation (excluding catchment management to minimise nutrient and sediment loading). We also review the limited cases of submerged macrophyte restoration in New Zealand International experiences Biomanipulation Biomanipulation is a term frequently used to describe fisheries manipulation aimed to either reduce turbidity generated by benthivorous feeding fish, or fish predation on biota such as zooplankton and snails that otherwise graze on phytoplankton/epiphytic algae. Biomanipulation has involved stocking of piscivores to control benthivorous or plantivorous fish, or direct removal of benthivores or plantivores by piscicides and intensive fishing methods. Both approaches have had substantial success overseas in improving water clarity and enabling macrophytes to expand (Cooke et al. 1995). Wave/erosion reduction Floating booms of logs or old tyres are used to dampen wave action, with their construction dependant on the fetch, depth, substrate type and strength of wave action (Cooke et al. 2005). Constructed floating islands planted with wetland plants are suited to small water bodies and major earthworks have been used to create breakwalls or artificial islands, although at considerable expense (Cooke et al. 2005). On a smaller scale, c.2 m wide, V-shaped wave breaks have been constructed from plywood or other materials and staked on the exposed side of plantings or remnant clumps. Other methods include the use of mattresses or bundles of brush/branches (Cooke et al. 2005) or brush wattle fences (Moss et al. 1996) to dampen wave action. Recognised problems with larger constructed wave barriers in lakes include potential for algal blooms in the adjacent quiescent areas, drifting debris fouling and damaging structures, navigation hazards and the need to create boater access. Alternative erosion control within lakes has utilised coir geotextiles rolls and mats, in some cases planted with emergent plant species (Cooke et al. 2005). A review of the potential to re-establish macrophyte beds in Te Waihora (Lake Ellesmere) 26

36 Dredging In addition to removal of nutrient-rich sediments that contribute to internal eutrophication of lakes, dredging has been used to reduce sediment re-suspension, to improve substrates for rooted plants or uncover buried seed banks (Hilt et al. 2006). Nevertheless it is recognised as an expensive option that is feasible to apply to limited areas only. Other sediment treatments Treatments to isolate sediments from the water column include capping with nutrient inactivating substances (e.g. alum) or a physical barrier (plastic, sand/gravel). Their outcome for macrophyte re-colonisation is poorly known (Hilt et al. 2006), nor are they likely to be effective in lakes with extensive sediment re-suspension like Te Waihora. Lake level draw down Limited lake drawdown of cm can foster macrophyte development (Hilt et al. 2006), especially if it increases light availability during the early growth season (early spring). Extreme drawdown has been used to enhance sediment properties and rehabilitate macrophyte communities in shallow lakes (e.g. Helsel and Zagar 2003) by dewatering and consolidating the lakebed and making it less prone to wave resuspension. Effectiveness of drawdown as a restoration method for re-vegetation is likely to relate to sediment characteristics, with lakes of low density ( 0.2g ml -1 ), highly organic and flocculent sediments gaining the most benefit. For example sediments (45% organic matter) in Big Muskego Lake (Wisconsin, USA) nearly doubled in density (from to g ml -1 ) following drawdown and rewetting (Helsel et al. 2003). Likewise, experimental desiccation of Lawrence Lake (upper Mississippi River, USA) sediments resulted in substantial sediment consolidation, as the percentage organic matter declined (James et al. 2002). Founder colonies Smart and Dick (1999) provided guidance on the establishment of aquatic plants within un-vegetated reservoirs in North America in the form of founder colonies - actively planting robust propagules / transplants into protective exclosures at strategic sites within a reservoir. Depending on the apparent constraints of herbivory based on A review of the potential to re-establish macrophyte beds in Te Waihora (Lake Ellesmere) 27

37 monitoring at this stage (i.e. apparent spread beyond protected sites), it was then identified if a larger fenced area as required for colonies to expand, with founder colonies ultimately serving as propagule sources for adjacent areas. They emphasised the importance of using well established transplants or large propagules, if necessary cultured on from starter material (e.g. stem fragments, root crowns, tubers, seeds) under suitable growth conditions New Zealand experiences In New Zealand, actions to restore submerged macrophytes have been carried out only as a component of research investigations, usually in small lakes (<55 ha). However other anecdotal evidence may also be drawn from observations of submerged vegetation recovery. The following case histories summarise the initiatives undertaken and their outcomes. Lake Rotomanuka This small (13.6 ha, 8 m deep) lake supported large beds of submerged macrophytes (Egeria densa) over most of the lake bed until the late 1990 s. Since that time submerged plants have remained restricted to limited sites at <0.3 m depth. Barriers to vegetation recovery were investigated (Reeves and de Winton 2005) that included the seed bank, light climate and fish grazing/disturbance from coarse fish species (rudd, catfish). Seed banks were found to be sparse and germination was poor. However, measurements of light attenuation suggested plants should be capable of colonising to at least 2-3 m depth. Consequently small fish exclosures (2.5 x 2.5 m, 30 mm stretched mesh size) were placed in areas of 1.8 to 2 m depth and plants introduced both inside and outside of the exclosures. Four plant species recorded from the lake were established in pots under culture, including two charophytes and two pondweeds (Potamogeton species). Plants were introduced to the lake in individual pots at <0.5 g dry weight and heights of c m. Pondweed species were observed growing to the water surface within the exclosure after 4.5 months, but by nine months all pondweeds outside the exclosures had disappeared. Where exclosures were removed, the surface reaching plants were removed within five months. Plant growth has remained limited to the exclosures for 3.5 years despite the ongoing release of seed and vegetative fragments. The lack of A review of the potential to re-establish macrophyte beds in Te Waihora (Lake Ellesmere) 28

38 plant recovery in this lake is attributed to fish disturbance, particularly grazing by rudd. Lake Rotoroa (Hamilton Lake) A small (54 ha) shallow (mean depth <2.5 m) urban lake, Lake Rotoroa underwent a major vegetation decline in 1989/90 when the relatively diverse vegetation covering 80% of the lake was lost. It was not until 1998 that a natural plant recovery was seen that re-colonised c % of the lake bed, apparently linked to an abundant and responsive seed bank in some areas (Reeves and de Winton 2005). The lake contained populations of rudd, catfish and tench, which were implicated in the limited nature of vegetation recovery via their direct disturbance and grazing of seedlings and plants (de Winton et al. 2002). Dugdale et al. (2006) describe how a 1 ha devegetated bay was netted off and its fish populations were reduced by an estimated 86% (5115 fish, 451 kg removed). Plugs of intact charophytes (0.4m 2 ) were transplanted to the netted area on two occasions but decreased in size over time. Nevertheless, charophyte plants that had been preestablished in pots could persist, but did not grow as well as plants that were additionally protected by small fish exclusion cages. Increased fish growth rate and recruitment observed following the removal of larger fish was thought to have somewhat negated the fishing effort. In contrast, charophyte plugs introduced to fish exclosures (2.5 x 2.5 m, 7mm mesh size) in another de-vegetated region of the lake expanded to fill 75% of exclosure area within 1 year. When exclosures were removed the charophytes persisted at the same cover for in excess of 1 year. Lake Wainamu Lake Wainamu is a 15 ha dune lake north-west of Auckland that underwent a vegetation decline in the late 1900 s and became more turbid. A netting initiative to reduce the influence of coarse fish (perch, rudd, and goldfish) was hoped to improve water quality. Approximately 9000 fish were removed over (ARC data). Subsequently a submerged vegetation and sonar survey of the lake made in 2005 and repeated in 2007 showed large increases in macrophyte biomass and depth range (extended by 2.5 m). A review of the potential to re-establish macrophyte beds in Te Waihora (Lake Ellesmere) 29

39 Lake Serpentine South Lake Serpentine South is a small (1.4 ha), shallow (c. 4 m) peat lake in the Waikato region and part of the complex of three lake basins in the Serpentine group. In the early 2000 s the other two lakes supported some of the last examples of purely native submerged vegetation in the Waikato group, but Lake Serpentine South was largely de-vegetated. The discovery of rudd in the lakes and concern about their impacts on vegetation values led Department of Conservation and Environment Waikato to carry out an intensive netting campaign. In Lake Serpentine South a 7-fold reduction in rudd catch rate (first night CPUE, fish per net per night) was achieved over In late 2005 a major increase in native pondweeds saw over half the lake bed colonised by dense beds. The fishing effort was thought responsible for the recovery in the absence of any other lake or catchment changes. Lake Whangape A large (1079 ha) shallow riverine lake, Lake Whangape underwent a partial vegetation decline and plant community shift in 1987/88 (Wells and Clayton 1989). In 1985 six large cages (4 x 2.5 x 1.7 m high, 55 mm mesh) were erected in different areas of the lake to exclude swan browsing. Within one growing season the cages contained macrophytes that were taller (to the water surface) and generally had a higher biomass per area than immediately outside the cage. In addition, the cages supported plant species that were uncommon in the remainder of the lake. Caged plants persisted after macrophytes had declined to insignificant levels elsewhere in the lake (Wells and Clayton 1989) Application of restoration techniques to Te Waihora Large shallow lakes where light attenuation is driven by wave resuspension of bottom sediments are recognised as difficult restoration cases, while chances are further reduced in the case of saline influenced lakes (Moss et al. 1996). In Table 5 we review the applicability of various techniques to re-establish macrophytes in Te Waihora. A review of the potential to re-establish macrophyte beds in Te Waihora (Lake Ellesmere) 30

40 Table 5: Application to Te Waihora of methods and techniques for macrophyte restoration, including initial scale of actions, subjective assessment of chances for success, and consideration of major risks. Restoration Method Scale Success Risk Lake level management Setting optimal lake level for maximal macrophyte habitat in spring, followed by slowly increased levels over summer Lake-wide Low-moderate May not be able to manage for flood inflows or wind lash influencing water level at some shores Light climate, grazing and dessication may still restrict vegetation development Moderate-high Possible impacts on surrounding land use, or salt marsh areas. Biomanipulation Removal of coarse fish (tench, goldfish, perch rudd) to reduce benthivorous/planktivorous influences on lake Lake-wide Low Coarse fish are a minor component of the lake fisheries and unlikely to be having undue effect High Scope for impacts on valued fisheries. Extreme lake level drawdown Exposure of lake sediments to drying and oxidation Littoral margins with area depending on extent of drawdown Low Lake sediments described as sandy to silty loams (In Gerbeaux 1989), not suitable as not highly organic? High Scope for impacts on valued fisheries and unknown repercussions for surrounding salt marsh areas. Wave reduction Structures to deflect or break wave action Erosion control Use of geotextiles rolls or mats to reduce sediment resuspension Exclosures Structures to exclude fish and/or waterfowl grazing and disturbance. Either artificially planted or to protect emerging seedlings Founder colonies Introduction of robust plants or propagules to overcome limitations of sparse seed bank or lack of seedling establishment Site - bay Moderate Moderate For larger structure an increased risk of local algal blooms? structures easily damaged, navigation hazards, need to allow boater access across extended booms Site Site - bay Site - bay Low-moderate May not be appropriate for high energy environments such as Lake Ellesmere Moderate to high Good evidence for success in range of lakes, waterfowl grazing likely in Lake Ellesmere Moderate to high Improved chance of success when combined with exclosures above Low Low Navigation hazard Low A review of the potential to re-establish macrophyte beds in Te Waihora (Lake Ellesmere) 31

41 4. To assess the conditions required for macrophyte rehabilitation in the lake, focusing especially on defining suitable windows for successful establishment and canopy development In February 1986 and 1987, a survey was made of the biomass of macrophyte in Overton s Bay and Taumutu (Gerbeaux 1989). During this period there was an unusually extensive growth of macrophytes, with Ruppia dominating at Taumutu, perhaps as a result of the greater water clarity, and Potamogeton pectinatus in Overton s Bay. The presence of P. pectinatus in Overton s Bay indicated it had the ability to withstand the more turbid and turbulent conditions at that site, and has flexible growth forms able to adjust to reduced light conditions (Gerbeaux and Ward 1991). These authors suggested that the appropriate environmental conditions to enable macrophyte germination and growth included low salinity for germination in spring followed by two weeks of high water clarity and so that light could stimulate both horizontal (rhizome) and vertical (stem) growth. Prior to the present review, Sagar et al. (2004) had constructed a model to determine relationships between wind speed, lake level, and sediment resuspension - this was then linked to an optical model that allowed the suspended sediment concentrations to in turn be linked to attenuation of light within the water column. Suspended sediment was shown to be the main determinant of water clarity in the lake, and was in turn determined by wind action, and to a lesser extent by lake depth. The model predicted that varying lake levels had only a minor effect on suspended sediment concentrations and associated light attenuation properties. So, even at high lake levels, wind action was sufficient to agitate sediments over much of the lake and produce highly turbid water - only by taking the lake level well above its current high levels would clear water be possible. To re-establish plants would require the fortuitous coincidence of prolonged periods of low wind to enable water clarity to improve to the extent that seeds could germinate at depths of m where they would stand a greater chance of surviving subsequent wave action; they would then need to grow rapidly to reach the surface. Without human intervention, the likelihood of these requirements seems remote. A review of likely areas where macrophyte rehabilitation could be attempted (Johnson 2006) suggested that the Harts Creek Wildlife reserve area would be the logical place to try. Reasons given were: Sheltered by Timberyard Point Reduced salinity and greater germination potential A review of the potential to re-establish macrophyte beds in Te Waihora (Lake Ellesmere) 32

42 Reduced level fluctuations Absence of boats and other human activities Remnant plants remain High value of macrophytes to breeding and feeding waterfowl It was suggested that replanting would be required to achieve a sustainable quantity of macrophyte in the area. Since the Sagar (2004) report, NIWA has developed a more sensitive wind model, and that was run to see whether outcomes varied from the previous assessment, and to indicate the most likely areas of the lake where macrophyte re-establishment would be possible Wave modelling Exposure of the Te Waihora shoreline to locally generated wind waves was studied using the SWAN numerical wave model. The prevailing wind directions, as indicated by a wind-rose diagram (Figure 9) of wind speed and direction for the Lincoln Broadfields EWS recorder over the 8 year period (June 1999 June 2007), are northeasterly and south-westerly. The modelling considered a strong constant wind (taken as 20 m. s -1, which is close to the mean annual wind speed at Lincoln Broadfields), blowing from the north-east and the south-west, as separate cases. Bathymetry of the lake was obtained from the 1:25000 scale bathymetry chart of Te Waihora (NZ Oceanographic Institute DSIR Chart, Lake Series), the basis of which was a May 1988 echo sounding survey. Depth contours were defined on the chart at 0.1 m intervals. These were first digitised. Spatial interpolation was then applied to the irregular point data to obtain a regular 100 m grid that could be input into SWAN. The focus of the modelling was to obtain wave exposure of the shoreline at a typical normal lake level. The reduced level of the lake at the time of the 1988 bathymetry survey was 0.8 m a.m.s.l., which is close to the wind affected natural mean lake level ( ) of m. The 1988 survey level (RL 0.8 m) was therefore adopted without modification. The SWAN model used was a third generation mode model. This model simulates transfer of wind energy to waves (using linear and exponential growth terms) and dissipation of wave energy by bottom friction, white-capping and depth-induced wave A review of the potential to re-establish macrophyte beds in Te Waihora (Lake Ellesmere) 33

43 breaking. Non-linear wave-wave interactions were also represented. The model uses the empirical JONSWAP bottom friction model for fully developed wave conditions in shallow water and applies this as a constant coefficient of friction for the bed. No attempt was made to calibrate for bottom friction hence the model must be regarded as uncalibrated. Modelling was done using a 200 m computational grid comprising 131 by 79 cells covering the physical domain. 69 m/s Calm Percent Figure 9: Wind speed versus mean wind direction at the Lincoln Broadfields EWS recorder site, 30-Jun-1999 to 15-Jun Wave exposure The simulated mean wave height and direction fields for the two applied wind scenarios considered are shown in Figure 10. These cases are: constant wind (20 m. s - 1 ) from the north-east (top plot) and constant wind (20 m. s -1 ) from the south-east (bottom plot). The plot colour represents the mean wave height with red indicating high wave height (or energy) and blue, low wave height (or energy). In Figure 10, the stretches of shoreline indicated by thick red lines are areas of high wave energy. These are areas where new lakeside plantings would be at most risk of attack from waves. Hence, we recommend that these areas should be avoided for any new plantings. Note that as the model is uncalibrated for bottom friction, we caution against applying wave heights from the model in an absolute sense. Rather the results should be used to assess areas of high relative wave energy. The model was also run to simulate an extreme storm event (Wahine Storm) and also a period of strong north-westerly winds. During the Wahine Storm, the maximum wind speed recorded at Christchurch airport was 27.3 m. s -1 on 11/4/1968 (note: surface wind is taken as the average wind recorded over a 10 minute interval). For both these scenarios, a constant wind speed of 25 m. s -1 was used. The simulated mean wave height and direction fields for these scenarios are shown in Figure 11. A review of the potential to re-establish macrophyte beds in Te Waihora (Lake Ellesmere) 34

44 Figure 10: Mean wave height and direction generated from a constant wind field of 20 m.s -1 from the NE (top) and 20 m. s -1 from the SW (bottom). Wave height is in metres. Lake level is taken as 0.8 m a.m.s.l. Red lines along the shore indicate areas of high wave energy. A review of the potential to re-establish macrophyte beds in Te Waihora (Lake Ellesmere) 35

45 25 m/s southerly wind Lake Ellesmere Wave Height and Direction m/s north-west wind Lake Ellesmere Wave Height and Direction Figure 11: Mean wave height and direction generated from a constant wind field of 25 m. s -1 from the South (top) and 25 m. s -1 from the NW (bottom). Wave height is in metres. Lake level is taken as 0.8 m a.m.s.l. A review of the potential to re-establish macrophyte beds in Te Waihora (Lake Ellesmere) 36

46 From the 20 m. s -1 simulation, the areas of high wave energy during north-east winds are the reach from Irwell towards Harts Creek, and the eastern half of Kaitorete Spit; the most sheltered reach is Kaituna Lagoon and embayments either side of the Selwyn River delta. During 20 m. s -1 south-east winds, the Greenpark Sands are particularly exposed, while most sheltered areas are those along the western shoreline including the western portion of Kaiterete Spit. Results from the 25 m. s -1 simulations were rather similar to those for the 20 m. s -1. Strong southerly wind conditions like those experienced during the Wahine Storm again show high wave activity along Greenpark sands and into the Selwyn River embayments; the least affected areas are the western shoreline, although much of this shoreline would become dewatered for the duration of the gale. A period of north-west gales again impacts along the eastern half of Kaitorete Spit, with the least affected reaches being the Selwyn River embayments, and much of the eastern shoreline. The conclusion from these simulations are that the western shoreline is the area least exposed to high wave energy, especially within the vicinity of Harts Creek and the area south to Taumutu. Likewise the shoreline between the LII and Selwyn River mouths is relatively sheltered. Of course, the downside of being sheltered is the loss of water in these areas as it gets pushed to the opposite end of the lake. Although not well documented, experienced fishers on the lake speak of level changes of > 1 m being not uncommon. While many aquatic plants have some ability to tolerate some exposure to the air, their survival will largely depend upon the duration of this exposure, the combined effects of desiccation by wind and air temperatures, and the extent that the substrate remains wet; generally exposure of a few hours would be tolerable, although even this could be critical to young plants during any development phase. The wave height model was also run to generate wave length data for inclusion in the previous section on barriers to re-colonisation (Section 3.1). The simulations are shown in Appendix I, The wavelength varies according to the direction and strength of the wind, but under the prevailing north-east winds, the southern shoreline experiences wavelengths of 3 4 m relatively close to shore. As indicated in Section 3.1, such conditions mean that the likelihood of natural regeneration of macrophytes without some wave-buffering facility, is extremely low A possible scenario for re-establishing macrophytes An assessment of methods or techniques for restoring macrophytes at Te Waihora (Table 5, section 3) identifies a combination of founder colony plantings, grazing exclosures and wave baffles as having moderate to high chances of success and low to A review of the potential to re-establish macrophyte beds in Te Waihora (Lake Ellesmere) 37

47 moderate risk of unacceptable or deleterious impacts. The following provides a description of the possible steps, and considerations required if these restoration actions were pursued. Site selection Site selectivity for founder colonies should consider protection from winds and wave action, depth, slope, substrate (mostly as an indicator of the energy environment), and potential for root anchorage in the sediment e.g. to 15 cm (Smart and Dick 1999). For example, locating founder colonies within protected coves was strongly recommended. Suggested locations based on the wave modelling are Greenpark Sands, Harts Creek to near Taumutu and Kaituna. Other workers have suggested Taumutu, Overtons Bay, Lakeside, Kaituna Lagoon (Gerbeaux 1989) or Harts Creek Wildlife Reserve (Johnson 2006). Cho and Poirrier (2005) used a model for selecting potential restoration sites based on predicted Z max, Z min, water level fluctuation and shoreface slope. The same factors should also be considered in identifying an appropriate planting depth, although Smart and Dick (1999) suggest general depths of 0.5 to 1.0 m. Protection from grazing The use of exclosures to protect plantings from herbivory is strongly recommended, with the type of exclosure required dependant on the type and expected level of herbivory (Smart and Dick 1999). Small mesh cylinders offering protection to individual plants or clumps at 4-6 m intervals are useful under conditions of low to moderate herbivory, with larger sized exclosures, up to an entire fenced cove more appropriate for high numbers of larger grazers (e.g. goldfish), although exclusion of fish fry is difficult because of their small size. Specific construction methods are suggested in Smart and Dick (1999), but materials and design would be dictated by the conditions where exclosures are deployed. Mesh size to exclude small grazers was suggested at 2.5 cm and 5 x 10 cm mesh as adequate for larger grazers. While the structures suggested by Smart and Dick (1999) were generally open at the top, Moss et al. (1996) emphasise protection against waterfowl by either covering exclosures with netting, or for smaller exclosures by raising the sides 1 m above the water surface to deter birds landing from the air. A review of the potential to re-establish macrophyte beds in Te Waihora (Lake Ellesmere) 38

48 Protection from wave disturbance Wave baffles or breaks are additional structures likely to aid macrophyte establishment, with the type of structure used being appropriate to the site. Larger wave baffles or breakwaters may have the additional benefit of improving local water clarity by settling suspended sediments, although they carry higher risk of problems (Table 5). Propagule sources Planting of robust propagules or whole macrophytes has advantages under highly turbid conditions as in Te Waihora, where shoots and leaves need reserves to reach to or near the water surface and escape severe light limitation (Scheffer et al. 1992). Consequently, artificial plantings may circumvent the light barrier that prevents establishment of macrophytes from remnant seed banks. The selection of species for restoration should consider historic records for Te Waihora (Gerbeaux 1989, Gerbeaux and Ward 1991) and species suitability based on known tolerances, perennial versus annual life history and propagule characteristics as given in Table 6. Of the species listed, Stukenia pectinata is the best candidate for plantings as it is tall-growing, tolerant of salinity fluctuations, tends to persist as a perennial, and forms tubers containing considerable food reserves. Table 6: Macrophyte species ranked in order of suitability for tranplantings to Te Waihora. Species Historical presence Tolerances Life-form Propagule Stukenia pectinata Bed-forming Fresh-brackish Submerged perennial Tuber Ruppia megacarpa Bed-forming Brackish Submerged perennial Seed Ruppia polycarpa Bed-forming Fresh-brackish Submerged annual Seed Lepilaena bilocularis Frequent Brackish Submerged annual Seed Lamprothamnium macropogon* Frequent Fresh-brackish Submerged perennial Oospore Zanichellia palustris Present Fresh-brackish Submerged annual Seed Ranunculus fluitans Present Fresh Submerged perennial? Myriophyllum triphyllum Present Fresh Submerged perennial Seed Lilaeopsis sp. Present Fresh-brackish Amphibious perennial Seed Chara globularis Present Fresh-brackish Submerged perennial Oospore Myriophyllum sp. Present Fresh Submerged perennial Seed Formerly Potamogeton pectinatus *Formerly Lamprothamnium papulosum A review of the potential to re-establish macrophyte beds in Te Waihora (Lake Ellesmere) 39

49 Additional species (e.g. from additional plantings or seed bank) would contribute to a diverse macrophyte assemblage with better prospects for long term establishment. Collection of source plants suited to local conditions is best, although culturing to a robust size before transplanting is strongly advised (Smart and Dick 1999). Plantings are recommended at 0.4 to 0.8 plants m 2 (Smart and Dick 1999) or 10 stem fragments (10 cm long) or turions per m 2 (Moss et al. 1996). Elsewhere vegetative propagules per m 2 were recommended to cover large areas (Cooke et al. 2005). Methods to place or secure plantings/propagules at sites include sandwiching shoots between sheets of plastic netting weighted in place by stones (Moss et al. 1996), in baskets (with and without substrate), planting in clay within weighted pots, incorporating plants in artificial textile mats, and use of weighted nets to hold plants on the substrate (Hilt et al. 2006). Buoyant tubers must be planted c. 5 cm deep and covered completely (Smart and Dick 1999). Timing of restoration Spring is the optimal planting time (Moss et al. 1996, Smart and Dick 1999) to give sufficient establishment time over the growth season, but should be tempered by the timing of a suitable water level regime (Smart and Dick 1999). For example, Gerbeaux (1989) identified favourable periods for macrophyte establishment in Te Waihora as periods of low lake level (>0.5 m) at the beginning of the growing season followed by gradual increases, with a low risk of plant exposure and desiccation. He also identified low salinity levels of <4-8 ppt for the seedling growth of R. megacarpa and R. polycarpa and <10 ppt growth for S. pectinata (Gerbeaux 1989). Cores taken from Whakaki Lagoon, Wairoa, have likewise indicated salinity tolerances for germination of <3.5 ppt for R. polycarpa, ppt for L. bilocularis and ppt required for L. macropogon (NIWA unpublished data). 5. Identify any critical gaps in information that are needed to determine suitable restoration conditions Before field trials were commenced, further information should be obtained on the extent and viability of the seed bank. Although Ruppia seedballs are still present in the lake (e.g. Jellyman 2007), the proportion of germinating seed appears to be low. Additional work on the extent and viability of the seed bank would be an important precursor to determining whether any re-establishment would be possible using existing seed, or whether propagules from elsewhere would need to be imported. A review of the potential to re-establish macrophyte beds in Te Waihora (Lake Ellesmere) 40

50 Before any commitment was made to an extensive field evaluation, a small pilot scale project should be initiated. Monitoring of this would provide valuable information on rates of colonisation, nutrient uptake, and the periodic effects of partial or complete dewatering. 6. Concluding comments There is some potential to re-establish macrophytes within selected reaches of Te Waihora. However, while there are benefits that would arise from a successful programme, there are also a number of negative effects. Ultimately a decision will need to be made balancing the perceived benefits against the costs of attempting to establish a viable, self-sustaining area of macrophyte bed and the risks that another extreme weather event could nullify the effort and expenditure involved. This evaluation will, of necessity, consider the staged costs of such a restoration programme, and whether this is the most effective means of spending a not inconsiderable amount of money, or whether there are other more effective means of improving the water quality of the lake. 7. Acknowledgements We wish to thank ECan (Ken Taylor, Shirley Hayward) for the opportunity to collate information on this issue. Thanks also to Brian Sorrell and Graeme Horrell of NIWA for oversight of this project. 8. References Annala, J.H.; Sullivan, K.J. (1997). Report from the Fishery Assessment Plenary, May 1997: stock assessments and yield estimates. Wellington, Unpublished report held in NIWA library. 381 p. Anon. (1997). Catfish invasion causes concern at Te Waihora. Te Reo o te Tini a Tangaroa 40: p 8. Beentjes, M.P. (1999). Size, age, and species composition of commercial eel catches from South Island market sampling, NIWA Technical Report p. Beentjes, M.P.; Chisnall, B.L. (1998). Size, age, and species composition of commercial eel catches from market sampling, NIWA Technical Report 29: 124 p. A review of the potential to re-establish macrophyte beds in Te Waihora (Lake Ellesmere) 41

51 Beentjes, M.P.; Dunn, A. (2008). Catch per unit effort (CPUE) analysis of the South Island commercial freshwater eel fishery, to Draft Fisheries Assessment Report, Ministry of Fisheries, New Zealand p. Burnet, A.M.R. (1952). Studies on the ecology of the New Zealand longfinned eel, Anguilla dieffenbachii Gray. Australian Journal of Marine and Freshwater Research 3: Chambers, P.A. (1987). Nearshore occurrence of submerged aquatic macrophytes in relation to wave action. Canadian Journal of Fisheries and Aquatic Sciences 44: Chisnall, B.L.; West, D.W.; Boubee, J.A.T. (1992). Structure of the eel population in Lake Waahi New Zealand Freshwater Fisheries Miscellaneous Report p. Clark, D.J.; Partridge, T.R. (1984). The shoreline vegetation of Lake Ellesmere, Canterbury, New Zealand. Report prepared for the North Canterbury Catchment Board and Regional Water Board, Christchurch. Cooke, G.D.; Welch, E.B.; Peterson, S.A.; Nichols, S.A. (2005). Restoration and management of lakes and reservoirs. Third edition, Taylor and Francis, New York. 591 p. de Winton, M.D.; Clayton, J.S. (1996). The impact of invasive submerged weed species on seed banks in lake sediments. Aquatic Botany 53: de Winton, M.D.; Taumoepeau, A.T.; Clayton, J.S. (2002). Fish effects on charophyte establishment in a shallow, eutrophic New Zealand lake. New Zealand Journal of Marine and Freshwater Research 36: Dugdale; T.M.; Hicks, B.J.; de Winton, M.D.; Taumoepeau, A.T. (2006). Fish exclosures versus intensive fishing to restore charophytes in a shallow New Zealand lake. Aquatic Conservation: Marine and Freshwater Ecosystems 16: Gerbeaux, P.. (1989). Aquatic plant decline in Lake Ellesmere: a case for macrophyte management in a shallow New Zealand lake. Thesis. Lincoln College and University of Canterbury, New Zealand. A review of the potential to re-establish macrophyte beds in Te Waihora (Lake Ellesmere) 42

52 Gerbeaux, P. (1993). Potential for re-establishment of aquatic plants in Lake Ellesmere (New Zealand). Journal of Aquatic Plant Management 31: Gerbeaux, P.; Ward, J. (1991). Factors affecting water clarity in Lake Ellesmere, New Zealand. New Zealand Journal of Marine and Freshwater Research 25: Glova, G.J. (1996). Propsects for enhancement of the Lake Ellesmere trout fishery. Water and Atmosphere 4: Glova, G.J.; Sagar, P.M. (2000). Summer spatial patterns of the fish community in a large, shallow, turbid coastal lake. New Zealand Journal of Marine and Freshwater Research 34: Gorman, T.B.S. (1960a). Report on flatfish: Lake Ellesmere. Unpublished Report, Marine Department, Christchurch, New Zealand. 26. p. Gorman, T.B.S. (1960b). Lake Ellesmere flounder fishery. Unpublished Report, Marine Department, Christchurch, New Zealand. 25 p. Graynoth, E.; Jellyman, D.J. (2002). Growth, survival, and recruitment of juvenile shortfinned eels (Anguilla australis) in a large New Zealand coastal lake. New Zealand Journal of Marine and Freshwater Research 36: Hardy, C.J. (1989). Fish habitats, fish, and fisheries of the Ellesmere catchment. New Zealand Freshwater Fisheries Report p. Hayes, J.W.; Rutledge, M.J.; Chisnall, B.L.; Ward, F.J. (1992). Effects of elevated turbidity on shallow lake fish communities. Environmental Biology of Fishes 35: Hayes, J.W.; Rutledge, M;J; (1991). Relationship between turbidity and fish diets in Lakes Waahi and Whangape. New Zealand Journal of Marine and Freshwater Research 25: Helsel, D.R.; Madsen, J.; James, B. (2003). Big Muskego story: changes in sediment, water quality, and aquatic plants. Lakeline 23(1): Helsel, D.R.; Zagar, T. (2003). Big Muskego story: rehabilitating a large shallow lake. Lakeline 23(1): A review of the potential to re-establish macrophyte beds in Te Waihora (Lake Ellesmere) 43

53 Hilt, S.; Gross, E.M.; Hupfer, M.; Morscheid, H.; Mahlmann, J.; Melzer, A.; Poltz, J.; Sandrock, S.; Scharf, E-M.; Schneider, S.; van de Weyer, K. (2006). Restoration of submerged vegetation in shallow eutrophic lakes a guideline and state of the art in Germany. Limnologica 36: Howard-Williams, C.; Davies, J. (1988). The invasion of Lake Taupo by the submerged waterweed Lagarosiphon major and its impacts on the native flora. New Zealand Journal of Ecology 11: Hughes, H.R.; McColl, R.H.S.; Rawlence, D.J. (1974). Lake Ellesmere. A review of the lake and its catchment. New Zealand Department of Scientific and Industrial Research. 27 p. James, W.F.; Barko, J.W.; Eakin, H.L. (2002). Experimental determination of the impacts of sediment desiccation and rewetting on sediment physical and chemical characteristics in Lawrence Lake, Pool 8, Upper Mississippi River. 20 p. Jellyman, D.J. (1992). Lake Ellesmere - an important fishery with an uncertain future. Freshwater Catch No. 48: 3-5. Jellyman, D.J. (2001). The influence of growth rates on the size of migrating female eels (Anguilla australis) in Lake Ellesmere, New Zealand. Journal of Fish Biology 58: Jellyman, D.J. (in prep.). What causes the high interannual variability in the species composition and abundance of flatfish (Rhombosolea spp.) in Lake Ellesmere (Te Waihora), Canterbury, New Zealand? New Zealand Journal of Marine and Freshwater Research. Jellyman, D.J What the?. Water and Atmosphere 15 (1): 6. Jellyman, D.J.; Graynoth, E. (2005). The use of fyke nets as a quantitative capture technique for freshwater eels (Anguilla spp.) in rivers. Fisheries Management and Ecology 12: Jellyman, D.J.; Smith, C. (In press). Native Fish and Fisheries of Te Waihora.. Te Waihora/Lake Ellesmere - the 2007 state of the lake and management futures report. 14 p. A review of the potential to re-establish macrophyte beds in Te Waihora (Lake Ellesmere) 44

54 Jellyman, D.J.; Chisnall, B.L.; Todd, P.R. (1995). The status of the eel stocks of Lake Ellesmere. NIWA Science and Technology Series 26: 62. Johnson, C. (2006). Aquatic vegetation at re-establishment and shallow lake management. Pp In: Te Waihora Lake Ellesmere Research Projects. Geog Management of Physical Earth Systems. Kelly, D.J., Jellyman, D.J Changes in trophic linkages to shortfin eels (Anguilla australis) since collapse of submerged macrophytes in Lake Ellesmere, New Zealand. Hydrobiologia 579: Maindonald, D.R. (1990). Black swan nesting Kaitorete Spit, September Internal report to the North Canterbury Acclimatisation Society, Christchurch. Mason, R. (1946). Report on weed banks in Lake Ellesmere. Botany Division, DSIR report prepared for the North Canterbury Acclimatisation Society. McDowall, R.M. (1994). Gamekeepers for the nation. The story of New zealands acclimatisation societies Christchurch, New Zealand, Canterbury University Press. 508 p. Moss, B.; Madgwick, J.; Phillips, G. (1996). A guide to the restoration of nutrientenriched shallow lakes. WW Hawes, UK. 180 p. Noordhuis, R.; van der Molen, D.T.; van den Berg, M.S. (2002). Response of herbivorous water-birds to the return of Chara in Lake Veluwemeer, The Netherlands. Aquatic Botany 72: O'Connell, D. (1996). Nature and extent of Te Waihora customary eel fishery. Mahinga tuna kei Te Waihora. Te Taumutu Runanga and Te Waihora Eel Management Committee. 43 p. Reeves, P.; Craggs, R.J.; Stephens, S.A.; de Winton, M.D.; Davies-Colley, R.J. (2002). Environmental changes at Lake Waikare, North Waikato. Wave climate, water quality and 'biology'. NIWA Client Report No: EVW02235, prepared for Environment Waikato. 69 p. Reeves, P.; de Winton, M. (2005). Restoring lake vegetation. Water and Atmosphere 13(3): A review of the potential to re-establish macrophyte beds in Te Waihora (Lake Ellesmere) 45

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56 Webb, B.F. (1982). Report on the growth of the Ruppia megacarpa in Lake Ellesmere. North Canterbury Acclimatisation Society Annual Report Wells, R.D.S.; Clayton, J.S Lake Whangape : the macrophytes. Ministry of Agriculture and Fisheries, Unpublished report. Wood, H. (2008). An investigation of the benthic invertebrate communities of Te Waihora. M.Sc thesis, University of Canterbury. A review of the potential to re-establish macrophyte beds in Te Waihora (Lake Ellesmere) 47

57 Appendix I.: Results of wave length modelling at the same wind scenarios used to generate wave height information (Section 4.2). Wavelength is given in metres 20 m/s north-east wind Lake Ellesmere Wavelength m/s south-west wind Lake Ellesmere Wavelength A review of the potential to re-establish macrophyte beds in Te Waihora (Lake Ellesmere) 48

58 25 m/s southerly wind Lake Ellesmere Wavelength m/s north-west wind Lake Ellesmere Wavelength A review of the potential to re-establish macrophyte beds in Te Waihora (Lake Ellesmere) 49

59