Tuna and native fish populations of Lake Moawhitu and wetland drains, Greville Harbour 1. Background This report summarises the findings of surveys of tuna (eel) and native fish populations in Lake Moawhitu and the adjacent wetland drain habitat undertaken in 2005, 2007 and 2013. It also reports on the condition of aquatic habitats and develops options for future enhancement of habitat and the fish populations. Of special interest is the health and future management of the tuna population because of its great importance to Ngati koata as a customary fishery and sustainable use options - for longfin tuna in particular which a species At Risk (Allibone et al 2010). First inspection of the lake and wetland was undertaken in early December 2005 prior to its destocking in May 2006 following its purchase by the Crown for inclusion within D'Urville Island Scenic Reserve. The initial 2005 freshwater inspection was to gain a general picture of the lakes fish population and consisted of some limited fyke netting and minnow trapping, with additional spotlighting observations around the lake margins. Following the initial freshwater inspection in 2005 a more detailed survey of tuna and other native fish formed part of a general ecological baseline survey of the property in May 2007 (refer Moore 2007, 2008, Rutledge 2007) with a follow up fish survey in May 2013. Originally survey work was to assist a large scale restoration programme funded as part of the government's initiative under the Carbon Neutral Public Service Programme to maximise carbon sequestration. The programme was cut in 2009 so restoration actions have become limited- including those relating to wetland restoration which were developed in plans by Preece 2008 and Walls 2006. Some drain blocking has occurred to help wetland recovery and as discussed later, rush regeneration around the lake margins is showing positive effects on aquatic habitat. Long term recovery of aquatic, riparian and catchment vegetation will require decades, with the fish population showing a progressive benefit as habitat and water quality improve over time. 2. Survey methods Un-baited fyke nets and fine meshed Gee minnow traps baited with marmite were the main survey methods used in the surveys, with some spotlighting observations made in 2007. Spotlighting is an effective technique for detecting schooling pelagic (ie up off the bottom) fish species which are less prone to capture in passive fishing gear such as traps and fykes which target fish more benthic (ie bottom living) in their habits. Besides tuna the main benthic species is the common bully (Gobiomorphus cotidianus) while the two pelagic species known to occur in the lake are inanga (Galaxias maculatus) and common smelt (Retropinna retropinna). General sampling locations are indicated in Figure 1. Unfortunately dirty water in 2013 prevented spotlighting observations being made. In 2007 and 2013, 12 fyke nets were set in the lake. In the Wetland drains 3 fykes were set in 2007 and 8 in 2013. The increased number set in 2013 was to get a more detailed picture of the fish populations throughout the drains. Five, fine meshed (3mm) Gee minnow traps were set in the lake in 2013 and 7 in 2007 with the main purpose to sample small fish and juvenile eels. Catch data for each sampling date was aggregated and mean rates of catch and fish numbers calculated. To find out how healthy tuna were they were weighed and measured after being anaesthetised in clove oil. The relationship between eel length and weight provides an index of condition or fatnesss. A short fat eel is in good condition while a long skinny eel is in poor condition.
Eel condition factor was calculated using the relationship : condition = 106 W/L3; where W= weight in grams and L=length in mm ( Bagenal 1978). Water samples were also taken in the Lake and Wetland drains to provide some indication of any water quality changes between 2007 and 2013. Main findings, and comments are shown in Table 1, with more detailed discussion in Section 3 below. At the time of the 2013 survey it was also possible to speak to Tahua Rene an island resident to get his general observations on the history of tuna harvest. A former caretaker based at Moawhitu also provided her observations of elver movements at the lake outlet culvert in Jan 2012. This information has been incorporated into this report as it contributes to understanding of the tuna population. 3. Discussion 3.1 Eel catch rates and population features The tuna population is a mix of both the endemic New Zealand longfin eel (Anguilla dieffenbachii) classified as an At Risk species (Allibone et al 2010) and the shortfin eel (Anguilla australis) which is more widespread in the Pacific region and not currently considered At Risk. The need to provide for recovery of the longfin tuna nationally is currently being considered by the government in response to the recent report of the Parliamentary Commissioner for Environment (PCE 2013) that includes options for closure of the commercial fishery and increased restrictions on harvest. The survey work indicated that the eel population in the lake is comprised of about 65% longfin eels and 35% shortfins, while in the drains longfins comprised more than 90% of the catch. The dominance of longfins in the lake is an interesting feature of Lake Moawhitu as shortfins most often dominate lowland lake systems (e.g. Jellyman 1989 ). Catch rates of eels in the lake were very similar for both species in both 2013 and 2007 (Table 1) with the average catch rate of about 2 eels per net/ night low compared with catch rates for other lowland lakes in New Zealand. The catch rate of eels in the wetland drains was double that in the lake at about 4 eels/per net/night in 2013. This reflected a very good catch of 18 larger longfin eels in one particular net set immediately upstream of the bridge where it crosses the Main Outlet Drain.
LAKE MOAWHITU 2007 2013 Longfin eel Total caught 13 14 Mean Catch per net 1.08 1.16 Mean length* (mm) 503.7 699.4 Mean weight (grams) 338.4 1050.4 Mean condition factor 2.39 2.84 Shortfin eel Total caught 9 10 Mean Catch per net 0.75 0.83 Mean length(mm) 598 561.2 Mean weight (grams) 550.5 448.4 Mean condition factor 2.23 2.28 Common bully Total caught 2406 2229 Mean Catch per trap 479 318 Common smelt and inanga No inanga were caught in minnow traps during survey work and just 2 smelt were caught in traps in 2007. Longfin Shortfin Total caught 33 3 Mean Catch per net 4.1 0.37 Mean length (mm) 730.4 598.3 Mean weight (grams) 1271.9 868.5 Mean condition factor 2.91 2.30 Bullies and other fish in the drains Table 1: Summary of findings Comments * 5 large eels caught near the rushes significantly increased overall mean size and condition in 2013 compared to 2007. Longfin numbers caught in 2013 were very similar to those in 2007. No juvenile eels were caught in either fyke nets or minnow traps in 2007 or 2013. Observations made by turning over large cobbles and boulders on the lake margin in both years showed a few small elvers to be present. Shortfin numbers caught in 2013 were very similar to those in 2007. Eel condition was slightly better in 2013 with a shortfin of about 450mm with a bad fungal infection observed. No shortfin elvers caught. In both 2007 and 2013 large numbers of healthy bullies were caught in traps - they occur throughout the lake at all depths and provide a plentiful food supply for eels and birds such as shags and white faced herons. More abundant bullies in 2007 than 2013 probably reflects a better breeding season in 2007. Bullies overall size range was 20-79 mm with an average of about Spotlighting lake margins in 2007 showed abundant smelt and a few schools of inanga. However, dirty lake water in 2013 meant spotlighting for smelt, inanga and other small fish was not possible. Casual observations in 2013 showed large numbers of smelt feeding in the evening. It seems likely, therefore, that good numbers of smelt remain in the lake. The presence of inanga will depend on passage being available for them through the culvert in the whitebait season. So the population size is probably variable. WETLAND DRAINS 2013 Comments In 2007 only limited sampling was done in the drain complex with 3 fyke nets set and just one longfin eel captured. In 2013 the sampling effort in the drains was increased with 8 nets set which captured reasonable numbers of larger longfin eels with few shortfins. Most eels were caught in one very big catch in a net set immediately upstream of the bridge where it crosses the Main Outlet Drain. There was good eel cover provide by large logs. No juvenile eels were caught in fyke or minnow traps set in the drains. No fish were caught in minnow traps set in the drains in 2007 and just one common bully of 50mm was caught in 2013. 2 giant bullies (Gobiomorphus gobioides) 138 and 140mm long were caught in fyke nets set in the drains. The bully population in the drains appears to be of a low density. Other fish species thought to be inanga and possibly some juvenile banded kokopu have been observed in the drains during survey work but have not been caught in traps or nets. If the catch from this net was excluded then the catch rate is similar to that for the lake at about 2eels/net/night. The cover provided by many large logs and the bridge accounts for the concentration of larger longfin eels at the Main Outlet Drain. In a similar way in Lake Moawhitu, the catch of several large longfin eels in a fyke net set near the regenerating rushes at the north end of the lake, shows the attraction that cover has for larger longfin eels. Overall, however, there is poor cover for eels in the drains (especially the smaller ones) which were originally draglined and excavated through the wetland complex as linear channels. Enhancing cover in the drains and lake could therefore be a good way of improving their carrying capacity for tuna (see 3.2.1.)
The lack of much change between 2007 and 2013 in catch rate for the lakes tuna population is not surprising as recovery of populations takes many years probably several decades for the longfin because of population decline, but a shorter time for recovery of the faster growing shortfin. The low tuna catch rate reflects a reduced population size caused by harvesting pressure- in particular the large scale commercial harvesting of tuna which has been reported to have occurred in Moawhitu in the past (Tahua Rene pers. comm.). Tuna are very vulnerable to commercial scale fyke netting - which is so efficient that 75 percent of the eels in a fished area can be caught in a single night (e.g Graynoth et al., 2008, Jellyman 2012). The lake is within d Urville Island Scenic Reserve so that commercial eel fishing and other harvest is prohibited unless expressly authorised by the Minister. It has been reported that formerly the lake sustained good harvests of tuna by tangata whenua using customary methods. Fig 3: 820 mm longfin caught in net set near regenerating rush patch (also see Fig. 4 and 10) Fig 4: Good eel cover in rush habitat at north end of the near lake (arrow indicates location of net) Fig 5: The drains generally have poor cover for eels Fig 6: More longfins were caught where bridges provided cover in the drains Nationally longfin eels are declining and they are an At Risk species (Allibone 2010) with declining runs and disproportionately low numbers of elvers and small size classes of eels (Jellyman 2009). This was a feature of both the shortfin and longfins caught in the lake (refer Figures 8 and 9 below) and in the wetland drains(figure 7) with an under representation of fish in the smaller- medium size class. There were just 2 longfin caught in the 30-40cm category and no shortfin caught in this category at all. No elvers or small eels were caught in fine meshed fyke nets or minnow traps and no elvers were seen during spotlighting surveys. The only observations made of elvers were a few fish about 150-250 mm long observed when larger cobbles and small boulders on the lake margins were turned over in both 2007 and 2013. For Lake Moawhitu recovery of the eel population pre-supposes there is ongoing recruitment of young eels (elvers) into the lake. The very limited number of elvers and small eels (ie 40cm or less) in the lake is therefore of significant concern. Besides the nationally declining runs of longfin elvers there are additional factors at Moawhitu that limit recruitment. These are the overhanging culvert at the lake outlet and the intermittent flows occurring through it.
Wetland drain longfin length classes 10 number of fish 8 6 4 2 0 40-50 50-60 60-70 70-80 80-90 90-100 100-110 length class (cm) Figure 7: Wetland drains longfin length classes Moawhitu longfin length classes number of fish 7 6 5 4 3 2 1 0 30-40 40-50 50-60 60-70 70-80 80-90 90-100 length class of fish Figure 8: Figure 7: Moawhitu longfin length classes Moawhitu shortfin length classes 10 number of fish 8 6 4 2 0 30-40 40-50 50-60 60-70 70-80 80-90 90-100 length class Figure 9: Lake Moawhitu shortfin length classes The placement of boulders at the outlet in 2008 to provide a more continuous climbing surface is likely to have assisted elvers to enter the culvert when water does flow. But the frequency of outflows through the culvert from the lake depends on rainfall and lake height both of which are probably low during the main elver migration season from about mid January through to the end of March. However, on one occasion following heavy rain several hundred elvers were observed migrating through the culvert by the DOC Caretaker during January 2012. Because favourable conditions for elver migration are likely to be infrequent, elver recruitment is likely to be limited and irregular perhaps occurring just every few years. 3.2 Population enhancement options for eels Possible actions that would improve elver recruitment and speed up recovery of the population in the lake include the trapping of elvers downstream of the culvert during dry years when the culvert does not run and then transferring the elvers into the lake. The trap and transfer of elvers to locations upstream of obstacles is a common management tool used to relocate elvers that
have their migration pathway blocked at dams (e.g. see PCE 2013). It involves capturing them in a trap and then letting the elvers go upstream of the dam. This technique could be trialled during the elver migration season during periods when there is no lake outflow. This would require people to be on site to set and service traps during the elver migration season (December- March ). It would also be helpful to know the frequency with which the lake outlet was flowing to inform future management approaches. To this end if any DOC staff or local observers could report any such information back to the DOC Sounds Office that would be useful. The maintenance of the rock ramp at the base of the culvert to assist elvers to climb upstream into it (when the culvert has flowing water) is important. Ensuring that the boulders are maintained in place by cementing them into a matrix of concrete is also recommended as a future action. Based on current information on the eel population size, its age structure and recruitment levels it will take many more years for tuna populations to rebuild. Probably shortfin will recover more quickly in the short term than longfins as shortfin elver runs are currently much stronger and more reliable than longfins nationally and shortfin grow faster and have a shorter generation time than longfin. 3.2.1. Future tuna harvest management Commercial and all other tuna harvesting in Moawhitu is prevented under the Reserves Act. Whether there is any ongoing unauthorised tuna take is unknown, but there may be some take by the general public unaware that take is not allowed within the Reserve. Uncontrolled ongoing take of tuna would constrain the recovery of the stocks. It also means there are less tuna available to Ngati koata who may wish to access tuna on a customary basis in the future which may be authorised under the Reserves Act by a special authority. Commercial scale tuna harvesting that has occurred in the past most likely accounts for the depleted tuna stock in Moawhitu. Longfin as an At Risk species need additional protection from harvesting pressure in order to allow the stocks to recover. There are relatively few places that tuna receive such protection and the Parliamentary Commissioner for the Environment (PCE 2013) recently recommended that commercial fishing for longfin be stopped nationally, at least for a time. Currently land held under the National Parks, Conservation and Reserves Acts provide the main refuges for tuna from commercial harvest. In terms of customary harvest options while the main wetland outlet drain showed a concentration of a population of larger sized longfins around cover, elsewhere in the drains the population is sparse. The longfin population if targeted in the main drain would quickly be exhausted and not be a sustainable long term option for harvest. On the basis of protecting longfin tuna stocks and allowing their long term recovery locally and nationally, the main option for customary harvest if desired by Ngati koata would be to investigate the targeting of shortfin tuna in the lake. The out-migration season of mature adult shortfin (called silver eels or tunaheke) from Moawhitu would be expected during February and March. At this time shortfin tunaheke would be accumulating at the outlet end of the lake and seeking to leave via the culvert and then travel further downstream and enter the sea via the main outlet drain. This would be a good time to get an impression of the population size to evaluate harvest potential of the migrant population. It is understood that tuna were taken in this manner under customary techniques. Fish are usually in their prime condition at this time. There may also be some longfiin tunaheke leaving the lake in March- April but these should be easily distinguishable from shortfin by their larger size and be able to be excluded from harvest.
3.2 Tuna size and condition related to habitat factors While increasing the number of elvers able to access the lake is a key tool to assist tuna population recovery, along with limiting harvest to sustainable levels, enhancing the quality and amount of tuna habitat and their access to food sources is another opportunity that will help the populations recover and remain healthy. Cover, food supply and water quality are key factors contributing to the growth, size and condition of tuna. These are considered below. 3.2.1 Cover Of all the sites surveyed the highest density of longfin occurred in the main outlet drain in association with heavy cover provided by large logs (washed back up the outlet stream by tidal surges) and the bridge. They were also the largest longfins and in the best condition. Likewise in the lake, the fyke net set near the heavy rush cover yielded the most large longfins and these were in good condition. This illustrates the importance of cover especially for larger longfins. The blocking up of drains and getting a more closed riparian cover over the drains and the natural accumulation of woody debris and root wads will take many years to provide additional tuna habitat. However, in the short term, increasing the heavy cover that longfins like by placing and tethering large jumble piles of logs into the drains -especially the larger deeper drains, presents a good opportunity to increase the carrying capacity of the drains habitat for tuna now. Immediately upstream of the current log accumulation area could be a logical place to trial this method. Habitat for other native fish such as bullies, inanga and kokopu species is also poor in the drains so these species are also likely to benefit from increased cover. In the lake, in the absence of cattle grazing, rushes (mostly Eleocharis) are quickly regenerating with their expansion around the margins and further out into the lake obvious during the 2013 survey. These should continue to expand providing additional cover for tuna and small fish such as bullies, inanga and smelt- all of which have been caught or observed in the rushes over the time of survey work in the lake. Probably over time, subject to elvers recruiting, the more aggressive and larger longfins will predominate the rushes zone, while shortfins will utilise the sparser weed beds and muddier offshore sediments (which they are adept at burrowing into) further offshore. Getting shade out over or into the water from larger stature climax native forest (e.g kahikatea, pukatea) and smaller stature wet adapted vegetation including flaxes would complement the rushes in providing additional cover and also assist in filtering runoff into the lake. The lake still discolours from runoff from the roads around the lake margin. Large fallen trees and tree limbs that enter lakes provide excellent cover for tuna and other larger native fish such as kokopu species. While the practicality of artificially introducing and tethering large material like this to the lake margins is likely to be challenging, it would provide a method of boosting the carrying capacity of tuna in the lake. Fig 10: The expanding rush fringe at the north end of the lake provides excellent expanding habitat for tuna and small native fish
3.2.2 Food supplies and populations of small native fish Small fish provide the main food supplies for tuna and fish eating birds such as shags and herons using the lake. The results from minnow trap and spotlight surveys show that the lake supports an abundant population of common bully (Figures 11 and 12) with very high catch rates in traps set throughout the lake. The size of bullies over the 3 survey periods ranged from 20-79 mm. During spotlighting observations at the north end of the lake, amongst the large beds of the rush Eleocharis, higher densities of larger bullies were present, presumably reflecting the increased cover and potential bully spawning habitat associated with the rushes. The cover provided by the marginal rush zones and weed beds in the lake (primarily Potamogeton cheesmanii and Myriophyllum triphyllum) provide cover for bullies, smelt (Figure 13) and inanga (Figure 14) and also habitat for invertebrates such as snails, caddisflies, midge larvae and crustaceans which are food sources for fish. Pelagic schooling native fish such as smelt and inanga while seen during spotlight surveys and during daylight casual observations, were almost completely absent from traps owing to their reluctance to enter the bottom set traps. Large numbers of smelt were evident in 2005 and 2007, but dirty water during the most recent survey in 2013 prevented spotlight observations from being made, nevertheless, smelt were obvious during the evening, feeding in the lake margins on the water surface. So it seems that smelt will also contribute as a food source for tuna as will inanga when they are present. As discussed earlier, for inanga to get into the lake there would need to be a good flow through the outlet culvert to allow them to access the lake which is likely to make their numbers quite variable from year to year. Figure 11: Contents of one minnow trap set on lake margins approximately 400 common bullies Figure 12: common bully Figure 13: Common smelt Figure 14: Inanga
Figure 15: Giant bully Figure 16: Banded kokopu The wetland drains support only small permanent populations of native fish, however, during the periods of the year when fish are migrating (such as during the whitebait and smelt migration seasons) there maybe large numbers passing through the drain system that are targeted as food by the populations of longfin and shortfin. Common bully, giant bully, inanga, smelt and banded kokopu juveniles are the species that have been caught or observed in the drains system. Giant bully (Figure 15) and banded kokopu (Figure 16) have not been recorded from the lake. None of these species are particularly abundant in the drains, primarily because the drain habitat is very poor with limited cover for fish and very deep accumulations of thick anoxic mud. The mud was more than 1 metre deep in places with a strong hydrogen sulphide smell. As discussed earlier increased instream and overhead cover would benefit the populations of fish in both the wetland drains and the lake- in particular cover loving species such as tuna, banded and giant kokopu and giant bully. 3.2.3 Water quality and fish health Good water quality is an important factor for the health of tuna, other native freshwater fish and invertebrates. Important factors include adequate oxygen, suitable ph range, temperature (ie not too warm) water clarity (ie absence of suspended sediment) and the absence of pollutants. Animal wastes contribute nutrients and other contaminants such as ammonia which can have directly toxic effects on fish (e.g Hickey 2013) and bacteria and viruses in animal faeces can also affect fish health. Addition of nutrients from livestock faeces or fertilizer inputs can degrade habitat by stimulating nuisance growths of plants- algae, macrophytes and phytoplankton; these can smother habitats, cause oxygen depletion when they decompose and may also alter ph (Drake et al 2010,). The use of weed killers and other agrichemicals can also produce adverse effects on freshwater fish and other aquatic life. A recent study for example showed that the use of the weed killer 'Roundup' increased the parasite load of native fish (Kelly et al 2010). The extent of use of agricultural and other chemicals in the lake catchment prior to de-stocking is not known but may have been significant with ongoing deleterious effects on water quality especially if they have accumulated in the lake bed sediments. The lake bed sediments have not been tested for metals or other contaminants but if present these may be having some effect on the aquatic life in the lake and also the wetland drains which have very thick deposits of sediment. Moawhitu has limited flushing with its small catchment so contaminants will persist for a long time in the thick layer of fine sediment and mud on the lake bed. The first observations of the lakes fish population were made in December 2005 prior to de-stocking and at a time of extremely low lake levels. Observations showed that fish were under stress with pustules, fungus and parasites evident on tuna and common bullies. The more resilient shortfin observed appeared to be less affected in 2005. High nutrient loadings, warm water,
low oxygen and faecal bacterial inputs from cattle grazing and degraded habitat margins from trampling and browsing probably all contributed to this situation in 2005. In 2007 tuna did not show evidence of bacterial and fungal infections but in 2013 there were still some fish with fungus (e.g see Fig. 17). This tends to suggest that water quality conditions are not showing a continuing improvement over time. This is consistent with the water quality results discussed below. Figure 17: Fungal infection and parasite (circle) on a shortfin eel caught in the 2013 survey Figure 18: Slip contributing sediment into lake via road formation While water sampling is based on a one sample snapshot of water quality in May 2007 and May 2013 the results indicated both the lake and wetland drains were significantly enriched with nitrogen (N) and phosphorus (P) compared with natural levels. While faecal coliform levels in the drains had declined in 2013 compared to 2007 levels, there was an increase in N and P levels in 2013. For the lake, N, P and faecal coliform levels all showed an increase in 2013 compared with 2007. The 2013 sample from the lake also showed a potentially toxic species of cyanobacteria (" picocyanobacteria" ) was present. The lake water quality environment, therefore, appears to show no consistent improvement which may reflect limited catchment inflows to dilute and flush out nutrients or contaminants accumulated in the lake bed sediments which then get re-suspended from wave action. Also, at the time of the 2013 survey, there had been recent rain. Rainfall events typically increase nutrient and faecal waste inputs by flushing them into lakes and streams so recent rain may be responsible for elevated levels in the 2013 sample. The lakes turbidity in May 2013 had a yellowish hue presumably reflecting the colour of clay and soils originating from the local catchment where there were signs of fresh slips (Figure 18). It was also noted that suspended fine sediment was draining along and across the roads around the lake edge and not being stilled or controlled by water tabling or filtering vegetation. Use of the roads during wet weather disturbs fine sediment deposited on the roads and contributes to sediment runoff into the lake. Road formation maintenance, resurfacing with metal, water tabling and targeted planting of native vegetation filter strips provide options for reducing sediment inputs into the lake. 4. Summary and recommendations 4.1 Habitat and water quality Aquatic habitat and water quality in Lake Moawhitu and the wetland drains will take in the order of decades to recover from past land use activities without a more active habitat restoration and planting programme along the lines recommended by Preece 2007 and Walls 2006. Providing more and better quality cover
(especially in the wetland drains) will increase the carrying capacity for tuna and other native fish such as inanga, smelt and kokopu. The natural regeneration of the rush beds in the north east sector of the lake and the extra habitat they are providing for tuna and small fish is the most positive benefit aspect of habitat recovery noticed to date. The presence of toxic cyanobacteria, high nutrients, elevated faecal coliforms and ongoing fungal and parasitic infection of fish in the lake show that it is not a healthy environment. Presence of cyanobacteria and elevated faecal coliform levels are also a human health concern (and dogs in particular are very vulnerable to cyanaobacteria). Water quality could be most actively improved by increasing the natural regeneration of vegetation in the catchment and around the lake margins with targetted plantings to reduce sediment run off and nutrient inputs. As discussed above measures to reduce sediment runoff from the roads would also help water quality recover and reduce the deposition of fine silt on the lake bed which can degrade aquatic habitat. Ongoing deer and pig control would also provide benefit by improving the regeneration of native vegetation. It is recommended that options to reactivate the restoration programme are explored with Ngati koata and potentially others with an interest in the lake as a partnership project. 4.2 State of the tuna stocks and harvest options As discussed in 3.2.1 above, past commercial scale tuna harvesting in the lake (as reported anecdotally) is the most likely explanation for depleted stocks. This and all other harvest is now forbidden under section 50 of the Reserves Act. There appears to be plenty of food in the lake to feed tuna and fish eating birds with the very large biomass of common bullies and other small fish. The absence of small size classes of both longfin and shortfin tuna reflects (in the case of longfin) the national decline of this species but also the local effect of the perched culvert outlet and limited flows to allow elvers of both species and other migratory fish such as inanga to recruit into the lake. As discussed in 3.2 above it is recommended that the existing rock ramp is cemented together to improving the reliability of the climbing surface for elvers and other fish. If resources allow it would also be useful to investigate the option discussed in 3.2 of using fine meshed minnow traps placed downstream of the culvert to capture elvers and then release them into the lake to boost the population recovery rate. Observations on how often the culvert does flow and any movement of elvers during the elver migration season would help gain further understanding of the population. Ngati koata are seeking access to the traditional customary tuna fishery that existed at Moawhitu. Given the national decline of longfins, the limited number of small longfin (and shortfin) in the lake and the concentration of big longfin in the limited cover available at the bridge over the main outlet drain, the harvest option that is most sustainable is that of out-migrating shortfin tuna (tunaheke). To evaluate the number and condition of fish (harvest of tuna with fungal infections and parasites obviously would not be recommended) these fish could be caught during the migration season using fyke nets set at the outlet end of the lake (if the culvert is not flowing) or in the outlet drain immediately downstream of the culvert if it is flowing. If there is a good number of shortfin tunaheke available then some could be harvested but allow some of the large female shortfins to escape to sea to spawn. This option should be discussed with Ngati koata along with the contents of this draft report to get their views on its findings and any of their knowledge of the lake and tuna that they may wish to share. Then the best options to make progress on protection and use of tuna in the lake including habitat restoration can be agreed to.
Acknowledgements The writer wishes to acknowledge the assistance provided to by DOC Sounds staff especially Mike Aviss and Richard Andrell, thanks also to Lawson Davey from Nelson Marlborough Fish and Game for his assistance in 2007 and to the batch owners who provided accommodation. Thanks to Tahua Rene and DOC cartetakers for sharing their knowledge of Moawhitu with us. References Allibone, R., B. David, R. Hitchmough, D. Jellyman, N. Ling, P. Ravenscroft and J. Waters 2010. Conservation status of New Zealand freshwater fish, 2009. New Zealand Journal of Marine and Freshwater Research. Bagenal, T.B. 1978. Age and Growth. In: Bagenal, T. (Ed.), Methods of Assessment of Fish Production in Fresh Waters. Oxford Blackwell Scientific Publication, pp: 101-136. Chisnall, B. L. 1996 Habitat associations of juvenile shortfinned eels (Anguilla australis) in shallow Lake Waahi, New Zealand. New Zealand Journal of Marine and Freshwater Research, 1996: Vol. 30: 233 237 Drake, D.; Kelly, D.; Schallenberg, M.; Ponder-Sutton, A.; Enright, M. 2009: Shallow coastal lakes in New Zealand: assessing indicators of ecological integrity and their relationships to broadscale human pressures. NIWA Client Report, CHC2009-005, Christchurch. 67 p. Graynoth, E., Jellyman, D.J. and Bonnett, M.L. 2008. Spawning escapement of female longfin eels. New Zealand fisheries assessment report 2008/7. Wellington: Ministry of Fisheries. Hickey CW 2013. Updating nitrate toxicity effects on freshwater aquatic species. NIWA client report HAM2013-009 prepared for MBIE Wellington. 39 p Jellyman, DJ. 1989. Diet of two species of freshwater eel (Anguilla spp.) in Lake Pounui, New Zealand. New Zealand Journal of Marine and Freshwater Research 23: 1 10. Jellyman, D.J. 2012. The status of longfin eels in New Zealand An overview of stocks and harvest. Report prepared for Parliamentary Commissioner for the Environment. NIWA. Kelly D. W., Poulin R., Tompkins D and Townsend C. R. 2010 Synergistic effects of glyphosate formulation and parasite infection on fish malformations and survival. Journal of Applied Ecology 2010, 47, 498 504
Moore S.M. 2006 and 2008. General Inventory Surveys of Greville Harbour/Moawhitu Unpublished Internal Department of Conservation Reports PCE 2013 : On a pathway to extinction? An investigation into the status and management of the longfin eel. Office of Parliamentary Commisioner for the Environment April 2013. Preece, J. 2008 Moawhitu wetland restoration plan. Prepared for Department of Conservation, Nelson Marlborough Conservancy November 2008. 62 pages Rutledge, M.J. 2005. Greville Harbour Property- fisheries and aquatic habitat values including management recommendations. Unpublished Internal Department of Conservation Report. Walls G. 2005 Draft Operation Plan Moawhitu North Greville Harbour Geoff Walls, Ecologist Taramoa Ltd, Christchurch December 2005