Assessment of fish passage in the Hutt River gorge in response to reduced flows

Transcription

1 Assessment of fish passage in the Hutt River gorge in response to reduced flows Prepared for Greater Wellington Water Prepared by Henry Hudson (EMA) and Mike Harkness (MWH) Hutt River at Flume Bridge below Kaitoke Weir Environmental Management Associates October 2010

2 Hutt Gorge fish passage i Citation: Hudson, H.R.; Harkness, M Assessment of fish passage in the Hutt River gorge in response to reduced flows. Environmental Management Associates, Christchurch. Report pages. Summary This report is an amalgamation of previous reports, reviews and discussions with stakeholders, concerning a reduction in streamflow below the Kaitoke water intake on the Hutt River. Our report provides the scientific basis to show that a 200 L/s reduction in minimum flow will have a no more than minor effect on fish passage over the 10 km Hutt River gorge below Kaitoke Weir (river km 42). A companion report evaluates potential effects on habitat availability in the river below the gorge. During low flow and high demand periods streamflow will be reduced from 600 L/s to 400 L/s in the ~900 m reach from the weir to the Pakuratahi confluence. The Pakuratahi River contributes ~400 L/s at its mean annual low flow (MALF) with inputs of 160 to 180 L/s expected about 3% of the time. Therefore, the residual flow through the gorge below the Pakuratahi confluence would occasionally be reduced from 1000 Ls to 800 L/s; and possibly from L/s (present) to L/s. Downstream of the gorge residual flows are higher because of tributary inflows. Therefore, the focus of the fish passage investigations was on the gorge reach. We addressed the following issues: Are there potential fish passage barriers in the gorge? Are these potential fish passage barriers passable? What are the risks of impeding free fish passage? and Would changing the minimum flows make a material difference? Are there potential fish passage barriers in the gorge? The gorge is characterised by extensive deep pools and numerous rapids and bedrock steps. More than 60 rapids/bedrock steps in the gorge reach were identified in helicopter surveys and site inspections. While parts of the channel dewatered as flows decreased, relatively deep continuous threads of flow occurred throughout the gorge with a Kaitoke Weir flow of ~600 L/s. To quantify changes in hydraulic geometry, we measured width, depth and velocity across the channel at 13 cross sections in 5 reaches with flows ranging from 612 L/s to 2241 L/s at Kaitoke Weir. These reaches represent typical potential blockages and the two greatest potential blockages in the gorge. The hydraulic geometry relations, modelled in RHYHABSIM, show there is little change in the width, cross sectional area, depth and velocity of cross sections in five selected reaches as flows are reduced from the present to the proposed minimum flows (i.e. from

3 Hutt Gorge fish passage ii 600 L/s to 400 L/s at Kaitoke). We concluded it was unlikely that reducing flow by 200 L/s would prevent fish passage. Are potential fish passage barriers passable? Changes in fish passage based on published sustained swimming speed and depths for different species and life stages were modelled in RHYHABSIM in the five reaches. The contiguous (connected) and total (fragmented across the channel) passage width assessment showed there are no impassable barriers to native fish or to trout in the Hutt gorge, at least in the modelled reaches. This is a precautionary approach because fish also have a burst swim mode which can only be maintained for short periods before the fish becomes fatigued. Research suggests that the burst swimming speed may restrict the ability of fish to negotiate fast water which may limit distribution of fish. What are the risks of impeding free fish passage? We assess the risk that the modelling is inaccurate; and the risk that a blockage to free passage will occur at low flow. We found that for the critical contiguous fish passage width, the measured width and predicted width are essentially the same at the calibration flow. Where there are differences, the passage width is under-predicted, thus providing a precautionary assessment. Even if there were blockages in terms of the free passage swimming thresholds used, it is important to note: (1) Native fish can migrate through interstices in stones or vegetation either in or out of water, through shallower water, and some can climb wetted margins of waterfalls and rapids; (2) Salmonids are known to be able to pass through extensive shallow riffles with exposure of much of their body; (3) The timing of low flows in summer is not necessarily coincident with major periods of fish migrations; (4) Fish often migrate during freshes; and (5) On average there are around 30 freshes per year with a flow greater than three times the median flow and this will not materially change. Although we conclude it is unlikely that barriers to fish passage would occur under either the present or proposed minimum flows, recommendations are made to limit risk. Would changing the minimum flows make a material difference? Native fish have low sustained swimming speeds hence the proposed minimum flows are more favourable for native fish passage than the present minimum flow or higher flows. To retain the maximum contiguous passage width for juvenile and small trout through rapids in the Kaitoke to Benge Creek reach

4 Hutt Gorge fish passage iii requires on average a minimum flow of ~400 L/s or less. There are velocity limitations to fish passage at higher flows. Adult trout can sustain higher speeds, so are able to move upstream at higher flows than other species or life stages. There is little difference in contiguous passage width through rapids for large trout between the proposed and present minimum flows. Conclusions Based on precautionary fish passage criterion we conclude there were no impassable barriers to native fish or trout in the Hutt Gorge at the proposed low flows, at least in the modelled reaches. Two of the study reaches were considered to have the greatest potential for impeding fish passage in the Gorge and other modelled reaches are considered to be representative of potential blockages. The risk of blockage to fish passage is minor for the following reasons: Fish can pass through shallower and faster water than modelled; Low flows and fish migration are not necessarily coincident; Fish often migrate during freshes; and The Hutt River typically has a large number of freshes. Therefore, it is unlikely that barriers to fish passage would occur under either the present or proposed minimum flows. Recommendations We recommend a variation to the resource consent to lower the minimum flow downstream of Kaitoke Weir to 400 L/s. An inspection of the river should be undertaken on the first occasion that flows are reduced to 400 L/s to confirm fish passage is not impeded. If the inspection shows there are migration blockages, flows could be temporarily manipulated to allow fish passage. Flow manipulation (i.e. increasing or decreasing flows to facilitate fish passage) may require further investigation. Alternatively, the blockage may be removed (e.g. moving logs or rocks in the low flow channel). The information in this report and any accompanying documentation is accurate to the best of the knowledge and belief of the Consultant acting on behalf of Greater Wellington Water. While the Consultant has exercised all reasonable skill and care in the preparation of information in this report, neither the Consultant nor Greater Wellington Water accept any liability in contract, tort or otherwise for any loss, damage, injury or expense, whether direct, indirect or consequential, arising out of the provision of information in this report.

5 Hutt Gorge fish passage iv Table of Contents 1 INTRODUCTION 1 2 FISH PASSAGE Species and types of migration Upstream migration periods Fish migration distances Fish passage crtierion 1 3 METHODS 6 4 STREAMFLOWS 12 5 HYDRAULICS & FISH PASSAGE 13 6 DISCUSSION 18 7 CONCLUSIONS 23 8 RECOMMENDATIONS 23 9 ACKNOWLEDGEMENTS REFERENCES 24 APPENDIX 1: PHOTOGRAPHS OF THE SURVEYED TRANSECTS 27 APPENDIX 2: HYDRAULIC GEOMETRY PLOTS 41 APPENDIX 3: FISH PASSAGE 47 List of Tables Table 2-1 List of common native fish species in the Wellington region (Joy & Death 2004); plus fish reported in the Hutt River; and type of diadromy (McDowall 1988)...2 Table 2-2 Upstream migration periods...1 Table 2-3 Average water velocity standard for fish passage through culverts (based on ODFW 2004)...2 Table 2-4 Swimming ability classification of some New Zealand freshwater fish species (modified from Mitchell & Boubée 1989 cited in Boubée et al. 1999)...3 Table 2-5 Swimming speeds, migration rates and velocity preferences of native New Zealand freshwater fish species (Boubée et al. 1999)...4 Table 5-1 Summary of hydraulic geometry for present and proposed minimum flows (rounded values) Table 5-2 Fish passage widths for combinations of depth and velocity List of Figures Figure 2-1 Swimming speeds of New Zealand fish (from Boubée et al. 1999)...5 Figure 3-1 Te Marua gorge reach rapids, river km 33 (top: 2600 L/s; bottom: 1204 L/s)...7

6 Hutt Gorge fish passage v Figure 3-2 Aerial oblique photograph of the lower Kaitoke Weir reach, with transects in the pool (K3), upper rapid (K2) and lower rapid (K1)...8 Figure 3-3 Aerial oblique photograph of the lower Te Marua gorge study reach (T1 & T2) to Benge Creek reach (B) Figure 3-4 Aerial oblique photograph of the lower Kaitoke Weir reach (K1-3), Swingbridge reach (S1-3) and Pakuratahi reach P1-3) Figure 3-5 Depths and velocities were measured with a wading rod and current meter along a tag line strung between headstakes Figure 6-1 A comparison of the calibration, modelled (simulated) and gauged cross section at Te Marua Figure A 1 Kaitoke lower transect (K1) Figure A 2 Kaitoke middle transect (K2) Figure A 3 Kaitoke upper transect (K3) Figure A 4 Swingbridge lower transect (S1) Figure A 5 Swingbridge middle transect (S2) Figure A 6 Swingbridge upper transect (S3) Figure A 7 Pakuratahi lower transect (P1 right bank) Figure A 8 Pakuratahi middle transect (P2) Figure A 9 Pakuratahi upper transect (P3) Figure A 10 Te Marua lower transect (T1) Figure A 11 Te Marua upper transect (T2) Figure A 12 Benge Creek lower transect (B1) Figure A 13 Benge Creek upper transect (B2) Figure A 14 Hydraulic geometry Kaitoke Figure A 15 Hydraulic geometry Swingbridge Figure A 16 Hydraulic geometry Pakuratahi Figure A 17 Hydraulic geometry Te Marua Figure A 18 Hydraulic geometry Benge Creek Figure A 19 Contiguous fish passage width Kaitoke Figure A 20 Contiguous fish passage width Swingbridge Figure A 21 Contiguous fish passage width Pakuratahi Figure A 22 Contiguous fish passage width Te Marua Figure A 23 Contiguous fish passage width Benge Creek... 51

7 Hutt Gorge fish passage 1 1 Introduction Greater Wellington Water (GWW) is responsible for providing high quality water to meet the reasonable needs of the people of greater Wellington, in a cost effective and environmentally responsible way (GWW 2004). Hutt River at Kaitoke Weir has been the major water source for greater Wellington since Water from the weir is piped to Upper Hutt, Porirua and Wellington; and is supplemented by Wainuiomata River abstraction; and from aquifers in the lower Hutt River floodplain (Waterloo). Usually Upper Hutt, Porirua and Wellington's northern suburbs are supplied from Kaitoke, Lower Hutt is supplied from Waterloo and Wellington's central business district and southern and eastern suburbs are supplied by a combination of Waterloo and Wainuiomata. GWW is going to undertake works to increase the storage capacity and seismic enhancement of the Stuart Macaskill Lakes. This requires the draining of the lakes. The draining of the lakes significantly increases the risk of GWW being able to meet its supply obligations in a dry year. To reduce the risk GWW is seeking a variation to its resource consent conditions to reduce the low flow at the Kaitoke Weir from 600 L/s to 400 L/s. Prior to 2001 there was no requirement to leave a residual flow below Kaitoke Weir. However, the abstraction consent issued in 2001 required a residual flow of at least 600 L/s. Investigations considering the impacts of reducing the low flow at Kaitoke Weir were instigated prior to the proposal to undertake work on the Stuart Maccaskill Lakes. As part of these investigations workshops were held, and investigations proposed to evaluate the potential effects on the aquatic environment of reducing flows at Kaitoke Weir, or at Te Marua (Hudson 2006a, b, c; 2008). Instream flow objectives were proposed for the Hutt River: To maintain aquatic habitat in the Hutt River using food production and/or brown trout as a critical (or target) species; To maintain adequate water depth for fish passage; and To maintain water quality for aquatic ecosystem purposes. This report addresses the second objective maintenance of fish passage. 2 Fish passage Several aspects of fish passage are considered: Species and types of migration; Upstream migration periods; 1 In 1939 the untouched areas of Kaitoke Regional Park and the Hutt Water Collection Area were purchased to supply water to the Wellington region. World War II interrupted construction works and it was not until April 1957 that the dam-like water intake weir, an underground aqueduct and the first treatment plant were completed. Accessed August 2007.

8 Hutt Gorge fish passage 2 Fish migration distances; and Fish passage criterion. 2.1 Species and types of migration Many of New Zealand's native fish species are diadromous requiring access to and from the sea to complete their life cycles. Amphidromy and catadromoy are the most common strategies, with a few anadromous species (McDowall 1988; Table 2-1). Amphidromous species spawn in fresh water, the hatched larvae drift to the sea and return to freshwater as juveniles to develop into adults and spawn (e.g. kokopu). Catadromous fish return to sea as mature adults to spawn and progeny return to freshwater as juveniles to grow and mature (e.g. eels). Anadromous fish are usually mature adults when they leave the sea and enter rivers to spawn (e.g. salmon and lamprey). Table 2-1 List of common native fish species in the Wellington region (Joy & Death 2004); plus fish reported in the Hutt River; and type of diadromy (McDowall 1988) Scientific name Common name Strategy Anguilla australis Shortfin eel catadromous Anguilla dieffenbachii Longfin eel catadromous Cheimarrichthys fosteri Torrentfish amphidromous Galaxias brevipinnis Koaro amphidromous Galaxias divergens Dwarf galaxiid non-migratory Galaxias fasciatus Banded kokopu amphidromous Galaxias maculates Inanga catadromous* Galaxias postvectis Shortjaw kokopu amphidromous Galaxias argenteus Giant kokopu amphidromous Gobiomorphus cotidianus Common bully amphidromous Gobiomorphus hubbsi Bluegill bully amphidromous Gobiomorphus huttoni Redfin bully amphidromous Gobiomorphus basalis Cran s bully non-migratory Gobiomorphus breviceps Upland bully non-migratory Geotria australis Lamprey anadromous * Inanga are marginally catadromous, since they migrate down only as far as estuaries to spawn Torrentfish have not been reported in the Hutt River, and modeling using the river environment classification suggests they are not likely to occur in the Hutt River (gw.govt.nz/story/10803.cfm). Banded kokopu occur in the bush-lined streams behind Naenae and Stokes Valley in the lower Hutt Valley, requiring access via Waiwhetu Stream (gw.govt.nz/story/10796.cfm), which enters the Hutt River at the mouth.

9 Hutt Gorge fish passage 3 Shortjaw kokopu are reported as occurring in the Hutt River above Kaitoke Weir (Chadderton et al. 2004). However, they are not reported in the New Zealand Freshwater Fish Database (NZFFD) (accessed in May 2009), and they are not predicted to occur in the Hutt River (gw.govt.nz/story/10802.cfm). 2.2 Upstream migration periods The fish migration calendar for the Waikato Region (Hamer 2007) was used as a basis for summarising upstream migration periods. The calendar identifies the peak period of upstream migration, and the range when the majority of migration occurs (Table 2-2). The calendar was modified for the Hutt River for eel migration and trout. The greatest threat to fish migration in the Hutt River probably occurs in March and April as the result of a long hot summer extending the period of high demand (McCarthy 2006). The emphasis on fish passage is on upstream fish passage where fish fight the current, rather than downstream migration. The rationale is that fish migration is determined by a combination of water depth and velocity. Our surveys show water depth is unlikely to be limiting in the Hutt gorge, but that small fish passage is limited by high velocity. As described in August & Hicks (2008) Each year from late winter to early summer, large numbers of shortfin and longfin glass eels enter the rivers and streams of New Zealand (Jellyman 1979). Low numbers of glass eels arrive during August, increasing to a maximum in September and October, followed by a decline in November and December (Jellyman 1977, 1979; Jellyman and Todd 1982; Jellyman et al. 1999; Jellyman and Lambert 2003). Brown trout movements may occur year round, but spawning migrations in the Hutt River are concentrated in the May-June period with lesser spawning migration in April (Corina Jordan, Fish & Game, written comm.). Importance is also placed on high flow events at the end of summer-beginning of autumn to cue the trout runs. The importance of river conditions to trout migrations is illustrated in radio tagging investigations in the upper Motueka catchment. Young et al. (in review) investigated adult trout movement over an 11 month period. He noted that Individual trout moved up to 41 km during the study. However, the majority of the tagged fish remained relatively stationary, with only 39% having home ranges >1 km. They also report on the importance of migratory cues stating Rates of movement declined steadily over the spring/summer period as flows decreased and water temperatures increased. Movement rates were positively related with average daily flow during the interval between tracking occasions, while the percentage of fish moving showed a negative relationship with average daily water temperature, beyond a threshold. All fish moved to some extent when water temperatures were <13 C, but the percentage of the tagged fish moving declined steadily at warmer temperatures.

10 Hutt Gorge fish passage 1 Table 2-2 Upstream migration periods Upstream Summer Autumn Winter Spring Migration Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Lamprey Eel (short & longfin) Koaro Dwarf galaxiid non migratory Banded kokopu Inanga Shortjaw kokopu Giant kokopu Common bully Bluegill bully Redfin bully Cran s bully non migratory Upland bully non migratory Brown trout adult Peak Range

11 Hutt Gorge fish passage 1 It is unlikely that brown trout spawning migrations would be coincident with periods of high abstraction and low flows. Higher flows, frequent freshes and lower water temperatures occur in the spawning migration period. There is potential overlap of peak upstream migration of common bully, low flows and high water demands. However, it is unlikely that common bully spawning migrations would be impeded because common bullies have only been reported in the lower river, not the gorge. If common bullies were found in the Hutt gorge, it is possible they are non-migratory (Bleackley (2008). Further, to a limited extent common bullies are able to climb the wetted margins of waterfalls, rapids and spillways (Table 2-4). Therefore, even if common bully migration through the gorge occurred, it is unlikely that migration would be impeded by reducing flows at Kaitoke weir to 400 L/s for limited periods. 2.3 Fish migration distances In New Zealand, in the absence of physical barriers to fish migration, such as waterfalls or dams, the proportional occurrence of diadromous fish declines with increasing elevations and river distances (e.g. McDowall 1998; Joy & Death 2002). Eikaas (2004) found that maximum slope encountered during upstream migrations was a good predictor of migratory galaxiid occurrence. He noted found that slope, independent of distance inland, is likely to be a better predictor of migratory fish occurrences than elevation above mean sea level, as the different species will have different slope-thresholds that they can overcome. The rate of decline, using this species trajectory approach, differs between species, and has been interpreted as a reflection of migratory capabilities and instincts, ease of access to upstream habitats, and habitat suitability (McDowall 1990, McDowall & Taylor 2000). Based on the distance and slope criteria reported in Eikaas (2004) and Leathwick (2007), it is unlikely that river channel slopes in the lower Hutt River and gorge limit the distribution of diadromous species. However, based on their findings, it is likely that only a few species reported in the Hutt River would be expected to migrate as far as the Hutt gorge (longfin eel, dwarf galaxias, crans bully and koaro). Of these species, longfin eel, and to some extent koaro, are anguiliforms and are able to worm their way through interstices in stones or vegetation either in or out of water. Koaro are able to climb wetted margins of waterfalls and rapids. Thus, these species are unlikely to be impeded by the proposed lower flows at Kaitoke weir. 2.4 Fish passage crtierion New Zealand studies have used the criteria of Thompson (1972) to determine streamflows required for trout and salmon fish passage (e.g. Mosley 1982; Duncan & Hicks 2001). Thompson (1972) proposed a minimum water depth of ~24 cm (0.24 m) for adult Chinook salmon, and ~18 cm (0.18 m) for steelhead trout; with a maximum velocity of ~2.4 m/s. Bjornn & Reiser (1991) used the

12 Hutt Gorge fish passage 2 same thresholds for upstream migration of adult salmon and steelhead; and included a minimum depth of 18 cm (0.18 m) and maximum velocity of 2.44 m/s for large trout and 12 cm (0.12 m) and 1.22 m/s for other trout. They reported maximum darting (burst) speeds of 1.89 to 3.87 m/s for brown trout; and 3.29 to 6.83 m/s for Chinook salmon, with maximum jumping heights of 0.8 and 2.4 m, respectively. ODFW (1997) have taken a more precautionary approach for stream crossings, requiring a minimum water depth of 30 cm (0.30 m) for Chinook salmon, 25 cm (0.25 m) for large trout and 20 cm (0.20 m) for smaller trout and juvenile salmon. As noted by Mosley (1982), salmon and trout are regularly observed moving upstream in water very much shallower than these depths over distances of some metres; but the fish may suffer abrasion and loss of condition as a result. He notes there are no data on the number of such shallow obstructions that may be negotiated before a serious reduction in fish condition occurs, or on the influence of such factors as water temperature, length of riffle, distance of travel, fish fatigue, and the availability of resting areas. While it is clear that fish can navigate through shallow water, consideration must be given to the length of the barrier that fish must navigate. Some guidance is given by Oregon Department of Fish and Wildlife (ODFW 1997 & 2004) guidelines for fish passage at stream crossings. The Oregon guidelines specify velocities depending on the culvert length (Table 2-3). The ODFW guidelines are based on physical abilities of fish. The emphasis is on the weakest fish, usually juvenile fish as small as 50 mm in length (Paul et al. 2002). Passage Length (m) Table 2-3 Average water velocity standard for fish passage through culverts (based on ODFW 2004) Salmon (m/s) Adult Trout (m/s) Juvenile Salmonids (m/s) < * * * Substrate and flow conditions in the crossing structure mimic the natural streambed above and below the structure Salmon and trout commonly move into relatively narrow headwater streams. Washington State use 2 to 3 feet (~ m) to define a minimum stream width for fish-bearing streams depending on location (WDFW 1998). Paul et al. (2002) evaluated juvenile and adult fish passage in Oregon streams with bankfull widths ranging from 0.5 to 13 m; with over half falling between 2 and 3 m. Recent bridges and open-bottom arches that were designed to the evolving ODFW standards had 100% success rates for fish passage (ODFW 2004). These types of crossings have, or mimic, natural stream channels beds. Therefore, there is support to evaluate fish passage for salmonids based on the standards outlined in ODFW (1997 & 2004) as a precautionary approach for salmonid fish passage.

13 Hutt Gorge fish passage 3 Boubée at al. (1999) reviewed fish passage requirements of native (and exotic) fish in New Zealand. While their focus was on fish passage through culverts, their findings are instructive in determining criteria for fish passage in the Hutt River. They note that New Zealand s diadromous native fish species mostly migrate up stream as small juveniles (e.g. 20 mm long bullies); some can spend a considerable amount of time out of the water; some also have a good climbing ability (e.g. elvers, koaro, and banded kokopu) or make use of the boundary layer and interstices between substrate particles to progress up stream (Table 2-4). This climbing behaviour gives the fish the ability to migrate over considerable barriers and penetrate far inland (Boubée at al. (1999). Table 2-4 Swimming ability classification of some New Zealand freshwater fish species (modified from Mitchell & Boubée 1989 cited in Boubée et al. 1999) While shallow depths are not necessarily a problem for some native fish given their size and swimming motion, there are clear water depth preferences for where these fish are found. Jowett & Richardson (1995) established habitat suitability criteria for numerous species including several species found in the Hutt River. They found small (<300 mm) shortfin and longfin eels, common bully, crans bully, bluegill bully, redfin bully, drarf galaxias and koaro are all found in very shallow water (mm s to a few cm). In terms of widths and depths for non-migratory galaxiid passage, Allibone & Townsend (1997) discuss fish pass requirements; and

14 Hutt Gorge fish passage 4 Allibone (2000) suggests a fish pass could consist of rock-lined guttering or drainage pipe less than 50 cm in width with a water depth up to 5 cm in the centre of the channel and shallower towards the edges. Velocity preferences and maximum velocities were quantified for native fish (Table 2-5) and plotted in Figure 2-1. For 70 mm inanga and smelt, Boubée et al. (1999) state that maintaining a zone with water velocities below 30 cm/s would allow fish to travel through a culvert without the need to rest. For culverts they propose that for New Zealand s smaller juvenile upstream migrants, a 50 to 100 mm (0.05 to 0.10 m) wide zone, with velocities below 30 cm/s, would ensure free passage. Table 2-5 Swimming speeds, migration rates and velocity preferences of native New Zealand freshwater fish species (Boubée et al. 1999)

15 Hutt Gorge fish passage 5 Figure 2-1 Swimming speeds of New Zealand fish (from Boubée et al. 1999)

16 Hutt Gorge fish passage 6 Passage above Kaitoke Weir is not examined in this report. Large weirs (>3m height), such as Kaitoke, are generally complete barriers to salmonids passage, whereas lower barriers can potentially be negotiated (Townsend & Crowl 1991). Kaitoke Weir however, may not impede all fish. For example, juvenile koaro are capable of scaling waterfalls up to 60 m in height (McDowall 1990). Use of burst speed would increases the acceptable velocities for fish passage, but impacts on the length of an impediment a fish can pass through. Sustained swimming speeds provide for free migration for sustained periods hence distances, and hence, to be precautionary, the following sustained swimming speed and depth criteria are applied in this report: 25 cm (0.25 m) minimum depth for large trout, with maximum velocities less than 1.2 m/s; 20 cm (0.20 m) minimum depth for smaller trout and juveniles, with maximum velocities less than 0.6 m/s; and 5 cm (0.05 m) minimum depth for native fish, with velocities less than 0.3 m/s. Based on passage requirements at constrictions, the following widths of fish passage were adopted in this report: 5 cm (0.05 m) for juvenile native migrants 50 cm (0.5 m) for salmonids. 3 Methods Fish passage barriers were identified as a potential issue in the Hutt River gorge which extends from Benge Creek (km 32) to Kaitoke Weir (km 42). In order to further assess the character of the gorge, low elevation aerial photographs and video of the gorge were taken at low flow, and segments of the gorge were walked and viewed from walkways. In the 10 km reach more than sixty potential barriers were identified; with an average spacing of around 150 m. In most cases the barriers are relatively short (e.g. rapids upstream of Te Marua; Figure 3-1), but some are relatively extensive. For example, below Kaitoke Weir the rapid (as defined by the broken water surface and protruding boulders and cobbles) extends more than three channel widths upstream (~75 m) (Figure 3-2).

17 Hutt Gorge fish passage 7 Figure 3-1 Te Marua gorge reach rapids, river km 33 (top: 2600 L/s; bottom: 1204 L/s)

18 Hutt Gorge fish passage 8 Figure 3-2 Aerial oblique photograph of the lower Kaitoke Weir reach, with transects in the pool (K3), upper rapid (K2) and lower rapid (K1)

19 Hutt Gorge fish passage 9 The characteristics of all the potential fish passage bottlenecks in the gorge were examined and five study reaches were selected to quantify the potential fish passage barriers. Two of the study reaches were considered to have the greatest potential for impeding fish passage the Kaitoke reach and the Pakuratahi reach. The Kaitoke reach has the longest rapid; and the Pakuratahi reach is the only significant divided channel. The other study reaches are considered representative of other sites with the greatest potential for impeding fish passage. Numerous other lesser potential impediments occur. The study reaches were called Kaitoke (K), Swingbridge (S), Pakuratahi (P), Te Marua gorge (T) and Benge Creek (B) (Figure 3-2 to Figure 3-4). Kaitoke and Swingbridge were located in the reach of Hutt River below the Kaitoke Weir and water supply intake, and above the confluence with the Pakuratahi River. Pakuratahi was in the Hutt River immediately downstream of the Pakuratahi confluence. The Te Marua gorge reach was in the lower gorge above the Te Marua water treatment plant and Benge Creek was where the Hutt River becomes less confined. Survey reaches were selected by examining aerial photographs, and viewing sections of river, and transect locations were determined on site by EMA and Greater Wellington Regional Council (GWRC) hydrology staff. Two or three transects were placed in each reach (Figure 3-1 to Figure 3-4). 2 The upper transects were placed across a wadable section of the pool to accurately measure flow and to determine the riffle entrance and holding pool characteristics. The middle transects were in the upper part of the rapid lower end of the pool transition; and the lower transects were placed across the shallowest part of the rapid. The lower transects were positioned to quantify depths and velocities across the crest of the rapid as flows dropped to the proposed low flow and the margins of the channel dewatered. Headstakes were placed at each end of the transects as temporary benchmarks (for the duration of the study) and depths and velocities were measured across the channel (Figure 3-5). GWRC undertook the streamflow measurements on three separate site visits in March In the pools more than 20 depths and velocities were measured at fixed intervals. In the rapids the average number of depth and velocity measurements across the channel was 43; with a range from 21 to 112. Data and photographs from the surveys were supplied by GWRC in an electronic format. MWH compiled the data into a format suitable for importing to the river hydraulics and habitat simulation programme RHYHABSIM (Jowett 1989). MWH undertook the hydraulic analyses using RHYHABSIM. EMA undertook further analysis on fish passage using the fish passage module with data export to Excel for further analysis and presentation. 2 It is not possible to obtain an accurate flow gauging from measurements undertaken on the diagonal across boulder strewn cross sections. The measurements in the pools are required to provide an accurate gauging of flow which is essential for developing the rating relation. The Te Marua gauge flow was used for the Benje Creek and Te Marua surveys.

20 Hutt Gorge fish passage 10 Figure 3-3 Aerial oblique photograph of the lower Te Marua gorge study reach (T1 & T2) to Benge Creek reach (B)

21 Hutt Gorge fish passage 11 Figure 3-4 Aerial oblique photograph of the lower Kaitoke Weir reach (K1-3), Swingbridge reach (S1-3) and Pakuratahi reach P1-3)

22 Hutt Gorge fish passage 12 4 Streamflows Figure 3-5 Depths and velocities were measured with a wading rod and current meter along a tag line strung between headstakes The format of the data for each cross section consisted of the offset, depth and velocity for a number of points across the section for the first survey (21 March 2007) and subsequent surveys (March 23 and 28). For the hydraulic analysis the water level for the first survey at each section was assumed to be 0 metres. The representative water level for each survey was based on the water depth in the deepest part of the channel a fixed distance along the transect. This is a precise measure because it is on a fixed line between headstakes, measurements were taken at fixed distances, and the bed was stable (bedrock or boulder). In effect this is the same as a temporary staff gauge fixed to the bed; but avoids the difficulty of hammering a stake into the bedrock/boulder bed in the deepest part of the channel where there is a risk of damage or destruction in high flows. With placement of a temporary pin at the edge of the channel there is also a risk of dewatering at low flows (e.g. Figure 3-1). Once imported to RHYHABSIM the data was checked and appropriate rating curves selected. Hydraulic geometry (width, depth and velocity) and fish passage widths were computed in RHYHABSIM. Multiple runs of fish passage criteria were computed to predict passage opportunities for various combinations of depth and velocity, and various flows. A flow range of 50 to 2000 L/s was modelled rather than just the present and proposed minimum flows. Each RHYHABSIM run generates a table and plot, so the outputs were exported to Excel and the data combined into a suite of curves of fish passage at various flows. The extended flow range, and combination of outputs, facilitates an assessment of the sensitivity of fish passage to flow; and comparison within and between study reaches. For the purposes of this investigation, the Hutt River is divided into three segments: (1) the gorge from Kaitoke Weir (km 42) to the Pakuratahi River confluence (km 41.15); (2) the gorge from the Pakuratahi confluence to Te Marua (km 32.4); and (3) the Hutt River below the gorge (i.e. downstream of Te Marua). Predicted water depths and velocities, and fish passage widths, are assessed against

23 Hutt Gorge fish passage 13 the estimates of low flow in the gorge from Kaitoke Weir to the Pakuratahi, and from the Pakuratahi confluence to Te Marua. For some 850 m, from the Kaitoke Weir to the Pakuratahi confluence, residual flows of 600 L/s occur for a period of days each year. The MALF of the Pakuratahi River at the mouth is estimated to be 410 L/s (Wilson 2006). Thus, in the gorge below the Pakuratahi the present nominal Hutt River MALF is about 1000 L/s (i.e. a 600 L/s residual flow below Kaitoke Weir and 410 L/s from the Pakuratahi). With a 400 L/s residual flow below Kaitoke Weir, the flow through the lower gorge would reduce to ~800 L/s if the Pakuratahi contributes ~400 L/s. However, for 3% of the time the input from the Pakuratahi is less than the MALF. The minimum measured flow at Truss Bridge is 111 L/s, with an equivalent flow at the mouth between 159 L/s and 182 L/s depending on which of Wilson s relationships are used. That is, the flow in the Hutt River gorge below the Pakuratahi confluence could be as low as ~760 to 780 L/s (current) or 560 to 580 L/s (proposed). However, low flow from the Pakuratahi may not be coincident with low flow at Kaitoke (Wilson 2006) or with high rates of water abstraction. Surveys were carried out on 21 March, 23 March and 28 March GWRC reported streamflows of 2241, 953 and 612 L/s in the Kaitoke study reach; and 2600, 1740 and 1204 L/s for the Te Marua gauge, respectively, at the time of survey. The Pakuratahi River, which flows into the Hutt River at river km 41.15, between the Swingbridge and Pakuratahi study sites, was also gauged. Flows from the Pakuratahi River were 739, 477 and 391 L/s, respectively. These gaugings are not directly comparable because of the timing of the gaugings with respect to the pumping operations of the treatment plant. The plant has base load pumping until 10 am, when full pumping occurs with lower electricity charges. 3 The lowest of the survey flows are close to the present minimum flow below Kaitoke Weir (612 L/s v. 600 L/s); the Pakuratahi mouth MALF (391 L/s v. 410 L/s); and the Te Marua MALF (1204 L/s v. 988 L/s reported in Wilson 2006). The minimum flow of the Hutt River below the gorge, as specified in the Regional Freshwater Plan (RFWP, WRC 1999), is 1200 L/s at Birchville (km 25). 5 Hydraulics & fish passage Fish passage potential is determined by comparing modelled width, depth and velocities with the abilities of fish to migrate through various combinations of depths and velocities of water. In Table 5-1 the width, cross sectional area (CS Area), depth and velocity are summarised for the five survey reaches. For Kaitoke and Swingbridge, the present minimum flow is 600 L/s and the proposed minimum flow is 400 L/s. For the downstream sites, the present minimum is 1000 L/s and the proposed minimum flow is 800 L/s during MALF conditions. Photographs and plots of the hydraulic 3 Alastair McCarthy, GWW, pers. comm.

24 Hutt Gorge fish passage 14 relations are appended (Appendix 1 Figure A 1 to Figure A 13; Appendix 2 Figure A 14 to Figure A 18). Table 5-1 Summary of hydraulic geometry for present and proposed minimum flows (rounded values) Location Transect Width (m) CS Area (m 2 ) Proposed Difference Proposed Difference Kaitoke K K K Swingbridge S S S Pakuratahi P P P Te Marua T T Benge C. B B Average Depth (m) Average Velocity (m/s) Proposed Difference Proposed Difference Kaitoke K K K Swingbridge S S S Pakuratahi P P P Te Marua T T Benge C. B B Changes in hydraulic geometry are relatively small. On average the lower rapid (K1, S1, P1, T1 & B1) transects decrease in width from 14.2 to 12.8 m; in cross sectional area from 2.3 to 2.0 m 2, in depth by 0.01 m; and in velocity by 0.02 m/s (0.25 to 0.23 m/s). On average the upper section of rapids pool transitions (K2, S2, P2, T2 & B2) decrease in width from 17.0 to 16.0 m; in cross sectional area from 3.9 to 3.5 m 2, in depth by 0.02 m (0.25 to 0.23 m); and in velocity by 0.02 m/s (0.19 to 0.17 m/s). The pools (K3, S3 & P3) have little change in width (0.1 to 0.5 m for an average width of 24 m); with a small change in cross sectional area (from 9.8 to 9.2 m 2 on average); with a 0.02 to 0.03 m decrease in depth from an average depth of 40 cm; with 0.01 to 0.02 m/s decrease in velocity from an average of 0.05 m/s.

25 Hutt Gorge fish passage 15 Changes in flow patterns are attributable to the type of mesohabitat. For example, the pool transect at Kaitoke (K3) has the largest cross sectional area (wider, deeper and slower flow) and is more U shaped, which limits the flow spreading out but increases the depth more rapidly as flow increases. For example, the transect in the rapid (K1) increases in channel width and velocity at a greater rate than the pool, which limits the increase in depth with higher flows (Figure A 1; Figure A 14). At lower flows the flowing water is concentrated into a narrower channel and can be funnelled into chutes between boulders; with side areas of slow flowing water. In the Pakuratahi K1 rapid, at low flows, two distinct channels flow along the banks (Figure 3-4). At high flows the mid channel bar would be drowned out with little increase in average depth, but with higher velocities (Figure A 16). As flows decrease more rocks are exposed in the active channel (Figure A 7) and the width rapidly reduces at low flows as the flows concentrate into the two distinct channels with relatively high velocities (Figure A 16). Various combinations of fish passage depths and velocities are modelled to provide a suite of fish passage curves to evaluate the needs of various species and life stages discussed in Section 2. In Table 5-2 the top section summarises changes in the contiguous width, and the bottom section changes in the total width for critical depth and velocity combinations. The contiguous (connected) width is the single widest thread of water in the channel that has suitable depth and velocity for fish passage. The total width is the sum of each of the threads of suitable depth and velocity that occur across the channel. These passable sections may be separated from other passable sections by water that is too fast or too shallow, or by dry areas. The contiguous width provides a precautionary measure of fish passage. For example, while both edges of a channel may have suitable passage depths and velocities, only one edge would be counted because the passage width has to be connected across the channel. The total width would count both edges as passable. As expected there is little change in the contiguous or total fish passage width in the pools (K3, S3 & P3). For native fish (>5 cm depth & <30 cm/s velocity), contiguous and total fish passage width exceeds 17 m, and the change in width is 0.1 to 0.2 m as minimum flows are reduced from 600 to 400 L/s. For juvenile trout (>20 cm & <60 cm/s) contiguous and total fish passage width does not change at Kaitoke (K3: 19.3 m); reduces slightly at Swingbridge (S3: from 12.8 to 12.1 m in both widths); and reduces from 24 to 11 m contiguous width at Pakuratahi, but with a ~5m loss of total width from ~29 m. For large adult trout (>25 cm & <120 cm/s) there is no change in contiguous or total passage width at Kaitoke (K3: ~19 m); a small change at Swingbridge (reducing from 12.1 to 10.8 m); with no change at Pakuratahi retaining ~11 m of contiguous width and ~24 m of total passage width (Table 5-2).

26 Hutt Gorge fish passage 16 Location & Transect Table 5-2 Fish passage widths for combinations of depth and velocity Depth >5 cm Velocity <30 cm/s Contiguous width (metres) Depth >20 cm Velocity <60 cm/s Depth >25 cm Velocity <120 cm/s Proposed ± Proposed ± Proposed ± Kaitoke K K K Swingbridge S S S Pakuratahi P P P Te Marua T T Benge C. B B Location & Transect Depth >5 cm Velocity <30 cm/s Total width (metres) Depth >20 cm Velocity <60 cm/s Depth >25 cm Velocity <120 cm/s Proposed ± Proposed ± Proposed ± Kaitoke K K K Swingbridge S S S Pakuratahi P P P Te Marua T T Benge C. B B In the pool rapid transitions the contiguous fish passage width for native fish stays the same (S2 & P2) or increases (K2, T2 & B2) as flows reduce from the present to proposed minimum flows (Table 5-2). On average the contiguous width increases from 5.5 to 7.8 m; with the greatest increase at Benge Creek (from 3.9 to 10.5 m). On average the total passage width increases from 12.2 to 12.5 m as the flow decreases from the present to proposed minimum flow. The upper reaches are 0.5 to 1.4 m narrower; and the lower reaches are 1.2 (T2) to 3.5 m wider (B2). Juvenile trout contiguous fish passage (>20 cm & <60 cm/s) in the pool-rapid transitions decreases from an average width of 8.0 m to

27 Hutt Gorge fish passage m as the flows reduce from the present to proposed minimum flow. Kaitoke (K2) has the greatest change in contiguous width (12.5 m to 6.3 m), but a smaller change in total width (from 13.8 to 11.5 m) (Table 5-2). Swingbridge (3.4 m) and Pakuratahi (6.9 m) contiguous widths do not change, and the lower reaches widths decrease by about a metre (to 7.3 m at Te Marua and 8.1 m at Benge Creek). Similarly, Swingbridge and Pakuratahi total widths do not change with reduced minimum flows; and the lower reach total widths reduced by about a metre at Te Marua and Benge Creek. Large adult trout fish passage is more restricted (>25 cm & < 120 cm/s) in the pool-rapid transitions than the pools (Table 5-2). On average the contiguous passage width decreases from ~6 to ~5 m with reduced minimum flows, but this varies between reaches. Kaitoke (4.3 m) and Pakuratahi (6.3 m) contiguous passage widths do not change with reduced flows. Te Marua (5.1 m retained) and Benge Creek (7 m retained) have decreased contiguous passage widths by 2.2 m and 1.1 m, respectively. The narrowest reach of contiguous fish passage occurs at Swingbridge, where the loss in width is 1.3 m, with retention of 2.1 m, at the proposed minimum flow. The same change in total passage width occurs at Swingbridge. On average total passage width decreases from 7.7 to 6.5 m with reduced minimum flows; with no change at Kaitoke. The most restricted passage is expected in the shallow rapids. The average contiguous passage width for native fish increases from 3.2 to 3.3 m with the proposed minimum flow (Table 5-2). Kaitoke has the largest passage width, but changes little at the lower proposed flow reducing from 6.5 to 6.3 m. There is a minor change at Swingbridge from 1.2 to 1.1 m; and a decrease at Pakuratahi from 4.2 to 3.3 m with reduced minimum flows. Lower reaches lose about one metre of contiguous width, retaining 2.6 m at Te Marua and 3.0 m at Benge Creek with reduced minimum flows. Total passage widths are greater than contiguous widths for native fish passage; decreasing in width from 6.4 to 6.0 m on average with the reduction in minimum flows. The narrowest total passage width occurs at Te Marua where width increases from 2.2 to 3.0 m as flows are reduced. Here the flow concentrates into a chute along the right bank at low flows (Figure 3-1). Juvenile trout contiguous passage averages 2.8 m in the shallow rapids at the present minimum flow; and this is predicted to decrease to 2.3 m on average at the proposed minimum flow. No change is predicted at Kaitoke, which has the greatest fish passage width at 6.3 m, and Pakuratahi (1.1 m) and Benge Creek (2.5 m) (Table 5-2). The contiguous fish passage width at reduced minimum flows is fairly consistent around 1 m at Swingbridge, Pakuratahi and Te Marua. The average total fish passage width for native fish reduces from 4.4 to 3.7 m. There is little change at Kaitoke with total width ~9 m. Other sections have smaller total passage widths. The narrowest total passage width occurs at Te Marua where the width is reduced from 1.9 to 0.8 m with the proposed flow reduction. Large trout contiguous fish passage through the rapids is reduced on average from 3.0 to 2.4 m; and from 3.9 to 3.1 m total width as the

28 Hutt Gorge fish passage 18 6 Discussion flows reduce (Table 5-2). Kaitoke has the greatest retained fish passage width, decreasing from 5.7 to 4.2 m contiguous width and from 7.3 to 5.8 m total width with the minimum flows. There is little change in contiguous passage width at Swingbridge, Pakuratahi and Te Marua; and in total width at Pakuratahi and Te Marua at the proposed minimum flow. 4 The narrowest passage widths occur at Swingbridge and Pakuratahi with ~1 m contiguous width retained. There are few differences between the contiguous and total with in the lower reaches. Given the proposed decrease in minimum flows through the gorge, four critical questions are addressed: Are there potential fish passage barriers in the gorge? Are these potential fish passage barriers passable? What are the risks of impeding fish passage? and Would changing the minimum flows make a material difference? Are there potential fish passage barriers in the gorge? Preliminary investigations indicated there were numerous rapids in the narrow gorge, interspersed with extensive deep pools. Based on a visual assessment of the channel character it was thought that fish passage barriers were unlikely to occur with the proposed minimum flows. Essentially, it was thought that as flows decreased, water would concentrate into chutes between the boulders providing fish passage. To more fully examine the extent of potential fish passage barriers through the less accessible parts of the gorge, low elevation aerial photographs and video of the gorge were taken under low flow conditions. This imagery confirmed the findings from the preliminary evaluation of the gorge undertaken by walking segments of the riverbed, viewing the gorge from adjacent walkways, and examining regional scale aerial photographs. There are more than 60 rapids in the gorge reach which are potential fish passage barriers. To evaluate fish passage, width, depths and velocities were measured at numerous points across the channel for three flows ranging from 612 to 2241 L/s at Kaitoke Weir. These data were used to calibrate a hydraulic model (RHYHABSIM) to predict hydraulic geometry and fish passage at various flows in the 50 to 2000 L/s range. Transects were placed across the shallowest part of the channel in rapids, and in pool-rapid transitions, in five study reaches. In three of these reaches, cross sections were also placed across the wadable section of pools to provide accurate flow estimates and illustrate fish passage and habitat. 4 At Swingbridge free passage for large trout is lost at 350 L/s for the 25 cm depth and less than 1.20 m/s velocity threshold (Figure A 20). The loss of free passage is due to decreased depth rather than velocity. If the depth threshold is reduced to 20 cm, the free passage width is maintained.

29 Hutt Gorge fish passage 19 Are potential fish passage barriers passable? Measurements and prediction of hydraulic geometry under low flow conditions show that there is little change in the width, cross sectional area, depth and velocity between the present and proposed minimum flows in the five selected reaches. This strongly suggests that there would be little change in fish passage opportunities with the proposed reduction in minimum flows. In addition to the hydraulic geometry analysis, RHYHABSIM was also used to predict changes in fish passage contiguous (connected) width and total (fragmented) width for various combinations of water depths and velocities. As discussed in Section 5, the contiguous width values provide a precautionary measure of fish passage. Using the best available information on fish passage from the literature, we determined fish passage opportunities for various thresholds including a 5 cm minimum depth for native fish, with velocities less than 0.3 m/s; 20 cm minimum depth for small trout and juveniles, with maximum velocities less than 0.6 m/s; and 25 cm minimum depth for large trout (>50 cm), with maximum velocities of 0.6 to 1.2 m/s. For these precautionary evaluation thresholds, where sustainable swimming speeds rather than burst swimming speeds were used, there were no instances where a barrier to fish passage occurred. In all cases the minimum width recommended for fish passage were exceeded, often by more than an order of magnitude. It is likely that fish passage opportunities will be greater than predicted because fish also have a burst swim mode which can only be maintained for short periods before the fish becomes fatigued. 5 Stevenson et al. (2008), suggest that the burst swimming speed of a fish species may restrict its ability to negotiate fast water, which may limit its distribution. What are the risks of impeding fish passage? Two aspects of risk are addressed here: (1) the risk the modelling is inaccurate; and (2) the risk that a blockage to free passage will occur. Regarding the first point, calibration and verification are not synonymous terms in hydraulic-habitat modelling. In RHYHABSIM calibration refers to the adjustment of the rating curve so as to define the observed changes of water levels for particular flows and the cease to flow water level. Verification refers to independently testing the results of the model. This step is almost invariably neglected in hydraulic-habitat modelling (Hudson et al. 2003). 5 For example, in Figure A19 increasing the velocity threshold from 20 cms to 30 cm/s (V30) or 60 cms/s for a depth of 5 cm (D5) increases the contiguous passage width at K1 from 2.6 m to 6.5 m to 8.0 m, respectively.

30 Depth (m) Hutt Gorge fish passage 20 In the reviewer comments the stepped nature of the Te Marua hydraulic geometry relations was raised as a possible indicator of problematic modelling (Figure A-17). The relationship is stepped because the T1 cross section is not a conventional gauging cross section. The cross section, which was chosen to describe potential fish passage problems, reduces in width significantly as flows decrease as illustrated in the figure below and in Figure 3-1. Te Marua T Gauged 1204 L/s -0.6 Simulated 1200 L/s -0.7 Calibration 2600 L/s Offset (m) Figure 6-1 A comparison of the calibration, modelled (simulated) and gauged cross section at Te Marua The critical issue is whether the predicted hydraulic geometry, hence the predicted fish passage, is comparable to the measured hydraulic geometry. In this regard, a wide range of flows were gauged at each of the cross sections. For example, at the site that is considered to be most problematic (Te Marua), the calibration flow was 2600 L/s; the intermediate measurement was at 1740 L/s and the low flow measurement at 1204 L/s. The proposed minimum flow in MALF conditions is 800 L/s. This is not a huge difference between the target flow and lowest measured flow. Thus, a fair amount of confidence can be gained from the independent measurement of water depths at 1204 L/s compared to the predicted depth at 1200 L/s. Even if there were blockages in terms of the free passage swimming thresholds used, it is important to note: (1) Greater passage widths are generally available if burst speeds were used to model passage width; (2) Native fish can migrate through interstices in stones or vegetation either in or out of water, through shallower water, and some can climb wetted margins of waterfalls and rapids (Boubée et al. 1999);

31 Hutt Gorge fish passage 21 (3) Salmonids are known to be able to pass through extensive shallow riffles with exposure of much of their body (Mosley 1982); (4) The timing of low flows in summer is not necessarily coincident with major periods of fish migrations (MCarthy 2007; Table 2-2); (5) Fish often migrate during freshes (Boubée et al. 1999); and (6) On average there are around 30 freshes per year with a flow greater than three times the median flow (Duncan & Woods 2004), and this will not be materially affected by the proposed reduction in low flows (Hudson 2010). Although we conclude it is unlikely that barriers to fish passage would occur under either the present or proposed minimum flows, recommendations are made to limit risk. Would changing the minimum flows make a material difference? To assess the effects of increasing or decreasing the minimum flows, and to assess the sensitivity of the estimates of present and proposed minimum flows, an analysis of the fish passage was undertaken for flows from 50 L/s to 2000 L/s. The critical area for fish passage is in the shallow rapids rather than in the deeper poolrapid transitions or pools. Therefore, emphasis was placed on the rapids and on the contiguous width. The fish passage curves presented in Appendix 3 show that increasing the flow from the present or proposed minimums would have little positive effect, and may have an adverse effect. For example, at the Kaitoke rapid (K1), there is little difference in native fish passage width (D5-V30) for the present (600 L/s) and proposed minimum flows (400 L/s). Further reductions in minimum flows would have little effect (e.g. at 200 L/s there is 6 m contiguous width). However, increasing the flows above 800 L/s would cause a decline in contiguous passage width (from more than 6 m to less than 3 m) (Figure A 19). At Swingbridge there is greater contiguous fish passage width at 200 L/s than at the proposed minimum flow of 400 L/s or present minimum flows of 600 L/s. The passage width is relatively constant to 1300 L/s, thereafter it declines (Figure A 20). Therefore, the proposed minimum flow of 400 L/s in the Kaitoke Weir to Pakuratahi River confluence reach is precautionary with regard to maintenance of native fish passage. Based on retaining maximum contiguous passage width for native fish in the upper reaches above the Pakuratahi River confluence, the minimum flow could be ~200 L/s; higher minimum flows than at present would lead to a decrease in fish passage width. In the reach below the Pakuratahi the present minimum flows in MALF conditions is estimated as 1000 L/s based on a 400 L/s MALF flow input from the Pakuratahi River and 600 L/s minimum from the Kaitoke Weir. Little or no gain in flows through the gorge to Benge

32 Hutt Gorge fish passage 22 Creek occurs in low flow conditions (Wilson 2006); therefore the proposed minimum flow would be 800 L/s (or possible L/s). Decreasing the minimum flows to 600 L/s at the Pakuratahi rapids (P1) would have little effect for native fish passage; and increasing the minimum flows above 1000 L/s would cause a decline in contiguous fish passage width (Figure A 21). Similarly, decreasing the minimum flows at Te Marua to 600 L/s would provide more contiguous fish passage than the present minimum flows or larger minimum flows (Figure A 22). At Benge Creek a flow of 500 L/s would provide at least as much contiguous fish passage as the present minimum flows and higher flows (Figure A 23). These findings indicate the proposed minimum flow of 800 L/s in the reach downstream of the Pakuratahi River confluence is precautionary with regard to native fish. Based on retaining maximum passage width for native fish, the minimum acceptable flow is 500 to 600 L/s; higher minimum flows than at present can lead to a decrease in native fish passage width. For juvenile and small trout, with a threshold of >20 cm depth and <60 cm/s velocity, there is a rapid increase in contiguous passage width as flows increase from zero, with a plateau from 400 to 1300 L/s for the K1 rapid, and decline in passage width at greater flows (Figure A 19). The Kaitoke pattern is repeated at Swingbridge with the S1 contiguous passage peak starting from 500 L/s, with the decline again at 1300 L/s. At Pakuratahi the plateau in contiguous passage width occurs from 250 L/s to 1200 L/s, with a small increase in contiguous width at higher flows (~0.3 m). To maintain contiguous passage width for juvenile and small trout from Kaitoke to Benge Creek requires on average a minimum flow of ~400 L/s or less. Although large adult trout require deeper water for passage, and can swim through faster water (>25 cm depth and <120 cm/s velocity); modeled passage widths have a similar pattern to the juvenile and small trout flow-passage width relations. In the Kaitoke reach there is effectively no change in fish passage width in the 350 to 750 L/s flow range (Figure A 19) because the rapid (K1) and the pool-rapid transition (K2) limits fish passage (4.3 m contiguous passage width in the flow range). At Swingbridge there is ~1 m of contiguous passage width at flows of 400 to 950 L/s, but there is no fish passage at lower flows (Figure A 20), unless the depth threshold is reduced to 20 cm. Therefore, in the upper gorge, the proposed minimum flow of 400 L/s provides about as much contiguous passage width for large trout as the present minimum flow of 600 L/s; and there is effectively no gain in passage width until flows exceed 750 to 950 L/s. At Pakuratahi large trout fish passage is insensitive to flows with ~1 m of contiguous passage width throughout the 300 to 1200 L/s flow range (Figure A 21). At Te Marua, there is also a plateau with 2.7 m of contiguous fish passage width in the 550 to 1100 L/s flow range (Figure A 22). Benge Creek has a more stepped flow-width relationship (Figure A 23). The contiguous width is 3.1 m from 600

33 Hutt Gorge fish passage 23 7 Conclusions to 800 L/s and increases to a plateau of 4.5 m contiguous passage width from 1000 to 1350 L/s. A minimum flow of 600 L/s in the lower gorge would provide as much contiguous fish passage width for large trout as the proposed minimum flow of 800 L/s. Increasing the minimum flow above the present 1000 L/s would not provide an increase in fish passage width until flows were in the 1250, 1150, and 1400 L/s range at Pakuratahi, Te Marua and Benge Creek, respectively. Using precautionary evaluation thresholds, where sustainable swimming speeds rather than burst swimming speeds were used, there were no instances where a depth and/or velocity barrier to fish passage are predicted at the proposed minimum flow. In fact, flows for all but large adult trout at Swingbridge could be reduced below the proposed minimums and still maintain fish passage. At Swingbridge large trout free passage is maintained for a 20 cm depth threshold. In all cases the minimum width recommended for fish passage through culverts were exceeded. The risk of impeding fish passage is low. Free-passage thresholds are used, which is precautionary because fish can migrate through much shallower and faster water than the thresholds applied here. Fish behaviour is also important, with migration at times that are not coincident with low flows, and with migration often triggered by freshes. Reducing the minimum flow from 600 L/s to 400 L/s at Kaitoke Weir would not make a material difference for fish passage opportunities based on these analyses and it is our opinion that the potential adverse effects and risks would be minor. 8 Recommendations (1) We recommend that consent to lower the minimum flow downstream of Kaitoke Weir to 400 L/s is granted. (2) An inspection of the river should be undertaken on the first occasion that flows are reduced to 400 L/s to confirm that there is no impediment to fish passage. (3) If the inspection shows there are migration blockages, flows could be temporarily manipulated to allow fish passage. Further investigations may be required in order to determine if greater or lesser flows facilitate fish passage. Alternatively, the blockage may be removed (e.g. moving logs or rocks in the low flow channel). 9 Acknowledgements The contributions of various individuals and organisations are gratefully acknowledged, notably Alastair McCarthy, Greater Wellington Water, who initiated the project, and for his contributions to various meetings and workshops and review and site visits;

34 Hutt Gorge fish passage References Greater Wellington Regional Council hydrology staff who undertook the cross section measurements (Jon Marks, and his team of Nick Boyens, Jake Brown, Wendy Purdon and Laura Watts-Keenan); GWRC staff for discussions at the workshops and in providing various reports (Amy Holden, Jon Marks, Murray McLea, Malory Osmond, Summer Warr and Laura Watts-Keenan); the Department of Conservation (Nadine Gibbs-Bott); Fish and Game (Blake Abernethy, Nic Cudby, Corina Jordan, Peter Taylor and Phil Teal); Joe Hay & Dr John Hayes (Cawthron Institute); David Cameron (MWH), Dr Greg Ryder (Ryder Consulting) and Dr Russell Death (Massey University) for their contributions to discussions, the workshops and review; and Ed Breese (Tonkin & Taylor), for his project management, useful discussion throughout the process, site visits, and review of the reports. Allibone, R.M Water abstraction impacts on non-migratory galaxiids of Otago streams. Science for Conservation pages. Allibone, R.M.; Townsend, R.M Distribution of four recently discovered galaxiid species in the Taieri River, New Zealand: the role of macrohabitat. Journal of Fish Biology 51: August, S.M.; Hicks, B.J Water temperature and upstream migration of glass eels in New Zealand: implications of climate change. Environmental Biology Fish 81: Bleackley, N.A Biology of common bully (gobiomorphus cotidianus) populations in the Tarawera and Rangitaiki rivers: reproductive isolation by inland distance or effluent discharges? Master of Science thesis, University of Waikato. 128 pages. Boubée, J. A. T.; Jowett, I. G.; Nichols, S.; Williams, E. K Fish passage at culverts: a review with possible solutions for New Zealand native species. Wellington, Department of Conservation. 63 pages. Bjornn, T.C.; Reiser, D.W Habitat requirements of salmonids in streams. American Fisheries Society Special Publication 19: Chadderton, W.L.; Brown, D.J; Stephens, R.T Identifying freshwater ecosystems of national importance for biodiversity: criteria, methods and candidate list of nationally important rivers. Discussion document. Department of Conservation, Wellington. Closs, G.P.; Smith, M.; Barry, B.; Markwitz, A Non-diadromous recruitment in coastal populations of common bully (Gobiomorphus cotidianus). New Zealand Journal of Marine and Freshwater Research 37: Duncan, M.J.; Hicks, D.M D habitat modelling for the Rangitata River. NIWA Client Report CHC01/ pages. Duncan, M.; Woods, R Flow regimes. Pages in Harding, J.; Mosley, M.P.; Pearson, C.; Sorrell, B.; editors, Freshwaters of New Zealand. New Zealand Hydrological Society and New Zealand Limnological Society.

35 Hutt Gorge fish passage 25 Eikaas, H.S The effect of habitat fragmentation on New Zealand native fish: a GIS approach. Doctor of Philosophy thesis, University of Canterbury. 126 pages. Goldsmith, R.; Ryder, G Hutt River flushing flows and algal growth. Ryder Consulting Limited report prepared for Greater Wellington Regional Council. 18 pages. GWW Asset management plan. Greater Wellington Water, Greater Wellington Regional Council. 76 pages. Hamer, M The Freshwater Fish spawning and migration calendar report. Environment Waikato Technical Report 2007/11. Hudson, H.R. 2006a. Hutt River water supply instream flow investigation workshop. Environmental Management Associates Report pages. Hudson, H.R. 2006b. Hutt instream flow assessment proposed investigations. Environmental Management Associates Report pages. Hudson, H.R. 2006c. Hutt instream flow preliminary assessment. Environmental Management Associates Report pages. Hudson, H.R Assessment of potential effects on instream habitat with reduced flow in the Hutt River at Kaitoke. Environmental Management Associates, Christchurch. Report Jowett, I.G River hydraulic and habitat simulation, RHYHABSIM computer manual. New Zealand Fisheries Miscellaneous Report 49, Ministry of Agriculture and Fisheries, Christchurch, New Zealand. Jowett, I.G.; Richardson, J Habitat preferences of common, riverine New Zealand native fishes and implications for flow management. New Zealand Journal of Marine and Freshwater Research 29: Joy, M.K.; Death, R.G Predictive modelling of freshwater fish as a biomonitoring tool in New Zealand. Freshwater Biology, 47: Joy, M.K,; Death, R.G Predictive modelling and spatial mapping of freshwater fish and decapod assemblages using GIS and neural networks. Freshwater Biology 49: Kennedy, M.; McCarthy, A Water source development strategy. Greater Wellington Regional Council Report pages. Leathwick, J Fish-finding with statistical models. Water & Atmosphere 15(1): McDowall, R.M Diadromy in fishes: migrations between freshwater and marine environments. Timber Press, Croom Helm, London. 308 pp. McDowall, R.M New Zealand freshwater fishes - a natural history and guide. Revised edition. Heinemann Reed, Auckland. 553 pp. McDowall, R.M Fighting the flow: downstream-upstream linkages in the ecology of diadromous fish faunas in West Coast New Zealand rivers. Freshwater Biology 40,

36 Hutt Gorge fish passage 26 McDowall, R.M.; Taylor, M.J Environmental indicators of habitat quality in a migratory freshwater fish fauna. Environmental Management, 25: Mosley, M.P Critical depths for passage in braided rivers. New Zealand Journal of Marine and Freshwater Research 16: MWH Biological survey of the Hutt River in the vicinity of the Kaitoke water abstraction weir. Consultant Report prepared by David Cameron for Greater Wellington Regional Council, January ODFW Guidelines and criteria for stream-road Crossings. Oregon Department of Fish and Wildlife 7 pages. ODFW Fish passage criteria. Oregon Department of Fish & Wildlife. 4 pages. Paul, J.; Dent, L.; Allen, M Oregon Department of Forestry: compliance with fish passage and peak flow requirements at stream crossings. Oregon Department of Forestry Forest Practices Monitoring Program Technical Report pages. Snelder, T.; Biggs, B.; Weatherhead, M New Zealand river environment classification user guide. Ministry for the Environment, report ME 499, Wellington. Stevenson, C.; Kopeinig, T.; Feurich, R.; Boubée, J Culvert barrel design to facilitate the upstream passage of small fish. Auckland Regional Council technical Publication No pages. Townsend, C.R.; Crowl, T.A Fragmented population structure in a native New Zealand fish: An effect of introduced brown trout? Oikos 61: Thompson, K Determining stream flows for fish life. Pages In: Proceedings instream flow requirements workshop. Pacific Northwest River Basins Commission. Vancouver, WA. WDFW Fish passage barrier assessment and prioritization manual. Washington Department of Fish and Wildlife Habitat and Land Services Program. 57 pages. Wilson, S Low-flow hydrology of the Hutt Catchment. Greater Wellington Regional Council unpublished report WGN_DOCS # v2. 54 pages. WRC Regional freshwater plan for the Wellington Region. Wellington Regional Council Pub. No. WRC/RP-G-99/ pages. Young R.G.; Wilkinson J., Hay J.; Hayes, J.W. in review: Movement and mortality of adult brown trout in the Motueka River, New Zealand: effects of water temperature, flow and flooding. Transactions of the American Fisheries Society.

37 Hutt Gorge fish passage 27 Appendix 1: Photographs of the surveyed transects

38 Hutt Gorge fish passage 28 Figure A 1 Kaitoke lower transect (K1)

39 Hutt Gorge fish passage 29 Figure A 2 Kaitoke middle transect (K2)

40 Hutt Gorge fish passage 30 Figure A 3 Kaitoke upper transect (K3)

41 Hutt Gorge fish passage 31 Figure A 4 Swingbridge lower transect (S1)

42 Hutt Gorge fish passage 32 Figure A 5 Swingbridge middle transect (S2)

43 Hutt Gorge fish passage 33 Figure A 6 Swingbridge upper transect (S3)

44 Hutt Gorge fish passage 34 Figure A 7 Pakuratahi lower transect (P1 right bank)

45 Hutt Gorge fish passage 35 Figure A 8 Pakuratahi middle transect (P2)

46 Hutt Gorge fish passage 36 Figure A 9 Pakuratahi upper transect (P3)

47 Hutt Gorge fish passage 37 Figure A 10 Te Marua lower transect (T1)

48 Hutt Gorge fish passage 38 Figure A 11 Te Marua upper transect (T2)

49 Hutt Gorge fish passage 39 Figure A 12 Benge Creek lower transect (B1)

50 Hutt Gorge fish passage 40 Figure A 13 Benge Creek upper transect (B2)