Whakapapa River: 2014 Drift Dive Survey Author: Adam Daniel Publication date: June 6, 2014 Auckland/Waikato Fish and Game Hamilton New Zealand 1
SUMMARY Two drift dive surveys were carried out in the headwaters of the Whakapapa River during February of 2014. A new reach near the Owhango Bridge was added to monitor a more accessible section of the river in terms of fishing pressure. The new site should provide information on the impact of the rapidly increasing pressure at the Owhango Bridge site compared to the less accessible and more stable pressure at the gauging bridge site. Water clarity was acceptable in both locations at 7.2 m and 4.2 m respectively. Rainbow and brown trout numbers appear to have stabilized and the river may now be at its maximum carrying capacity under the existing flow regime. The controlled flow regime in the Whakapapa headwaters can have a considerable impact on the number of days that the river is fishable over summer. During dry summers the headwaters will rarely be fishable due to high flows, while in wet summers the headwaters will often be one of the few rivers in the region providing fishing opportunities due to the hydro extraction. 1.0 INTRODUCTION Fish & Game s management objective for the upper Whakapapa River is to maintain a trophy trout fishery. Fish & Game would also like to increase access to the ample but isolated public lands along the upper Whakapapa River. An investigation into possible solutions to the lack of access on the upper Whakapapa is ongoing with DoC and the local council. Since 2002 there has been a strong trend of increasing trout numbers in the headwaters of the Whakapapa River. Anecdotal reports indicate that there has also been a corresponding increase in angler usage especially by guided parties. We will have good data on angle use of the Whakapapa after the National Angling Survey due to be released in 2014. In addition Fish and Game will undertake a monitoring program during 2014-2015 to evaluate the impact of increased access on the Whakapapa and Whanganui Rivers. 2.0 STUDY AREA The Whakapapa River is formed by the joining of the Whakapapa-nui and Whakapapa-iti Streams. Both streams drain small glaciers and snowfields on the western slopes of Mt Ruapehu and are cold, swift and clear. The Whakapapa Intake of the Tongariro Power Scheme is located about two kilometers downstream from the confluence of the Whakapapanui and Whakapapaiti streams. Below the intake the Whakapapa is a turbulent river alternating between steep rapids and short deep pools. Here the river is quite narrow, generally about 10 to 20 m wide. Access to the intake is via a private road running through the Taurewa Station operated by Landcorp. Downstream from the intake, access is restricted for the first 5 km by steep banks which support native forest and scrub. The Owhango bridge site is located 20 km form the intake and is the first publicly accessible section of the river. The Owhango Bridge site is easily accessible and heavily fished. This location is also known for illegal fishing activities such as netting, set lines 2
and fishing with bait. 3.0 METHODS An additional drift dive location near Owhango was added for the 2014 survey. The Owhango site was added to increase the chances of detecting population level changes and to monitor the impact of potential increases in fishing pressure from the 42 nd traverse entrance at Owhango. The Owhango dive site was 1 km long (Figure 1) and the traditional gauging station dive covered 2.5 km of river downstream from the gauging bridge (Figure 2). Only pools were dived due to the rapids between pools, as a result very little habitat suitable for small fish was investigated in either dive location. In total fifteen pools were dived below the gauging station and seven pools were dived above the Owhango Bridge. The drift dive involved three divers floating downstream using snorkelling equipment and counting trout as they became visible. Individual pool counts were not successfully conducted in 2014 but are planned for the 2015 dives. Figure1. Drift dive monitoring area above the Owhango Bridge 3
Figure 2. Drift dive monitoring area downstream from the upper Whakapapa River gauging bridge. Water clarity was measured using a black disk, which was placed in the water and then viewed underwater in a horizontal direction using a periscope. The maximum distance that the outline of the black disc could be viewed underwater provided an estimate of water clarity. River flow at the time of the dive was obtained from the Genesis Energy website. 4.0 RESULTS The Owhango dive was started at 2 pm February the 25th and the gauging station dive was started at 11.00 a.m. on the 26th of February 2014. Water clarity was good at the gauging station at 7.2 m but clarity was significantly reduced at Owhango with a visibility of only 4.2 m. The Whakapapa River had a median flow of 6.8 m³/s for the year to date prior to the surveys and was flowing at approximately 8.3 m³/s for both dives (data provided by Genesis Energy). All but one trout observed on the gauging station dive survey (upper) were large adult fish at least 40 cm in length (Table 1). However a large number of age 0 fish were observed in slow moving backwaters in the Owhango section of the river indicating a successful recruitment year. Trout were observed in nearly every pool with trout numbers varying from 1 to 19 fish per pool. In total 187 trout were observed on the Owhango dive and 72 trout were observed at the traditional gauging station site. The majority of the rainbows were located at the Owhango site and most of the browns were located upriver at the gauging station site. No native fish were seen although daytime drift dives are not a suitable method for observing or counting native fish. 4
2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 Clarity (m) Table 1. The size and frequency of rainbow (R) and brown (B) trout seen in the Whakapapa River above Owhango on the 25 of February 2014 and below the Gauging station during drift dive surveys. Fish under 20 cm are not identified to species. Site <10 cm 10-20 cm 20-30 cm 30-40 cm 40+ cm Owhango 17R 28R 28R 10 101 1B 2B 4B Gauging 2R 1R 10R 0 1 Station 0.4B 12.8B 13.2B 5.0 DISCUSSION Water clarity at the gauging station site was good for drift diving in 2014 but down compared to previous years (Figure 3). However, the long term trend in water clarity is one of the only sites in the Auckland/Waikato Region that is stable or increasing in clarity. The Owhango site had reduced water clarity at 4.2 m likely due to the influence of the Piopiotea Stream. Although the recent Horizon s Regional Council One Plan should provide the regulatory framework to help improve water quality in this catchment Auckland Waikato Fish & Game will focus on assisting land owners to reduce erosion and nutrient loss where practical. 12 10 8 6 4 2 0 Year Figure 3. Water clarity in the upper Whakapapa River as observed on 11 drift dive surveys undertaken since 2002. Dotted line indicates 12 year trend in water clarity. Trout numbers were slightly down from the previous two surveys but the decline was not significant and was well within the variability between surveys that is expected with driftdiving (Figures 4). In recent years there has been a trend of increasing trout numbers but numbers have tapered off in the last two surveys and the river may now be at its maximum carrying capacity under existing conditions. If this is the case angling pressure should be the main driver on the density of large fish. 5
1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 Number of trout 30 28 26 24 22 20 18 16 14 12 10 8 6 4 2 0 Rrainbow (Gauge) Brown (Gauge) Rainbow (Owhango) Brown (Owhango) Year Figure 4. Number of large ( 40 cm) trout per kilometer of the upper Whakapapa River at the gauging station bridge (gauge) and above the Owhango access (Owhango). In response to concerns of increased angling pressure Fish & Game will review the national angler survey due out in 2014 and will be conducting an extensive angler monitoring program on the Whakapapa and Whanganui Rivers during 2014/2015. The angler monitoring program will include 30 random monitoring locations to identify both legal and illegal fishing activity on the river during a 12 month period. This data will be compared to the current and past angler survey data to quantify actual angling pressure. Although there are limited regulatory options available to reduce angler harvest considering the upper Whakapapa River fishery is currently managed as a trophy fishery with no legal take of rainbow trout. Auckland Waikato Fish & Game are currently focusing funding and staff time towards obtaining public access to the Whakapapa upriver of Owhango. If fishing pressure is high throughout the backcountry fishery the management priority will shift to maintaining or improving water quality. Flows in the upper river are substantially modified by the intake weir operated by Genesis Energy which maintains a minimum flow of 3 m³/s and can divert up to 40 m³/s. The intake weir reduces the magnitude of floods such that moderate rainfall usually has negligible effect on flows resulting in dramatic increases and decreases in flows (Figure 5) rather than rain induced spikes followed by a gradual reduction in flow. Although such a dramatic hydrograph could cause mass stranding s of fish the steep channelized morphology of the Whakapapa River bed does not allow for lateral flooding that would likely strand fish. 6
1 February 2013 13 February 2013 26 February 2013 11 March 2013 23 March 2013 5 April 2013 18 April 2013 30 April 2013 13 May 2013 26 May 2013 7 June 2013 20 June 2013 3 July 2013 15 July 2013 28 July 2013 10 August 2013 22 August 2013 4 September 2013 17 September 2013 30 September 2013 12 October 2013 25 October 2013 7 November 2013 19 November 2013 2 December 2013 15 December 2013 27 December 2013 9 January 2014 22 January 2014 3 February 2014 16 February 2014 300 Flow (m3/s) 250 200 150 100 50 0 Figure 5. Flows in the upper Whakapapa River (below intake weir) for the year prior to February 2014. Genesis Energy is required to maintain a flow of at least 29 m³/s downstream in the Whanganui River below Taumarunui at Te Maire. Consequently during periods of low rainfall Genesis Energy has to stop diverting water from the upper Whakapapa to maintain the Te Maire flow requirement, and hence the river returns to natural base flows, about 8 m³/s, which is well above the flow preferred by anglers in the upper reaches. For anglers that wade the upper Whakapapa a safe fishable flow is about 5 m³/s. Fishing pressure on the upper Whakapapa is likely to be increased during wet summers, due to hydropower takes reducing flows on the Whakapapa and increased natural flows in other local rivers. The current state of the Tongariro River fishery is also a major factor regulating pressure on the upper Whakapapa with poor fishing on the Tongariro resulting in increased pressure on the upper Whakapapa. Although the 2013/2014 summer was considered a drought there was a dramatic increase in the number of days the upper Whakapapa was fishable (Figure 6) likely resulting in increased pressure on the trout population. 7
Number of fishable days Whakapapa 140 120 100 80 60 Whakapapa 40 20 0 2007/08 2008/09 2009/10 2010/11 2011/12 2013/2014 Date Figure 6. Number of fishable days per season (defined as days with flows less than 5 m³/s) on the upper Whakapapa River (below intake weir) within the fishing season (Oct 1-June 30). Overall the fish population on the Whakapapa is healthy with ample large rainbow and brown trout. Angler use monitoring over the next year will greatly increase our understanding of the fishing pressure on the Whakapapa. In terms of long term management the Whakapapa will be a key focus area for Auckland Waikato Fish & Game due to the high quality of the fishery. 8