Research Document - 2002/119
Review of 2001 Chinook Returns to the West Coast Vancouver Island, Forecast of the 2002 Return to the Stamp River / Robertson Creek Hatchery Indicator Stock, and Outlook for other WCVI Chinook Stocks
By B.E. Riddell, W. Luedke, J. Till, S. Taylor, A. Tompkins
Abstract
The intensive assessments and resulting abundance forecasts of the Robertson Creek Hatchery (RCH) and Stamp River chinook are undertaken annually for management of ocean and terminal fisheries, and as an indicator of the expected returns to the naturally spawning chinook populations along the west coast of Vancouver Island (WCVI). Forecasts presented in this paper indicate a continued conservation concern for naturally spawning WCVI chinook in 2002.
Review of return of the WCVI chinook in 2001
The terminal return of chinook to the Stamp River/RCH indicator stock was estimated to be 40,574 adult (age 3 and older) chinook, plus 4,612 age 2 males (jacks). The adult return represented a 600% increase from the 2000 return level.
Returns were monitored in another 18 WCVI streams for natural chinook spawning escapement. Changes in numbers of spawners, relative to 2000 levels, were variable. Returns for the 7 PSC indicators stocks continued to decline while returns for the other 11 extensively surveyed systems increased relative to 2000. Over all these systems, total escapement to all natural stock indicators increased by 60% from 2000 to 2001.
Forecast for the 2002 terminal return of the WCVI chinook:
The forecasting methods applied have been reviewed and accepted previously by PSARC. For 2002, the forecasted total return of Stamp River/RCH chinook to the terminal area of Barkley Sound and Alberni Inlet is estimated to be 80,300 based on averaging the forecast models. The mean absolute percent error in the average forecast (returns) is 21%. The age structure of the 2002 return is projected to be: 25% Age 3, 73% Age 4, and 2% Age 5 chinook, with an expected sex ratio of 40% females. At this time, the forecast only assumes fishing mortality in Southeast Alaska (SEAK). Harvest rate factors in SEAK were based on the Pacific Salmon Treaty agreements and we initially used a harvest rate scalar of 0.755 in SEAK troll fishery. The remaining cohort is identified as the expected terminal run assuming no fishing mortality on this stock in Canada.
Overall, returns are expected to increase by more than 500% relative to 2001, due to the expected large age 4 component. The number of females predicted in the terminal run is 31,800, which would result in 127 million egg deposition for the Stamp River/RCH, double the base period level.
A slightly larger terminal run is predicted if the forecast is expressed as a cumulative probability distribution. Based on the annual deviations from forecasts observed between 1988 and 2001, the 50% value of the cumulative distribution is 82,500 chinook in the terminal run and the 50% confidence interval is 66,000 - 96,100 adult chinook (Appendix Figure 3). However, given that this distribution is based on only 14 years of observations, the authors recommend continuing with past methods and applying the average forecast model that predicts 80,300 adult chinook.
The more serious concern for conservation is the expected run size to the naturally spawning chinook populations along the WCVI. The 2002 outlook in the WCVI indicator streams assumes a 90% increase from 2001 levels and expected 40% female component in the total return. However, the results show some chinook populations along the WCVI with less than 100 females (Table 10).
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