Research Document - 2001/155
Review of 2000 Chinook returns to the West Coast Vancouver Island, forecast of the 2001 return to the Stamp River/Robertson Creek Hatchery indicator stock and outlook for other WCVI Chinook stocks
By B. Riddell, W. Luedke, J. Till, R. Ferguson
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 2001.
Review of return of the WCVI chinook in 2000
The 2000 terminal return of chinook to the Stamp River/RCH indicator stock was estimated to be 6,450 adult (age 3 and older) chinook, plus 2,970 age 2 males (jacks). The adult return represented a 80% decline from the 1999 return level.
Returns to another 22 WCVI streams that were monitored for chinook spawning escapements also declined considerably from 1999 levels. Over all these systems, the decline was approximately 60%. The overall age composition was very similar to that in the Stamp River/RCH terminal run.
Forecast for the 2001 terminal return of the WCVI chinook
The forecasting methods applied have been reviewed and accepted previously by the Pacific Scientific Advice Review Committee (PSARC). For 2001, the forecasted total return of Stamp River/RCH chinook to the terminal area of Barkley Sound and Alberni Inlet is estimated to be 14,200 based on averaging the forecast models. The mean absolute percent error in the average forecast (1985-2000 returns) is 15%. The age structure of the 2001 return is projected to be: 78% Age-3, 9% Age-4, and 13% Age-5; with an expected sex ratio of 20% females. At this time, the forecast only assumes fishing mortality in South East Alaska (SEAK). Harvest rate factors in SEAK were based on the Pacific Salmon Treaty agreements and we initially used a harvest rate scalar of 0.5 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 double relative to 2000, due to the expected large age 3 component. However, the number of females, and resulting eggs, will decrease by an expected 25% from the 2000 level. The resulting level of egg deposition to the Stamp River/RCH will be substantially below the base period level (approximately 20% of base). The expected number of eggs would be the lowest on record since 1985 (Appendix Table 3), when the indicator stock program began.
A slightly more conservative 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 2000, the 50% value of the cumulative distribution is 13,200 chinook in the terminal run and the 50% confidence interval is 11,600 to 15,100 adult chinook (Fig. 8). However, given that this distribution is based on only 13 years of observations, the authors recommend continuing with past methods and applying the average forecast model that predicts 14,200 adult chinook.
The more serious concern for conservation is the expected run size to the naturally spawning chinook populations along the WCVI. The 2001 outlook in the WCVI indicator streams assumes a 120% increase from 2000 levels and expected 20% female component in the total return. The results show many chinook populations along the WCVI with less than 100 females (Table 9).
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