Research Document - 1999/125
Forecast for Southern British Columbia Coho Salmon in 1999.
By B. Holtby, J. Irvine, R. Tanasichuk and K. Simpson
Abstract
This Working Paper documents forecasts of marine survival, abundance and distribution for the coho salmon of southern British Columbia (Thompson River, lower Fraser, Strait of Georgia, and west Vancouver Island).
Marine survival: Our recommendations for the marine survival forecast for the five hatchery indicators and one wild coho indicator are given in the following Table. Survivals are expected to remain poor for all Strait of Georgia wild and hatchery stocks and are forecast to be either unchanged or lower in 1999 compared to 1998 (following Table). Survival of Black Creek coho, the single wild indicator on the Strait of Georgia for which there is a forecast, is one of the survivals expected to be lower in 1999 compared to 1998. Nevertheless, survival rates appear to be substantially greater toward the north end of Georgia Strait compared to the lower mainland and the Fraser. CWT escapement data are not yet available for Thompson coho and consequently there is no forecast of marine survival for this aggregate. There are no indications in the magnitude of the escapement that survivals improved in 1998 and the forecast of abundance remains dismal. The forecast survival for wVI coho is slightly lower than in 1997 and 1998.
| Indicator | Model | Ŝ1999 | (50% CI) | Change Relative to 1998 |
|---|---|---|---|---|
| Biq Qualicum | LLY | 0.003 | (0.0013-0.008) | same |
| Quinsam | LLY | 0.021 | (0.013-0.034) | same |
| Chilliwack | RAT3 | 0.017 | (0.010-0.027) | lower |
| Inch Creek | LLY | 0.005 | (0.003-0.010) | same |
| Black Creek | 3 YRA | 0.0042 | (0.031-0.056) | lower |
| Robertson Creek | Sibling regression | 0.029 | (0.020-0.041) | lower |
Abundance forecast: Without fisheries information, forecasting abundance is highly problematic, and because we are using time-series models the forecast is dependent on the highly uncertain estimate of abundance in 1998. With those caveats the RAT3 forecast of the StG-Fr aggregate is 2.0´ 105 (50% CI: 1.5´ 105-2.8´ 105). This forecast portends a very worrisome further deterioration in the status of Strait of Georgia coho.
The LLY forecast for the wVI aggregate is 4.5´ 105 (50% CI: 3.1´ 105-6.5´ 105). This forecast is 77% of the overall average abundance of 5.9´ 105.
The abundance forecast for Thompson coho is for continued severe depression. Brood year escapements in the Thompson were very low and there is no indication that marine survival will improve either in the southern Strait of Georgia or the west coast of Vancouver Island. We conclude that it is unlikely that stock size will increase appreciably for either the North or South Thompson aggregate in 1999.
Distribution forecast: The predicted proportion of catch inside the Strait of Georgia (pinside) should there be no fishing restrictions is 0.33 (50%CI 0.25-0.42), which can be characterized as a moderately strong outside distribution. The confidence interval suggests that an extreme outside year (pinside < 0.2) is about as likely as a return to a "normal" distribution (pinside>0.4).
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