Research Document - 2002/094

Forecast for southern British Columbia coho salmon in 2002

By K. Simpson, D. Dobson, S. Lemke, R. Sweeting, R.W. Tanasichuk, and S. Baillie

Abstract

This research paper documents forecasts of marine survival, abundance and distribution for the coho salmon of southern British Columbia (Fraser River system, Strait of Georgia, and west Vancouver Island) for return year 2002.

Marine survival

Recommendations for the marine survival forecast for the five hatchery indicators and two wild coho indicators are:

For the 1999 brood in the Strait of Georgia, time series forecasts are for survivals to remain about the same (Vancouver Island hatcheries) or decrease (Lower Fraser hatcheries and Black Creek, the wild indicator on Vancouver Island). Trawl surveys in 2001 suggest that the four hatchery indicators will have a mean survival similar to the 2001 return. These survivals can be characterised as poor, relative to survivals experienced 10 to 20 years ago and in terms of the low exploitations that are necessary at these survivals for wild populations to sustain themselves.

Survival forecasts have been less accurate for west Vancouver Island indicator stocks. Until recently, Robertson Hatchery was the only indicator stock, where survival and exploitation measurements were taken. This year a sibling model predicted a survival of 3.1%, much less than the 9.6% last year but a time series model predicted the same survival as last year. Both have performed about the same in the past. We chose the sibling forecast because it is more conservative and because the second indicator, the wild Carnation Creek stock, has a similar forecast of 4%. The Carnation forecast is based on the abundance of an euphausiid prey species in Barkley Sound. This forecast also represents a decrease: survival in 2001 was 5.8%.

Abundance

Forecasting abundance of coastal stocks is highly problematic, particularly in the present regime of low exploitation. The forecasts have been sufficiently poor that we have chosen to discontinue them. Another method may be developed in the future. The forecast total abundance of Thompson River watershed coho uses time series analysis of measured abundances (direct estimates of catch and escapement) and it is still feasible. The forecast for 2002 is ~25,000, which is about half the observed abundance in 2001. It does represent a forecasted increase over the 1999 brood abundance of 18,700, however. The escapement in 2001 was the largest since 1989 and escapements in 2000 and 1999 were larger than brood year escapements. Greater proportions of fish that are surviving to maturity are returning to spawn because of the significant reductions in fishing pressure. Thus, assuming marine survivals and fishing pressures remain low, the outlook for Thompson and other interior Fraser coho is for slow but gradual improvement.

Distribution

In the hypothetical circumstance of historical patterns of fishing, the predicted proportion of catch inside the Strait of Georgia would be 0.35 (50% CI: 0.27-0.45), which can be characterized as a moderate outside distribution. A strong inside year is highly unlikely.

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