Research Document - 2000/107

Pre-season run size forecasts for Fraser River Sockeye in 2000.

By A. Cass

Abstract

Adult returns of sockeye to the Fraser River on the 2000 cycle line are the lowest of the four cycle lines averaging 4.5 million sockeye compared to an all-year mean of 9.4 million during 1970-99. The major stocks expected in 2000 based on brood year escapement estimates for 1996 are Chilko, Stellako, Early Stuart, Late Stuart, Birkenhead, and Weaver sockeye. Forecasts are made for each of four migratory timing groups and 19 individual stocks. Forecasting methods are unchanged from previous PSARC reviews and are based on a variety of explanatory variables and forecast models. The 2000 forecast, all stocks combined, at the 50% probability level (point estimate) is 4.1 million sockeye or near the long-term mean (1970-1996). At the 75% probability level the forecast is 2.3 million fish. The summer run group accounts for 65% of the forecast. Within that timing group, Chilko and Stellako sockeye respectively account for 33% and 16% of the forecast at the 50% probability level. A cautionary prognosis for 2000 returns is warranted. Estimates of jack returns in 1999 (2000 age-4 returns) to several of the major stocks on the cycle line were very low compared to brood year escapements and compared to jack returns on the previous year in this cycle. Temperatures in the north Pacific Ocean in the spring of 1998 were above average during the transition from intense El Nino conditions in 1997 to cooler La Nina conditions in the latter half of 1998. Ocean survival of sockeye that went to sea in 1997 was very low. The carry-over effect of above average temperatures in the spring of 1998 on juvenile sockeye survival for the 1996 brood (2000 age-4 returns) is unknown. Qualitative information reported in this document indicate that run sizes may be less than the 50% probability level.

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