Research Document - 2000/068

Stock status of Fraser River Sockeye.

By A. Cass, J.T. Schnute, L. Richards, and A. MacDonald

Abstract

We assess the status of 11 stocks using spawning escapement and adult return data (catch + escapement) compiled by the Pacific Salmon Commission. Together these stocks accounted for 93% of the total sockeye returns in a newly developed database covering spawning years 1948-93 and adult return years 1954-97. Three of the stocks, for which juvenile freshwater biomass data is available, are also assessed to evaluate the utility of these data to measure freshwater capacity. Sockeye spawning escapement increased gradually from an average of 1.5 million/yr in the 1950s to 10.7 million/yr during 1980-97. Increases in escapement mainly occurred in the large actively managed stocks and cycle lines. Returns increased nearly two fold from 6.4 million sockeye/yr to 11.3 million in the same periods. Off-cycle Late Shuswap sockeye abundance has remained at low levels. The abundance of Bowron, Raft and Cultus Lake stocks has declined in the last two decades. We considered two alternative stock-recruitment models in this analysis: 1) a two-parameter Ricker model that assumes no interaction among cycle lines, and 2) a modified Ricker model (Larkin model) that estimates cycle-line interaction terms for escapement at lags 1, 2 and 3 years. Parameter uncertainty is captured by sampling the Bayes posterior distribution. For the Ricker model fit to spawner and adult return data, parameter uncertainty of escapement at maximum sustainable yield (MSY) is large and most stocks show little evidence of diminished returns within the range of observed spawner abundance. The productivity parameter estimate is comparatively precise. Of the three stocks with juvenile biomass data (Quesnel, Chilko and Shuswap sockeye), only Shuswap Lake sockeye show evidence of diminished returns at high escapements. Negative, cross-cycle interaction is statistically evident in all the large, highly cyclic stocks as revealed by the interaction terms in the Larkin model. High priority should be put on decision analysis that recognizes the risk of strong cross cycle negative interactions.

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