Research Document 1997/139

Queen Charlotte Islands herring carrying capacity and sustainable harvest rates in different climate regimes

By D. Ware

Abstract

Over the last 44 years the prefishery biomass of the Queen Charlotte Islands (QCI) herring stock has averaged about 29 thousand t, and the catch about 8.1 thousand t. The productivity of this stock undergoes large changes in response to interannual and decadal timescale variations in spawner biomass (and other factors that remain relatively obscure), and the resulting recruitment to the stock four years later. Recruitment accounts for most of the stock productivity. The stock-recruitment curve suggests that QCI herring have been most productive when the average spawning stock biomass has been about 11 thousand t. Since 1950, the average carrying capacity of the stock appears to have been about 46 to 49 thousand t. The 1951 and 1977 year-classes were extraordinarly large. In contrast, the 1990, 1991 and 1992 yr-classes were extremely small. The failure of these yr-classes caused the stock biomass to decline below the cutoff threshold (10,700 t), so the commercial roe herring fishery was closed from 1994 to 1997 to allow the stock to rebuild. Previous studies have found that recruitment to the QCI herring stock is correlated with several environmental variables. However, the correlations that have been discovered so far aren't very useful for stock forecasting because they don't explain much of the residual, "density-independent" recruitment variability. More work needs to be done to extract other plausible forms of the stock-recruitment signal from the "noise" in the recruitment time series and, more importantly, to discover some relevant environmental and ecosystem variables that explain a significant fraction of the residual variation.

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