Research Document - 2001/101

An evaluation of a recruitment forecasting procedure for Strait of Georgia herring

By Tanasichuk, R.W.

Abstract

This report describes an evaluation of an extension of the recruitment forecasting procedure which has been used since 1987 to forecast recruitment (number of age 3 spawners) for the West Coast Vancouver Island (WCVI) stock of Pacific herring (Clupea pallasi) (Ware and Tanasichuk 1987). The extension is to forecast recruitment for Strait of Georgia herring. The forecasting procedure is based on the age composition of samples of herring trawled along the southwest coast of Vancouver Island during summer fisheries oceanography surveys. Results of analyses of data on observed proportion of age 3 fish in the subsequent spring's roe fishery showed that proportion of age 3 fish from the WCVI and Strait of Georgia stocks were correlated significantly, and that the functional relationship had a slope and intercept which were not significantly different from 1 and 0 respectively. Therefore, year-specific proportion at age 3 was the same for both stocks, which means that proportion at age 3 for either stock should not be biased by mixing on the summer feeding grounds. Results of regression analyses showed that proportion of age 3 fish observed in the prefishery can be predicted from the proportion of age 3 herring in the samples trawled the previous summer. Residual analysis showed no time trend in the residuals, or any effect of sampling time or the magnitude of the forecast. A retrospective analysis of the forecasts over 1994 - 2001 showed that observed proportions at age 3 were mostly within the 95% confidence interval of the forecast, and that the mean difference between forecasted and observed proportion at age 3 was less than 0.03 among the three stock assessment models. The approach provides acceptably accurate forecasts of recruitment for the Strait of Georgia stock. Recruitment for this stock is forecasted to be between average and good. Forecasts for WCVI herring are presented in an appendix. Recruitment for WCVI herring is forecasted to be poor.

Observations on fish species compositions and distributions made during the 2001 offshore survey were reported. Species compositions and herring distributions were different than in previous years. Whitebait smelt (Allosmerus elongatus) were common nearshore. There has only been one previous record of this species in Canadian waters. In addition, walleye pollock (Theragra chalcogramma) were common; they have been scarce since the early 1990's. Herring were concentrated on 40 Mile and especially Swiftsure Banks and were dispersed along the continental shelf slope to at least 49^o N.

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