Research Document 1997/141
A retrospective analysis of escapement model performance using different adult survival rate estimate
By R. Tanasichuk and J. Schweigert
Test fishing data, catch data and spawning biomass estimates for the west coast Vancouver Island herring stock were used to compare how three different methods of estimating apparent adult survival rate affect the performance of the escapement stock assessment model. The first, the procedure currently used, included all test fishing data from 1972 to the present. The second excludes data from samples taken before 1980. These data were excluded because survival curves suggested size-selective test fishing before 1980. The first two procedures use running mean age-specific survival rates. The third method uses a regression which estimates age- and year- specific apparent adult survival rates based on the 1980-1996 data. The current procedure overestimates returning adult biomass by 28%. The second and third procedures underestimate biomass by 19 and 17% respectively. Residual analysis showed that the second and third methods underestimated biomass consistently. All procedures provided returning biomass estimates of acceptable accuracy.
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