Species-specific stratification and the estimate of groundfish biomass in the eastern Bering Sea

Abstract

Species biomass estimates for eastern Bering Sea groundfish are calculated using data collected in the Resource Assessment and Conservation Engineering Division's annual demersal trawl survey. These data are analyzed with a stratified statistical sampling model. The survey is divided into 10 strata; a common stratification scheme is used for all species. In this study, the use of species-specific stratification schemes was examined to study how great a reduction in the variance of the biomass estimator might be achievable. New stratification schemes were developed by visually examining the distribution of catch per unit effort (CPUE) data averaged over several years. Geographically contiguous strata were drawn by grouping sampling stations with similar mean CPUEs. Reductions in the variances were achieved, but their magnitude was not large enough to yield biologically meaningful reductions in the sizes of the estimated confidence intervals of the species biomass. The use of non-geographically contiguous strata was also examined. Using this type of a stratification scheme resulted in even smaller reductions in the variance estimates.


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