Spatial and Temporal Variability in Size at Maturity of Walleye Pollock in the Eastern Bering Sea

Abstract

Size at maturity is incorporated into stock assessments of walleye pollock Theragra chalcogramma. For eastern Bering Sea walleye pollock, however, this important biological variable has not been examined since 1976 and possible interannual and geographic variations have never been evaluated. Maturity condition and fish length data were collected from 10,197 walleye pollock aboard factory trawlers during winter 2002 and 2003. We also analyzed a smaller, previously unanalyzed data set on maturity collected annually by National Marine Fisheries Service scientists during eight echo integration-trawl surveys over 1989-2002. Length at 50% maturity (L ₅₀) was estimated by year and area by logistic regression using maximum likelihood methods. Spatial (within subareas) and temporal (interannual) variability were found. For instance, fish matured at the smallest lengths north of the Pribilof Islands and in the years 1989, 1991, and 1995 and at the largest lengths in the southeastern Bering Sea and in the years 2001 and 2002. We found evidence that the variability in size at maturity was related to biological mechanisms. Length at 50% maturity appeared to be directly related to growth as measured by length at age. Moreover, an inverse relationship between walleye pollock biomass and L ₅₀ suggests that growth is density dependent. This spatiotemporal variability in size at maturity has implications for estimates of spawning stock biomass and should be incorporated into the annual stock assessments for setting annual catch specifications.