Patterns and implications of skip-molting for the Eastern Bering Sea snow and Tanner crab (Chionoecetes opilio and C. bairdi)

Abstract

Male skip-molting, defined for this study as when adolescent male snow or Tanner crab (Chionoecetes opilio and C. bairdi) do not molt during the winter/spring molting season, reduces the abundances of large mature males by requiring skip-molters survive at least one additional year of natural mortality before molting resumes and by increasing terminal molting at smaller sizes. These dynamics have obvious consequences for fisheries that only target and retain large mature males. Though considered common in Atlantic Canada (AC) snow crab and recently documented in Sea of Japan snow crab, skip-molting remains unexamined for Eastern Bering Sea snow and Tanner crab populations. Using chela height and shell condition data collected from 1989 to 2017, size-specific proportions of skip-molting were estimated for each species. Estimated size-specific proportions for snow crab resemble those reported for AC snow crab populations; estimated Tanner crab proportions are about twice that for snow crab when accounting for size differences between the species. Population simulations indicate skip-molting can reduce the biomass of large mature males by 12–47% relative to a population with no skip-molting, depending on species and assumptions of skip-molter survival. Regression models developed to identify important covariates of skip-molting explained only modest variation in the data (i.e., low deviance explained). Additional histological and physiological data are needed to validate classification of skip-molters from field measurements. Skip-molting proportions may have been underestimated due to shell condition misclassification of skip-molters. The stock assessments for EBS snow and Tanner crab do not consider skip-molting and assume all adolescent males molt annually. The effects of misspecified growth dynamics on assessment estimates warrant further research.