Temporal and spatial variability in size-at-maturity for the eastern Bering Sea snow and Tanner crab (Chionoecetes opilio and C. bairdi)

Abstract

Growth in size stops at the terminal molt (molt-to-maturity) for the eastern Bering Sea snow and Tanner crab (Chionoecetes opilio and C. bairdi) and understanding size-at-maturity (SAM) dynamics is critical to manage their fisheries, which target and retain large mature males. Bottom trawl survey data from 1982 to 2017 were analyzed with correlations and spatial regressions to identify temporal and spatial patterns of SAM and size-specific proportions mature (PM) for newshell (recently molted) crab of each sex of each species. While substantial annual variability in SAM was estimated, no long-term directional trends were estimated for either species. Recruitment variability was likely the major driver of temporal trends in SAM for both species, rather than variable ocean temperatures or other possible factors. Recruitment abundances and SAM were negatively correlated for female snow crab but positively correlated for male snow crab and both sexes of Tanner crab. Male and female snow crab SAM had similar periodicities as snow crab recruitment. SAM spatial patterns were relatively stable at decadal time-scales for both species. Annual temporal variability in PM was moderate to negligible for both species and decadal-sale variability was modest, indicating temporally stable size-specific maturation probabilities. Comparison of SAM and PM regression results show strong effects of ontogenetic movement on spatial patterns of SAM for male snow crab. Intra- and inter-species comparisons allowed for broader understanding of SAM dynamics. Strong periodicity of male snow crab SAM may have utility for fisheries management. Elucidating the mechanisms responsible for periodic recruitment for these species will help provide a more complete understanding of both SAM dynamics and general population dynamics. With increasing ocean temperatures in the EBS, the mean size of mature crab for these species may increase but would certainly be accompanied by a decrease in snow crab abundance, the more stenothermic of the two species.