Recent shifts in northern Bering Sea snow crab (Chionoecetes opilio) size structure and the potential role of climate-mediated range contraction

Recent historic lows in sea ice and cold pool extent in the Bering Sea have been linked to large-scale biogeographic shifts in many demersal taxa. Snow crab (Chionoecetes opilio) are associated with the cold pool and thus may be especially prone to northward range contraction with continued warming. Data from the 1988-2019 National Marine Fisheries Service eastern Bering Sea (EBS) bottom trawl surveys were used to examine the effect of climate warming on snow crab thermal habitat use and spatial distributions. The northern Bering Sea (NBS) was also surveyed in 2010 and 2017-2019, allowing us to examine NBS snow crab demographic structure relative to potential climate-driven range contraction into the NBS. Across the time series, trends in temperatures occupied by snow crab were tightly coupled with average bottom temperatures in the EBS despite extreme temperatures in 2018-2019 that exceeded cold-water thermal preferences of juvenile snow crab. Furthermore, we found that increased temperatures and a reduced, more northerly cold pool extent resulted in a smaller area occupied by snow crab across different ontogenetic stages, although there was no evidence for a northward shift in centers of distribution within the EBS. These findings suggest that the spatial extent and average temperature of snow crab distribution are likely constrained by the availability of cold water habitat in the EBS, and dramatic declines in juvenile snow crab abundance observed in 2019 may be attributed to potential direct or indirect temperature effects on survival of highly stenothermic early benthic stages of snow crab. Despite limited support for a directional range shift, survey abundance estimates indicated a similar to 600% increase in abundance of larger size classes of male snow crab (>= 61 mm carapace width) in the NBS between 2018 and 2019. Substantial shifts in juvenile abundances and NBS snow crab size structure in 2019 may have important management implications for the stock. While the increase in NBS mature male biomass may suggest the potential for a commercial fishery in more northern latitudes, concurrent declines in juvenile abundance suggest caution concerning the sustainability of the stock with continued warming.