Do Changes in the Spatial Distributions of the Newfoundland Snow Crab Fleets Affect CPUE Estimates?

Snow crab (Chionoecetes opilio) fisheries are economically very important in Newfoundland and Labrador, contributing 33% to the provincial total landings value of in 2013. This fishery has recently been MSC certified, with requirements for new estimations of stock reference points and evaluations of harvest control rules. As a step towards these goals, within-season depletion-based methods based on weekly, spatially-referenced CPUE data have been implemented as a way to back-calculate snow crab biomass levels at the start of the season. A key assumption of this approach is that the spatial distribution of fishery effort is spatially stationary within and among years. We evaluated this assumption by examining spatial metrics of effort distribution (deviations from annual centroids, minimal areas containing specified % of fishing effort) within and among seasons using data from no less than 9 fishing zones from 1990 to 2012.  These analyses revealed whether and where this assumption was valid and allowed us to test the hypothesis that periods of years with markedly different biomass-CPUE relationships were due to changing spatial distributions of fishery effort. The analyses contribute to improving the implementation of depletion-based methods into stock assessments and comparing these results to other metrics of crab stock biomass.