Research Document - 2014/082

Model-based estimation of commercial-sized snow crab (Chionoecetes opilio) abundance in the southern Gulf of St. Lawrence, 1980-2013, using data from two bottom trawl surveys

By H.P. Benoît and N. Cadigan

Abstract

There are two fishery-independent bottom-trawl surveys that provide relative abundance indices for snow crab in the southern Gulf of St. Lawrence (sGSL). One of the surveys is principally directed to snow crab and has been conducted annually since 1988 (henceforth called the crab survey, CS). The second is a research vessel bottom-trawl survey conducted annually since 1971 (henceforth called the research vessel survey, RVS), which was initially focused on demersal fish but which has provided information on snow crab in the catches since 1980. Benoît and Cadigan (2013) presented a model-based estimation framework that integrates data from the two surveys to produce an annual standardized index of commercial-sized sGSL snow crab abundance for 1980-2012. Though the reliability of the framework still needs to be assessed using simulations, it was used here to update the standardized index using the most recent survey data collected in 2013 and to provide a preliminary estimate of the relative catchability (fishing efficiency) of the new vessel used in the CS in 2013. Two model variants were considered, one in which the catchability of the 2013 CS vessel was estimated separately from that of the other CS vessels, and a second in which the catchability of the two most recent CS vessels was constrained to be the same. The first model variant produced an estimate of 2013 crab abundance that was greater than the 2012 estimate, and estimated the relative catchability of the 2013 CS vessel to be among the lowest of the vessels used in that survey. In contrast, the second model variant estimates a slight decrease in abundance from 2012 to 2013. With a single year of data, we cannot rule out the possibility that a year effect in the RVS in 2013 could have greatly influenced the model estimates of abundance for that year and the relative catchability of the 2013 CS vessel. With additional years of survey data involving this vessel and the RVS vessel, the reliability of the estimate of relative catchability will greatly improve. This situation highlights the importance of having some interannual consistency in the CS vessels in the absence of comparative fishing.

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