Research Document - 2012/015
A comparison of the abundance, size composition, geographic distribution and habitat associations of snow crab (Chionoecetes opilio) in two bottom trawl surveys in the southern Gulf of St. Lawrence
By H.P. Benoît
A framework review of the assessment methods for the southern Gulf of St. Lawrence snow crab (Chionoecetes opilio) stock took place in Moncton (NB) from November 19-23, 2011. The objectives of the meeting included reviewing the current sampling design and inference zone for the bottom-trawl survey used to assess crab abundance and distribution, and reviewing the methods for reconstructing a homogenous time series of biomasses and abundances from 1989 to present. To date, the snow crab assessment has been founded in large part on a dedicated bottom-trawl survey, undertaken annually since 1988, to provide a fishery-independent index of stock status. A standardized research vessel (RV) bottom-trawl survey of the southern Gulf has also been undertaken each September since 1971 and has principally been used for assessments and scientific research dedicated to marine fish. Though snow crab catches have been quantified in the RV survey since 1980, this information has yet to be incorporated into the assessment of southern Gulf snow crab. This document presents the first formal evaluation of the utility of the RV survey data for the assessment of southern Gulf snow crab, while addressing three aspects of the terms of reference (ToR) for the framework review. First, based on the broader geographic scope of the RV survey compared to the crab survey, it is shown that the snow crab survey presently covers the majority of the areas inhabited by snow crab in the southern Gulf, with the exception of deeper waters where low and temporally stable densities of crab are found. Second, based on the broader spatial and longer temporal scopes for the RV survey, it is shown that crab densities vary with both depth and bottom-temperature, though the relationship with depth is temporally more stable. Depth is therefore an appropriate environmental variable for defining the crab survey area and a valid auxiliary variable for the model used to estimate biomass in the assessment (kriging with external drift). Third, using the RV survey as a baseline, possible differences in catchability between the vessel used to undertake the crab survey from 1999-2002 and the one used since 2003 were examined. The analysis revealed evidence that the former vessel may have been more efficient at catching snow crab, particularly small but also large individuals, though the two vessels are presently assumed to be equivalent in the assessment. Overall, the RV survey was found to produce a picture of southern Gulf snow crab abundance, distribution and size composition that is very comparable to that obtained from the dedicated snow crab survey. The information from the standardized RV survey has a strong potential for helping to address key problems with the snow crab survey related geographic areas that were unsampled in the past, as well as in addressing possible changes in catchability that resulted from uncalibrated vessel changes. Furthermore, the RV survey has the potential to enhance understanding of stock productivity and dynamics by extending abundance indices back to 1980.
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