Research Document - 2002/051

A study of the catchability of snow crab by the Campelen 1800 survey trawl

By E. G. Dawe, B. R. McCallum, S. J. Walsh, P. C. Beck, H. J. Drew and E. M. Seward

Abstract

An experiment was carried out during Sept. 2001 to estimate the catchability (trawl efficiency) of snow crab by the NAFC's (Northwest Atlantic Fisheries Centre) standard survey bottom trawl. The experiment involved towing secondary trawls directly behind the footgear of the Campelen 1800 shrimp trawl, which has been the NAFC's standard survey bottom trawl since 1995. Overall, catchability was found to be much lower than 1.0 and highly variable, with variability being strongly related to substrate type and snow crab size. Catchability increased with crab size and was higher on soft than on hard substrates. Three substrate-specific catchability functions were defined; 1) catchability was highest (about 0.9), and least dependent on crab size, on the softest (mud) substrate within a relatively deep depth stratum; 2) catchability was lower (about 0.5-0.7) and linearly related to crab size throughout most of the deeper stratum on a soft (sand/mud) substrate; 3) catchability was lowest (about 0-0.5) and most strongly related to crab size on a variety of hard substrates within a relatively shallow depth stratum. We recommend that further experiments of this type be conducted to fully elaborate the relationship of trawl efficiency with crab size and substrate type. This would provide a basis for standardizing survey catches for catchability effects and refining survey-based estimates of biomass and exploitation rates of snow crab.

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