Capture efficiency of a multi-species survey trawl for Snow Crab ( Chionoecetes opilio) in the Newfoundland region

Abstract

An experiment was carried out during September of 2001 to estimate the relative catching efficiency of the NAFC's (Northwest Atlantic Fisheries Centre) standard survey bottom trawl, the Campelen 1800 shrimp trawl, for Snow Crab ( Chionoecetes opilio). Secondary trawls were mounted underneath the main trawl directly behind the footgear to estimate escapement of crab passing underneath the main trawl. Overall, the capture efficiency of the survey trawl was found to be much lower than 1.0 and highly variable, with variability being strongly related to substrate type and Snow Crab size. The probability of capture monotonically increased with crab size, and was higher on soft than on hard substrates. Three substrate-specific capture efficiency functions were defined; (1) efficiency was highest (about 0.73–0.95), and least dependent on crab size, on the softest mud substrate; (2) efficiency was lower (about 0.39–0.70) and linearly related to crab size throughout a soft mud–sand substrate; and (3) efficiency was lowest (about 0.05–0.33) on a variety of hard substrates within relatively shallow water. Our models over-estimated true efficiency because secondary trawls were not fully efficient, i.e., crab escaped underneath their footgear. Nevertheless, our experiment demonstrated that substrate type can affect capture efficiency and annual changes in trawl configuration or in Snow Crab distribution in relation to substrate type will affect survey biomass estimates. Additional studies using various approaches could provide a basis for standardizing survey catches for varying capture efficiency effects and refining survey-based estimates of biomass and exploitation rates of Snow Crab.