Hachinohe Laboratory, Tohoku National Fisheries Research Institute, Fisheries Research Agency, Hachinohe, Aomori 031-0841, Japan
When estimating biomass by the area-swept method of bottom trawl surveys, it is important for fisheries managers to estimate the change in capturing efficiency of the net (Q) with the body size of a target species. In this study, Q depending upon the foot rope of a bottom-trawl net was examined for snow crab Chionoecetes opilio using an experimental trawl net (a normal trawl net equipped with an auxiliary net underneath) off the Pacific coast of northern Honshu, Japan. The relationship between carapace width (CW) and Q was expressed by a logistic equation using a generalized linear mixed model (GLMM) because Q increased with CW. The model without the effect of “sex” was selected as the best one by AIC (Akaike's Information Criterion). The maximum value of Q determined by the equation reached 0.664, suggesting that more than 50% of snow crab with CW of over 70 mm were caught by bottom trawl. The CW-Q relationship should be used for estimating the biomass of snow crab in the trawl survey, because the biomass of smaller/larger crab tends to be under-/over-estimated when a constant value of Q is used.