1National Research Institute of Fisheries Engineering, Fisheries Research Agency, Hasaki, Ibaraki 314-0421, 2Tohoku National Fisheries Research Institute, Fisheries Research Agency, Hachinohe, Aomori, 031-0841, Japan
The stock abundance of snow crabs (Chionoecetes opilio and C. japonicus) has been estimated from the data of trawl surveys. However, the net efficiencies (the proportion of the number of captured crabs to crabs in the swept area calculated as the towing distance multiplied by the trawl net width between the wing-tips) of survey trawls have not been sufficiently measured. To estimate accurately the population density of snow crabs from survey trawls, net efficiency was estimated in this study. The surveys were carried out in the Pacific coastal water of northern Japan at depths ranging between 400 m and 700 m in June 2000. The population densities of the snow crabs were estimated by visual strip transects using the deep-sea video monitoring system on a towed sledge prior to trawling over the same ground. Numbers of the two crabs were combined because it was very difficult to distinguish between them from the video images. Net efficiency was estimated using regression analysis based on the relationship of the catch number per trawl swept area with the population density obtained from the video observation. Net efficiency was estimated to be 0.30 and its ninety-five percent confidence interval was between 0.23 and 0.37.