Spatial variation in abundance and catch composition of Cancer pagurus in Norwegian waters: biological reasoning and implications for assessment

Abstract

Abstract The edible crab (Cancer pagurus), which is abundant along the Norwegian coast north to Troms County, has been exploited in Norway since the start of the 20th century. The main fishery is in Mid-Norway and Helgeland (63°–67°N), which together land 75% of the Norwegian catch. The fishery is regulated by season, minimum legal size, and the discarding of soft or ovigerous females. Catches have never been systematically described, so this study presents data collected over 4 years by a reference fleet of professional fishers reporting the catch from four standardized trial traps set among their ordinary traps. Catch rate, catch composition, and size distribution from Mid-Norway/Helgeland are compared with the smaller crab fishery in Rogaland (59°N) and with new crab grounds off Vesterålen (69°N). Local variations in size and sex composition between and within region seemed partly to be caused by differences between exposed and sheltered regions, and between heavily and newly exploited grounds. Size distribution is used as an indicator to determine a suitable sampling strategy. The most efficient survey design is seemingly the collection of relatively small samples from many boats to reduce the between-vessel component of variation. A suggested strategy to obtain an abundance index would be to collect daily catch rates from all commercial landings.