A comparison of size selection of Atlantic cod ( Gadus morhua) and haddock ( Melanogrammus aeglefinus) by bottom longlines and otter trawls

Abstract

The size selection of Atlantic cod and haddock by bottom longlines was described in an experiment conducted off Nova Scotia, Canada, by comparing their catches at length with those from an otter trawl with a small mesh codend fished concurrently. Three longline configurations were tested that differed primarily in the size of circle hooks and baits used. Bait size was increased with increasing hook size. Size selection of diamond and square mesh otter trawl codends of 130 mm mesh size were tested in the same experiment. The size selection curves obtained for cod taken on longlines were asymptotic and selection parameters were calculated by fitting logistic curves to the data. For haddock, only the lower end of the selection curves could be described. Larger hooks/baits were less efficient at catching smaller cod, but more efficient at catching larger cod, than were smaller hooks/baits. The smallest hook/bait combination had a length at 50% selection for cod below that of the 130 mm diamond mesh trawl codend, the minimum mesh size allowed at the time of the experiment. This result was counter to expectations based on the observed length compositions of commercial landings. The largest hook/bait combination had a 50% selection above that of the 130 mm square mesh. Although cod up to 126 cm were caught during this experiment, few were larger than 90 cm, leaving unresolved the question of whether the selection curves described are applicable to the largest cod.