Evaluation of codends with sorting grids, exit windows, and diamond meshes: Size selection and fish behaviour

摘要

This paper presents the results of an experiment aimed at improving the size selection of cod (Gadus morhua) and haddock (Melanogrammus aeglefinus) in the Nordic bottom trawl fishery. Three systems simulating commercial conditions were tested: a 135mm diamond-mesh codend fitted with a 55mm sorting grid (Sort-V); a 135mm diamond-mesh codend fitted with two lateral exit windows; and a codend built entirely of 155mm diamond mesh. The selectivity curves showed similar selection ranges (SR) for the three systems. For cod, the mean SRs were around 8cm, while for haddock, they were around 6cm. All the estimated ld5d0 values were far above the minimum landing sizes (MLS). For cod, the mean ld5d0s were 56.1, 53.9, and 60.7cm for the sorting grid, exit windows, and diamond-mesh configurations, respectively, while for haddock, they were 50.2, 50.6, and 49.9cm, respectively. Underwater observations revealed that most of the fish escaped from the grid and exit-window codends as soon as they reached the vicinity of the sorting device. In contrast, fish remained inside the diamond-mesh codend for a longer time, and consequently were more exposed to physical damage before escape. In addition, many fish escaped from this codend during haul back and these fish were not likely to survive due to the rapid changes in pressure. Thus, the selectivity results obtained with this type of codend should be interpreted carefully.