The effect of smaller mesh sizes on catching larger fish with trawls

摘要

Selection curves of small mesh trawls were assessed with one alternate haul and two trouser trawl experiments. The effect of fish length on the ratio (pi) of the number of fish of length i caught in the large mesh cod-end to the number of fish of length i caught in both cod-ends was estimated with generalized linear modeling. The alternate haul experiment was conducted with beam trawls equipped with 2 and 18 mm stretched mesh cod-ends to assess size selectivity for perch Perea fluviatilis. None of the perch that entered the cod-end could escape through the mesh of the small mesh cod-end. For small perch (< ca. 10 cm total length), p(i) reflected the ratio in areas swept by the large and small mesh nets, but for larger perch, p(i) increased. Water flow through the small mesh net must have been lower than through the large mesh net, enabling relatively more larger perch (> ca. 10 cm total length) to out-swim the small mesh net. The trouser trawl experiments were conducted to study selectivity for smelt Osmerus eperlanus, using 12 and 22 mm stretched mesh cod-ends, and for American plaice Hippoglossoides platessoides, using 39 and 155 mm stretched mesh cod-ends (published data). During both trouser trawl experiments, smaller fish that entered the cod-end could escape through the large mesh cod-ends, so p(i) increased with increasing fish size. However, also beyond the size where fish could escape through the large mesh cod-end, p(i) continued to increase. This implied that the small mesh cod-end was less selective to larger fish. We demonstrated that the lower selectivity to larger fish in the cod-end with the smaller mesh could result in 20-50% overestimation of the length (L-50) at which 50% of the fish entering the cod-end escape through the mesh.