Improving gear selectivity of whiting (Merlangius merlangus) on board French demersal trawlers in the English Channel and North Sea

Abstract

In the English Channel and Southern Bight, French demersal trawlers target a mix of demersal species including whiting (Merlangius merlangus). The discard ban to be implemented in January 2016 motivated fishermen to look for technical measures to reduce unwanted catches of undersized individuals (<27cm). From 2008–2014, 18 different gear configurations were tested for bycatch reduction on board fleets of vessels of 16–20m and 20–24m. Selective devices tested included grids, square mesh panels (SMP), square mesh cylinders (SMC), various combinations of these devices and large mesh trawls. Using the catch comparison method, our results focus on six devices that proved efficient to reduce catches of undersized whiting, equally divided between the two vessel size groups. Large mesh trawls are not adequate to improve whiting size selectivity with losses of commercial size fish. The mandatory 80-mm SMP was efficient at letting undersized whiting escape from the trawl for both vessel length classes. Its efficiency was enhanced when it was positioned 6–9m ahead of the codline compared with that placed at 12–15m. The most appropriate bar spacing for whiting size-selectivity was found in a flexigrid with 23mmbar spacing, but this required a large portion of square mesh netting to be installed ahead of it (SMP and SMC) to limit catches of undersized individuals. The most promising device tested was an 80mm SMC, either 2m long and used alone or 1m long and used in association with the mandatory SMP; these setups allowed significant escapes of fish up to 25cm in length. For both SMP and SMC, a mesh size equal to or larger than 100mm led to losses of commercial size whiting.