Best practices II: Effect on future development of sole and plaice of changing mesh size from 80mm to 90mm in the beam trawl fishery

This study investigates the consequence for future development of stock size, catches, landings and discards of sole and plaice of changing the mesh size of the cod-end from 80mm to 90mm for the Dutch beam trawlers in theTBB 70-99 fleet currently fishing with 80 mm.This study investigates the consequence for future development of stock size, catches, landings and discards of sole and plaice of changing the mesh size of the cod-end from 80mm to 90mm for the Dutch beam trawlers in theTBB 70-99 fleet currently fishing with 80 mm. The question is addressed by means of long term stochastic simulations. Using the simulation framework developed to test the effect of implementing the landing obligation, the future fishery selection pattern (how the fishing mortality is distributed across ages) is modified based on the results of the selectivity experiment to represent the consequence of changing mesh size. Simulations were then run for the next 50 years for different assumptions on the survival rate for both stocks: a 0% survival rate, and the lower and upper bounds of the current estimates of survival for each species. The differences in the effect on sole and plaice of using a 90mm net are related to both the direct effect of exploiting the stock with a different selection pattern and of applying different Fmsy values. The effects of changing mesh size are larger for sole than for plaice, because the share of the landings taken by the Dutch beam trawlers currently fishing with 80 mm is much larger for sole than for plaice. For sole, fishing with the 90mm net results in lower discards (10 to 16%). Landings are also lower (up to 4%) in the short term, but the situation reverses and landings become higher in the medium and long term (up to 3% after 5 years). These results are explained by the fact that when the 90mm net is used, the cohorts are exploited at a slightly later age combined with a stronger targeting of the older ages. This exploitation patterns leads in the medium and long term to a larger stock (by 3 to 13%), which explains the higher landings. Those benefits (in the medium and long term) of using the 90mm net are largest for the 0% and 10% survival assumptions, but are smaller (especially for the landings) for the assumption with 30% survival: the higher the chance for a discarded fish to survive, the less it pays to increase the selectivity of the gear because fish caught and discarded have still a chance to join the stock and further grow and reproduce. For plaice, in the scenarios with 0% and 10% survival, the Fmsy value for the 90mm net is higher than for the 80mm net. As a result, stock size is lower and catches, landings and (despite the improved selectivity of the net) discards are higher if the 90mm net is used. For the scenario with 20% survival rate, Fmsy values are similar for the 80mm and 90mm mesh size and the improved selectivity of the 90mm net indeed results in slightly lower discards, which in the medium and long term result in a slightly larger stock with slightly higher landings. One important assumption in these simulations is that the stocks are exploited at Fmsy in the future. However, if the beam trawl fleet switches to the 90mm net, its catchability (at least for sole) will decrease, meaning that a higher fishing effort will be necessary to achieve a same fishing mortality on the stock. The present study does not model explicitly catchability and effort, and therefore cannot quantify the change in effort implied if the stocks were to be exploited at Fmsy with the 90mm net.