A modelling approach for assessing short-term catch losses as a consequence of a mesh size increase

Short-term effects of an increase in mesh size of a square mesh window codend were modelled with a stochastic size-selective simulation model using the 1999 fisheries data from the southern Baltic Sea (SD 25). Vessel-type-dependent selectivity estimates and catch per unit of effort (CPUE) data from Baltic cod ( Gadus morhua ) demersal trawl fishery were utilized to estimate the catch losses. When the window mesh size is increased by 15 mm (from 105 to 120 mm), overall catch loss of marketable size of fish during the first month would be 36–49%. Due to the marked effect of vessel type on overall codend selectivity, the decrease in marketable catch for side trawlers would be 42–60% and for stern trawlers 31–45%. With a 38-cm minimum landing size, discarding would decrease by 66–71%. In case fishermen would like to compensate the loss in the marketable catch by increasing their fishing effort, they would have to increase the effort by 55–90%. A 62–123% increase in fishing effort would be required to compensate the decrease in the marketable catch if both the window mesh size (from 105 to 120 mm) and the minimum landing size (from 38 to 40 cm) are increased. Fishermen would likely not be able to increase effort to such an extent. Instead, they might try to circumvent the regulations by intentionally decreasing the selectivity of their gear, i.e. by gear manipulation.