Modelling gear and fishers size selection for escapees, discards, and landings: A case study in Mediterranean trawl fisheries

Gear selectivity and discards are important issues related to fisheries management but separately modelled. This work examines for the first time the overall size-selection pattern on the total amount of individuals of a species entering the trawl codend. An innovative approach was used based on modelling the escapement through the codend in the sea and the subsequently selection process by the fisher on the deck of the fishing vessel resulting into the discards and landings. Three different trawl codends and three species were investigated in the case study conducted. A dual sequential model accounting for both gear size-selectivity and the subsequent fisher-size-selectivity was applied, under the hypothesis that a fish entering the codend can follow a multinomial distribution with three probabilities, the escape, the discard and the landing probability, respectively. The model described the escape probability through the gear and the landing probability by the fisher as S-shaped curves leading to a bell-shaped curve for the discard probability affected by both gear and fisher selection. The model described well the experimental data in all cases. Sampling scheme of three compartments proved adequate. The model provides at the same time selectivity and discards parameters useful in fisheries management.