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

Abstract

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.

How Can We Reduce the Overexploitation of the Mediterranean Resources?

“…Since undersized fishes of many large-sized species, such as M. merluccius, are highly vulnerable to the minimum mesh size enforced in the Mediterranean (40 mm square or 50 mm diamond) (Brčić et al, 2018;Mytilineou et al, 2018) and a further increase of the mesh size would lose shrimps, cephalopods, and medium-sized fish, the exploitation pattern can be enhanced through: (i) increasing the trawl net selectivity by adopting grids and separators that allow the undersized fish to escape (Coll et al, 2008;Massutí et al, 2009;Aydın and Tosunoglu, 2011;Vitale et al, 2018b); (ii) delaying the size/age of the first capture of juveniles through spatial and/or temporal closures to fisheries when and where the juveniles aggregate in order to improve the fraction of fish reaching sexual maturity (Caddy, 1999(Caddy, , 2009Fiorentino et al, 2003;Garofalo et al, 2011;Colloca et al, 2015;Despoti et al, 2020;Mytilineou et al, 2020;Milisenda et al, 2021); or (iii) a combination of the two approaches.…”

Marine spatial closures as a supplementary tool to reduce discards in bottom trawl fisheries: Examples from southern European waters

“…Improving gear selectivity is the primary measure to avoid unwanted catch (e.g., Bahamon et al, 2006;Ordines et al, 2006;Sardà et al, 2004). However, in multispecies fisheries like the Mediterranean bottom trawl fishery defining an optimum mesh size to fit all species can be quite challenging and even in some cases rather unrealistic (Bahamon et al, 2006;Mytilineou et al, 2018;Sala et al, 2008;Suuronen and Sarda, 2007). In addition, given that the ultimate goal of the LO is to promote fishers to adopt more selective fishing practices by avoiding areas or seasons associated with high quantities of unwanted catch, the spatial and temporal nature of discards attracts much attention.…”

“Crash landing” obligation for Mediterranean mixed fisheries: Evaluation of management strategies using bioeconomic modelling in the Aegean Sea

“…Another aspect that could be investigated is a selectivity scenario using the mesh panels or grids that have been successfully applied elsewhere. Mytilineou et al (2018) completed research on the selectivity of trawl fisheries in the Aegean Sea using different codend mesh sizes under a national research program, showing the importance of the 40 mm square and 50 mm diamond mesh currently used by the Greek fleet in comparison with the previous gear (40 mm diamond), which had fewer escapees and more discards, but none of them achieved a value of L 50 close to the MLS. The willingness to pay scenario was based on a survey held in two other European countries, Spain and Italy (Onofri et al 2018), where consumers assessed the value of the reserve price for a more sustainable fishery, but the consumption patterns and consumer choices may be very different for the Greek case.…”