Escape, discard, and landing probability in multispecies Mediterranean bottom-trawl fishery

Abstract

Commercial bycatch species and their exploitation pattern in the Mediterranean trawl fishery are little studied. The present work examines the overall size-selection pattern, both regarding the trawl in the sea and the fisher onboard the vessel, for seven commercial bycatch species using different codends. The applied selection model predicted the escape, discard, and landing probability for each species simultaneously, a useful method for providing information important for fisheries management under the ecosystem approach. Among the studied codends, the 40-mm diamond mesh codend, still in use in non-EU Mediterranean fleets, was found unsuitable for the stocks in all cases. The 40-mm square mesh codend (40S) was found appropriate for blue whiting (Micromesistius poutassou), comber (Serranus cabrilla), and bogue (Boops boops) sustainability. The 50-mm diamond mesh codend (50D) was more suitable than the square mesh codend only for the spotted flounder (Citharus linguatula). Both the 40S and the 50D codends were appropriate for picarel (Spicara smaris), whereas none ensured sustainable exploitation for the blackbelly rosefish (Helicolenus dactylopterus), and the streaked gurnard (Chelidonichthys lastoviza). The results are discussed in relation to juvenile protection, discard mitigation, and fisher selection behaviour, important factors for the sustainability of stocks and fisheries in the Common Fishery Policy for the Mediterranean Sea.

Impacts on biodiversity from codend and fisher selection in bottom trawl fishing

“…Thus, it seems that the 40S codend produced a retained catch with less discards and more legal landings, as has also been shown for some species in the published literature (e.g. Mytilineou et al, 2018a, Mytilineou et al, 2021aMytilineou et al, 2021b), and which is more in line with the current regulations on discard mitigation (Regulation (EU) No 1380/ 2013), and the legislated minimum reference conservation sizes of some species (Regulation (EU) 2019/1241).…”

“…Tsagarakis et al (2008), investigating data from a 10-year period, found a decreasing trend in the species richness and other diversity indices for the discarded fraction, but not for the whole retained catch, which may be explained by the shortness of the time-period. In the present study, it was also surprising that the Shannon-Wiener index in the escapees of the 40S and 50D, the two more selective codends (Mytilineou et al, 2018a;Mytilineou et al, 2021a;Mytilineou et al, 2021b), presented significantly lower values than the total assemblage entering, which was not the case for the less selective 40D codend. This could be explained by the fact that more individuals of some species can escape through the 40S and 50D codends, which may reduce their entropy.…”

“…This indicated that small-sized species benefit against the large-sized species. Furthermore, it is already known that trawl fishing retains the largest individuals of the species entering the codend as a result of the size-selection of this gear (Wileman et al, 1996), which has also been shown in the area by several selectivity studies of Mytilineou et al (2018a); Mytilineou et al (2021a); Mytilineou et al (2021b). This constant escape of small-sized species and small-sized individuals of some larger species, along with the simultaneous removal of the large-sized species and individuals, which are potential predators of the small-sized ones, firstly may support the dominance of the latter, changing community species composition and functionality, and secondly, may produce increased food requirements for them, which, if not available, may result in a ecological cascade (Coll et al, 2008b).…”

“…Map of the study area indicating the hauls of the experimental fishing. Mytilineou et al 10.3389/fmars.2022.1021467 Sampling was based on the three-fractions design including escapees, discards, and landings as described in Mytilineou et al (2018a); Mytilineou et al (2021a). In each fraction, all specimens were identified to the species level or the lowest possible taxonomic level (taxon namely as species hereafter), counted, and weighed.…”

Fishery dynamics of blue whiting, Micromesistius poutassou, a highly discarded bycatch species in the NW Mediterranean Sea

“…In the Mediterranean, blue whiting is not a bottom trawl target species but a bycatch of the Norway lobster fishery, and it does not generate high income (Martín et al 2014). Most bycatch species are unregulated and are not assessed (Mytilineou et al 2021). No minimum conservation reference size has been defined for blue whiting (EU 2019b) although on the Spanish Mediterranean coast a minimum landing size has been established at 15 cm total length (TL) (BOE 2006).…”