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

Abstract

Fisheries have important impacts on marine biodiversity. In this work, combined information on the abundance, species richness, diversity indices, species composition, trophic level and vulnerability index were examined for the first-time to detect differences in five units related to trawl fishing: the fish assemblage entering the trawl codend, and the escaping, retained, discarded and landed fractions, derived by the gear and fisher selection practices. The work was based on a case study conducted in the Mediterranean Sea, using three different meshes in the trawl codend (40mm-40D and 50mm-50D diamond meshes, and 40 mm-40S square meshes) and a cover of the codends with small mesh size. In general, trawl fishing produces an escaping fraction that was always lower in abundance, richness, and vulnerability index, similar in diversity indices and trophic level, and different in species composition compared to the fish assemblage entering the codend. In almost all cases, fishers selected as landings a fraction that was the lowest in diversity indices, and the highest in trophic level. In contrast, fishers discarded a fraction that was the highest in diversity and vulnerability index, and the lowest in trophic level. Although the three codends did not differ significantly in the fraction of escapees in terms of diversity indices, trophic level, and vulnerability index, the 40S codend showed a significantly higher percentage in the escaping number of species and individuals, and less differences in the species composition; in addition, lower percentage in abundance of discards and higher of landings in the retained catch (0.6:1) than did the other two codends (0.9:1). It was suggested that an urgent modification of the trawl for the elimination of the discarded highly vulnerable species (e.g. Elasmobranchs) is needed, and that trawl species-selectivity should be improved by allowing escape or avoiding catch of the discarded fraction to minimize biodiversity losses.

Every animal matters! Evaluating the selectivity of a Mediterranean bottom trawl fishery from a species community perspective

“…This percentage falls within the range (20–65%) reported by Tsagarakis et al [ 2 ] and gives a clear insight of the high impact exerted by this fishing gear on the benthic and benthopelagic community. The fate of the species entering the trawl codend is also quantified in the recent work of Mytilineou et al [ 13 ], who estimated that less than 30% of the species caught were selected to be marketed. Also, in the present study, there were more species entirely discarded (37 species) than the ones possibly landed (31 species).…”

“…These are often the most economically important species [ 11 ] or most vulnerable species requiring urgent management measures [ 12 ]. On the contrary, the multispecies nature of Mediterranean bottom trawl fisheries is rarely addressed, despite the growing need for ecosystem-based fishery management for conserving biodiversity [ 13 ]. However, the overall ecological impact of using a specific gear or technical modification cannot be determined if significantly important fractions of the catch are neglected, which may cause an underestimation of the unaccounted fishing mortality [ 14 ].…”

Age, growth and otolith morphometrics of Serranus hepatus (L., 1758) in two areas of the Eastern Mediterranean

“…Information on these topics is also necessary for discarded species, for which the existing excessive lack of adequate data may lead to inaccurate conclusions in stock assessment and management (FAO, 2020). No published work on the above-mentioned biological features of S. hepatus is known in the Eastern Ionian and southwestern Aegean Seas so far, where considerable abundance for this discarded species is known (Labropoulou, 2007; Mytilineou et al ., 2022).…”

Correlation between the Density of Acipenser sinensis and Its Environmental DNA

“…Traditional surveys such as bottom trawling and using fyke nets entail substantial human and material resource consumption [6,7] while also requiring taxonomic expertise, thereby posing a significant risk of ecosystem disturbance with respect to the riverbed. The publication of lists of banned fishing gear and the development of energy-efficient fishing gear have cut down on the widespread use of such survey methods and contributed to significant biodiversity improvements on the river bottom [8]. In addition, the tracking method of using a mark is complicated, costly, and damages fish; thus, the amount of tagging is limited [9].…”