Diamondvs.square mesh codend in a multi-species trawl fishery of the western Mediterranean: effects on catch composition, yield, size selectivity and discards

Abstract

Selectivity studies usually describe the effects on target species, whereas information on by-catch and discards is scarce. Nevertheless, large quantities of undersized individuals and invertebrates are discarded in the Mediterranean multi-species bottom trawl fishery. The present work analyses the data from two surveys carried out on the shallow and deep continental shelf (50-78 m, and 147-189 m, respectively) off the Balearic Islands (western Mediterranean). In these surveys, the traditionally used 40 mm diamond mesh codend and an experimental square mesh codend were used under commercial conditions. Catch composition, yields, size selectivity of both target and by-catch species, and discards were compared between the two mesh types. The mean selection length (L 50 ) clearly increased for most species when using square mesh, escaping many more individuals under their minimum landing size. Yield of Spicara smaris was significantly lower by using the square mesh, changing the composition of the retained catch. Escapement ratio and economic loss were significantly higher with square mesh, although economic loss was almost negligible for both meshes on the deep continental shelf. The use of square mesh significantly reduced the discards of algae in the shallow waters and fish on the deep continental shelf. The results confirmed that square mesh codend reduces the fishing pressure on small specimens as well as the impact of trawling on the ecosystem. These benefits would not lead to a reduction of the yields neither of the main target species, the fishes Merluccius merluccius, Mullus surmuletus, Zeus faber, and the cephalopods Loligo vulgaris and Octopus vulgaris, nor of the rest of commercial categories, except for Spicara smaris.

Performance of square-mesh codends in reducing discards and by-catch in the Shark Bay scallop fishery

“…Given that the focus of the Queensland mesh trials was to reduce by-catch by using square mesh, alternative square-mesh sizes were not tested; however, our study showed that largersized square-mesh codends than those tested in Queensland have the potential to concurrently reduce by-catch and smallsize scallops, with a risk of a small loss of commercial-sized scallops. These results are consistent with selectivity studies in other trawl fisheries where square-mesh codends have demonstrated their superior performance to diamond-mesh codends in reducing by-catch and discard rates, particularly in prawn and some fish trawl fisheries (Broadhurst 2000;Ordines et al 2006). Demersal trawl-gear selectivity studies for scallops are limited because most other scallop fisheries operate dredging gear.…”

Harvest Strategies for an Ecosystem Approach to Fisheries Management in Western Mediterranean Demersal Fisheries

“…Whereas, data from the BTF was available from previous studies (e.g., Ordines et al, 2006;Palmer et al, 2009;Quetglas et al, 2013), fishing tactics (a combination of target species, gear, and fishing location at a given time of the year; Pelletier and Ferraris, 2000), and fishery statistics from the SSF were determined within Myfish (for methodology, see Quetglas et al, 2016).…”

“…In the study area, commercial trawlers use up to four different fishing tactics (Palmer et al, 2009), which are associated with the shallow and deep continental shelf, and the upper and middle continental slope Ordines et al, 2006). Vessels mainly target striped red mullet (Mullus surmuletus) and European hake (Merluccius merluccius) on the shallow and deep shelf respectively.…”

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“…Improving selectivity, in particular reducing catches of the smallest individuals, results in medium to long term higher yields per unit of fishing effort and in greater catch values (Broadhurst et al, 1996;Fonseca et al, 2005a,b). However, economic performance improvements may not be so straight forward in multi-species fisheries, such as the Mediterranean fisheries (Ordines et al, 2006;Tosunoğlu et al, 2009).…”