One study found that unwanted fish catch depended on codend mesh size as well as configuration (square or diamond). One study found that square mesh codends retained more fish overall than diamond mesh but varied for individual species by fish shape and size. Four studies found no reduction in catches of unwanted small rockfish and flatfish, three of four commercially important bottom fish species, total unwanted catch (fish and invertebrates), or the total number of unwanted species (fish and invertebrates), compared to diamond mesh codends. Reduction of unwanted catch (16 studies): Ten of 16 replicated, controlled studies (including five paired, three randomized and three randomized and paired) in the Greenland Sea, Aegean Sea, Atlantic Ocean, Tasman Sea, Pacific Ocean , Mediterranean Sea, English Channel, Bristol Channel and Coral Sea, found that square mesh codends reduced the unwanted (non-target or non-marketable/discarded) catches of all fish species monitored, young individuals of half or most commercially targeted fish, total unwanted catch (fish and invertebrates), and discarded fish in deeper but not shallower fishing areas, compared to diamond mesh codends and two of those studies also found that there was a variable effect on unwanted catch between individual fish species/groups.The other study reported that skate caught in a square mesh codend had a higher overall survival likelihood than those caught in a diamond mesh codend. Survival (2 studies): One of two replicated, paired, controlled studies in the Aegean Sea and Bristol Channel found that the short-term survival of two of six fish species was higher after escaping through a square mesh compared to a diamond mesh codend.Five studies were in the North Atlantic Ocean (Canada, Portugal, USA), four were in the Aegean Sea (Greece, Turkey), three were in the Mediterranean Sea (Spain) and the Tasman Sea (Australia), two studies were in each of the English Channel (UK), the Adriatic Sea (Italy) and the South Pacific Ocean (Australia, Chile), and one study was in each of the Greenland Sea (Iceland), the North Pacific Ocean (USA), the Bristol Channel (UK), the Kattegat and the Skagerrak (Denmark) and the Coral Sea (Australia). Twenty-six studies examined the effects of using a square mesh instead of a diamond mesh codend in a trawl net on marine fish populations.(2008) Size selection by diamond- and square-mesh codends in multi-species Mediterranean demersal trawl fisheries. (1986) An analysis of length selection data from comparative fishing experiments on haddock and whiting with square and diamond mesh codends. (1986) Selectivity experiments with square mesh codends in bottom trawls. Interventions describing the use of sections of square mesh (or large diamond mesh) inserted into diamond mesh nets, see ‘ Fishing gear modification - Fit mesh escape panels/windows to a trawl net’ and ‘Modify the configuration of a mesh escape panel/window in a trawl net’. Evidence of this intervention when used in combination with other interventions to reduce unwanted catch in trawl nets is summarized under ‘ Fishing gear modification - Fit mesh escape panels/windows to a trawl net and use square mesh instead of diamond mesh codend’ and ‘Fit a size-sorting escape grid (rigid or flexible) to trawl nets and use a square mesh instead of a diamond mesh codend’. For interventions describing the effects of different mesh types in fishing gear more generally, but including trawl nets, see ‘Fishing gear modification - Use a larger mesh size’. Square mesh trawl codends instead of diamond mesh codends may therefore increase the escape of small fish from trawl nets and reduce fishing mortality.Įvidence for a similar intervention applied to trawl nets only is summarized under ‘Fishing gear modification – ‘Rotate the orientation of diamond mesh in a trawl net’. Square shaped mesh however, retains its shape better under strain and maintains a larger size of the gaps in the meshes, potentially allowing greater escape opportunities for smaller fish and invertebrates (Robertson & Stewart, 1986). It is difficult for many fish to escape from diamond mesh trawl codends because the mesh openings close under tension during fishing deployments (Isaksen & Valdemarsen (1986). However, many of the fish species or individuals are often small, immature, or unmarketable, and will not yet have had the chance to spawn and even if returned to the sea, might not survive (Sala et al. Commercial fishing trawlers conventionally use a net constructed of diamond shaped mesh due to its ability to capture and retain a wide range of sizes and species (Sala et al.
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