Discard mitigation increases skate survival in the Bristol Channel

Published source details Enever R., Revill A.S., Caslake R. & Grant A. (2010) Discard mitigation increases skate survival in the Bristol Channel. Fisheries Research, 102, 9-15.
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This study is summarised as evidence for the following.

	Action	Category
	Use a square mesh instead of a diamond mesh codend in a trawl net Action Link
	Use a larger mesh size Action Link

Use a square mesh instead of a diamond mesh codend in a trawl net

A replicated, paired, controlled study in 2009 in bottom fishing grounds in the Bristol Channel, UK (Enever et al. 2010) reported that bottom trawl nets fitted with a square mesh codend caught fewer discarded fish compared to a standard diamond mesh codend, and the overall survival likelihood of skate Rajidae spp. post-capture was improved. Data were not statistically tested. Overall numbers of discarded fish in the 100 mm square mesh codend were 68% lower than the 80 mm diamond mesh codend (square: 2,241 fish, diamond: 7,056 fish), and ranged between 25% to 100% for individual species/groups (see paper for data). The proportion of skate given a good initial ‘health’ score after capture (equal to 86% chance of survival) as a proxy for survival likelihood) was 47% in the square mesh codend and 25% in the diamond mesh codend. Catch data was collected in June/July 2009 on a commercial twin-rigged bottom trawler at 35–65 m depth. Sixteen paired trawl deployments (3–5 kn) were done with an experimental 100 mm square mesh codend towed simultaneously with a conventional 80 mm diamond mesh codend. Separate assessment of the post-capture visual condition and survival of 278 small-eyed skate Raja microocellata was used to determine a three-point ‘health’ scale as an indicator of survival. The scale was used to assess the health of individuals of five skate species (see paper for details) as the nets came aboard, 358 skate from the square mesh and 754 from the diamond mesh codend.

(Summarised by: Leo Clarke)
Use a larger mesh size

A replicated, paired, controlled study in 2009 in an area of fished seabed in the Bristol Channel, UK (Enever et al. 2010) reported that bottom trawls fitted with a larger mesh size, both diamond and square mesh shapes, typically had lower catches of discarded fish and improved overall survival and condition of skates and rays Batoidea post-capture, compared with a conventional 80 mm diamond mesh codend. Results were not tested for statistical significance. For diamond mesh, a larger mesh size had lower discarded catch numbers of five of seven fish species/groups compared to the standard size (large: 13–1,544 fish, standard: 109–5,371 fish) and two were higher (large: 11–313 fish, standard: 9–215 fish). For a square mesh of the same larger size, the discarded catch of all seven species/groups was reduced compared to the standard diamond mesh (large: 0–2,091 fish, standard: 3–5,082 fish). See original paper for species individual data. For 1,539 skates/rays caught, the proportion assessed as in good health (associated with improved post-release survival) was 47% and 34% in the large square and diamond meshes respectively, and 25% in the 80 mm diamond mesh. Post-capture survival (after 48 h) of 278 small-eyed skate Raja microocellata ranged from 59–67% in the larger meshes and 55–57% in the standard mesh size. Data were collected from 32 paired trawl deployments in the Bristol Channel in June–July 2009. Trawls fitted with either an experimental 100 mm diamond or 100 mm square mesh codend were towed simultaneously with a conventional 80 mm diamond mesh codends (16 hauls each). Small-eyed skate from each codend were assigned health scored and monitored for 48 h.

(Summarised by: Leo Clarke)