Study

Cod-end mesh size selection for rockfish and flatfish of the US West Coast

  • Published source details Perez-Comas J.A., Erickson D.L. & Pikitch E.K. (1998) Cod-end mesh size selection for rockfish and flatfish of the US West Coast. Fisheries Research, 34, 247-268.

Actions

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
Marine Fish Conservation

Use a larger mesh size

Action Link
Marine Fish Conservation
  1. Use a square mesh instead of a diamond mesh codend in a trawl net

    A replicated, randomized, paired, controlled study in 1988–1990 in fishing grounds in the Pacific Ocean off the west coast USA (Perez-Comas et al. 1998) found that bottom trawls fitted with a square mesh codend did not typically improve the selectivity or reduce the catch of unwanted small, rockfish and flatfish species compared to diamond mesh codends, but increasing the mesh size did, for both designs. The length at which half of fish were likely to escape capture was higher in square mesh codends than diamond mesh codends of the same mesh size for two of five rockfish but none of four flatfish (see paper for individual data – not statistically tested). Increasing the mesh sizes retained fewer undersized fish in both codend types: for four of four rockfish and three of four flatfish in square mesh increased from 114 mm to 127 mm; and for five of five rockfish and three of four flatfish in diamond mesh increased from 114 mm to 127–140 mm (data presented as selectivity curves). In 1988–1990 the West Coast Groundfish Mesh Size survey tested experimental diamond mesh codends with mesh sizes of 76 (chosen as the ‘standard’ for analysis), 114, 127 and 140 mm and square mesh codends of 114 mm and 127 mm. Codends were towed in randomized blocks of two or three codends at a time during each fishing season by commercial trawling vessels, totalling 493 deployments.

    (Summarised by: Leo Clarke)

  2. Use a larger mesh size

    A replicated, randomized, controlled study in 1988–1990 of bottom fishing grounds in the Pacific Ocean, USA (Perez-Comas et al. 1998) found that increasing the mesh size in trawl net codends improved the size-selectivity of rockfishes Sebastidae and flatfishes Pleuronectidae and Bothidae, for both diamond and square mesh. For five rockfish species, the lengths at which fish had a 50% chance of escape increased with increasing mesh size for both diamond (140 mm: 42–46 cm ,127 mm: 38–41 cm, 114 mm: 30–37 cm) and square meshes (127 mm: 34–44 cm, 114 mm: 29–42), compared to the standard 114 mm mesh size. For five flatfish species, the lengths at which fish had a 50% chance of escape also increased with increasing mesh size (diamond, 140 mm: 35–43 cm, 127 mm: 30–41 cm, 114 mm: 28–37 cm; square, 127 mm: 31–37 cm, 114 mm: 25–32). See original paper for species specific data. Decreases in the estimated percentages of discarded fish with increasing mesh size were found for eight of the ten species, but these differed between diamond and square meshes (data not statistically tested). Data were collected in 1988–1990 by the West Coast Groundfish Mesh Size survey. Experimental diamond mesh codends with mesh sizes of 140 mm, 127 mm, 114 mm, and 76 mm (the standard to sample all lengths of fish), and square mesh codends of 114 mm and 127 mm were tested. Codends were deployed in randomized blocks of two or three codends, together with the standard, during each fishing season by commercial trawling vessels.

    (Summarised by: Leo Clarke)

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