Study

Size selectivity of diamond and square mesh codends for four commercial Mediterranean fish species

  • Published source details Petrakis G. & Stergiou K.I. (1997) Size selectivity of diamond and square mesh codends for four commercial Mediterranean fish species. ICES Journal of Marine Science, 54, 13-23.

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, controlled study in 1993–1994 in two seabed areas in the Aegean Sea, Greece (Petrakis & Stergiou 1997, same experimental set-up as Stergiou et al. 1997 and Stergiou 1999) reported that a bottom trawl with a square mesh codend improved the size selectivity and escape rate of only one of four commercially important bottom fish species, compared to a diamond mesh codend of the same mesh size. Results were not statistically tested. The estimated length at which fish had a 50% chance of escape was greater in 20 mm square mesh codends than 20 mm diamond mesh codends for European hake Merluccius merluccius (15.1 vs 13.8 cm) and both were greater than the 14 mm diamond mesh (4.2 cm); but smaller for blue whiting Micromesistius poutassou (17.0 vs 21.2 cm), poor cod Trisopterus minutus capelanus (11.9 vs 13.7 cm) and four-spot megrim Lepidorhombus boscii (8.5 vs 10.3 cm) (selectivity for these species could not be estimated for the 14 mm diamond mesh codend). The proportion of fish retained in the trawl versus those that escaped was lower in square mesh codends for hake (square: 0.26, diamond: 0.35), but was higher for blue whiting (square: 0.61, diamond: 0.29), poor cod (square: 0.31, diamond: 0.18) and megrim (square: 0.90, diamond: 0.66). Experimental trawl deployments were conducted in the Trikeri Channel in October 1993 (5 stations) and the North Euboikos Gulf in March 1994 (seven stations). A trawl net was randomly assigned either a 20 mm square mesh codend or a 14 mm (the size currently used commercially) or 20 mm diamond mesh codend, and towed for 45–60 min at depths between 73–210 m. Each codend was deployed for 12 hauls. Small mesh (10 mm) covers over the codend sampled the escaping fish catch. Fish from the codend and cover were identified and counted, and their lengths measured.

    (Summarised by: Leo Clarke)

  2. Use a larger mesh size

    A replicated, randomized, controlled study in 1993–1994 in two areas of seabed in the Aegean Sea, Greece (Petrakis & Stergiou 1997, same experimental set-up as Stergiou et al. 1997) found that increasing the mesh size of diamond mesh trawl codends improved size-selectivity and reduced the undersized catches of European hake Merluccius merluccius compared to codends with conventional mesh size. The length at which hake had a 50% chance of escape was 13.8 cm for 20 mm diamond mesh and 4.2 cm for 14 mm diamond mesh codends. The percentage of undersized hake retained was lower in the larger diamond mesh (20 mm: 52%, 14 mm: 61%). In October 1993 and March 1994, experimental trawl deployments were conducted in two areas (Trikeri Channel and North Euboikos Gulf, twelve stations in total) using a trawl fitted with either a 20 mm diamond mesh codend or a conventional 14 mm diamond mesh codend used by the fishery (12 hauls of each at each sites). Codend type was randomly allocated and small mesh (10 mm) covers retained fish escaping through the meshes. For each deployment, the total number and weight caught by species in the codends and covers were recorded. Lengths of the main species were subsampled.

    (Summarised by: Leo Clarke)

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