Study

Size selectivity for four fish species of the deep groundfish assemblage off the Portuguese southwest coast: evidence of mesh size, mesh configuration and cod end catch effects

  • Published source details Campos A., Fonseca P. & Henriques V. (2003) Size selectivity for four fish species of the deep groundfish assemblage off the Portuguese southwest coast: evidence of mesh size, mesh configuration and cod end catch effects. Fisheries Research, 63, 213-233

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, controlled study in 1992 in a fished area of seabed in the Atlantic Ocean off the southwest coast of Portugal (Campos et al. 2003) reported that changing the configuration of mesh in a bottom trawl net to square from diamond resulted in lower retention and improved size selection of three of four commercial fish species. Across all hauls, the square mesh codend released more smaller and/or undersized individuals of European hake Merluccius merluccius, blue whiting Micromesistius poutassou and horse mackerel Trachurus trachurus than diamond mesh codends, but it was similar for four spot megrim Lepidorhombus boscii (data presented as size frequencies and selectivity curves). The length at which fish have a 50% chance of escape was higher in square mesh for hake (square: 25 cm, diamond: 17–19 cm). Almost all blue whiting escaped from the square mesh codend and all horse mackerel were retained in diamond mesh codends (compared to 40% escape in square mesh) meaning estimates of selectivity could not be calculated. Catch comparison surveys were done by a research vessel in August 1992 using a square mesh codend of 65 mm mesh size (10 hauls) and diamond mesh codends of mesh sizes of 65 mm (13 hauls), 70 mm (18 hauls) and 80 mm (19 hauls). Deployments were of 1 h, at 3.5 kn and in depths of 200–400 m. Covers fitted over each of the codends sampled fish escaping through the meshes. Codend and cover catches were weighed. All total lengths of hake and megrim were measured, and mackerel and whiting lengths sub-sampled.

  2. Use a larger mesh size

    A replicated study in 1992 of bottom fishing grounds in the Atlantic Ocean off Portugal (Campos et al. 2003) found that trawl nets of larger codend mesh size had improved size-selectivity for fish compared to smaller mesh sizes. The length at which fish had a 50% chance of escape was greater for the larger mesh sizes compared to the smallest, for hake Merluccius merluccius (80 mm: 19 cm, 70 mm: 19 cm, 65 mm: 17 cm), blue whiting Micromesistius poutassou (80 mm: 25 cm, 70 mm: 25 cm, 65 mm: 23 cm) and four spot megrim Lepidorhombus boscii (80 mm: 21 cm, 70 mm: 18 cm, 65 mm: 17 cm). A total of 50 deployments of three different diamond mesh codends (80 mm, 70 mm and 65 mm; 13–19 hauls/mesh size) were carried by a research vessel in August 1992. Trawl nets were towed at 3.5 knots for 60 minutes. All fish escaping through the meshes of the codends were collected by a cover fitted over the trawl. All fish were weighed. The lengths of all hake and megrim were measured, and mackerel and whiting lengths subsampled.

Output references

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