Study

The effects of mesh size, cod-end extension length and cod-end diameter on the selectivity of Scottish trawls and seines

  • Published source details Reeves S.A., Armstrong D.W., Fryer R.J. & Coull K.A. (1992) The effects of mesh size, cod-end extension length and cod-end diameter on the selectivity of Scottish trawls and seines. ICES Journal of Marine Science, 49, 279-288.

Actions

This study is summarised as evidence for the following.

Action Category

Decrease the circumference or diameter of the codend of a trawl net

Action Link
Marine Fish Conservation

Modify the design or configuration of trawl gear (mixed measures)

Action Link
Marine Fish Conservation

Use a larger mesh size

Action Link
Marine Fish Conservation
  1. Decrease the circumference or diameter of the codend of a trawl net

    A replicated, controlled study in 1986–1988 on bottom fishing grounds in the northern and central North Sea, UK (Reeves et al. 1992) found that fishing nets (trawls and seines) with a narrower codend diameter improved the size-selectivity of haddock Melanogrammus aeglefinus, cod Gadus morhua and whiting Merlangius merlangus compared to standard and wide diameter codends. Across three codend mesh sizes and lengths of the extension piece, the length at which fish had a 50% chance of escape was greater for narrow (2.2 m) diameter codends for haddock (18–32 cm), cod (20–36 cm) and whiting (21–39 cm), compared to standard (3.2 m) codend diameters (haddock: 14–28 cm, whiting: 18–35 cm, cod: 16–32 cm) and wide (4 m) codend diameters (haddock: 11–25, whiting: 15–28, cod: 12–28 cm). Data were collected during three surveys on two commercial vessels in 1986–1988: two deploying a seine net and one a bottom trawl. Deployments were made with codends of three nominal diameters: narrow, 2.2 m (seine: 107, trawl: 36 hauls), standard, 3.2 m (seine: 105, trawl: 35 hauls) and wide, 4 m (seine: 116, trawl: 36 hauls); three mesh sizes (nominal 80, 90 and 100 mm) and three extension piece lengths (nominal 0.1, 9.1 and 13.7 m). Covers over each codend retained the fish escaping through the meshes for sampling.

    (Summarised by: Rosslyn McIntyre)

  2. Modify the design or configuration of trawl gear (mixed measures)

    A replicated, controlled study in 1986–1988 on bottom fishing grounds in the North Sea, UK (Reeves et al. 1992) found that modifying the design of bottom trawls and seine nets (changing length of extension piece) resulted in an increase in size-selectivity with shorter extension pieces for haddock Melanogrammus aeglefinus, whiting Merlangius merlangus and cod Gadus morhua. For both gear types, extension length affected size-selectivity and increased with decreasing length (data reported as statistical results). Overall, the estimated lengths at which fish had a 50% chance of escape with the shortest extension piece (0 m) ranged from 15–32 cm for haddock, 20–35 cm for whiting and 16–36 cm for cod. For the longest extension length (13.7 m) these were decreased to 11–28 cm for haddock, 15–30 cm for whiting and 12–32 cm for cod. In addition, gear size-selectivity increased with increasing mesh size and narrower diameters of the codend, and their effect was greater than the effect of extension length. Data were collected during three surveys on two commercial vessels between 1986–1988 in the northern or central North Sea. Fishing was conducted with two seine nets and one trawl net configured with three different extension lengths (0, 9.1 and 13.7 m), mesh sizes (80, 90 and 100 mm) and codend diameters (2.2, 3.2 and 4 m). For seine nets, 114 hauls were completed with a 9.1 m extension, 104 hauls with a 13.7 m extension and 110 hauls with a 0 m extension. For the trawls, 37, 35 and 35 hauls were completed for the 9.1, 13.7 and 0 m extensions, respectively. Small mesh covers attached over the codends sampled fish escaping through the meshes. All fish in the codends and covers were identified, and length recorded. No cod were caught in trawl net deployments.

    (Summarised by: Rosslyn McIntyre)

  3. Use a larger mesh size

    A replicated, controlled study in 1986-1988 on bottom fishing grounds in the northern and central North Sea, UK (Reeves et al. 1992) found that larger mesh size trawl codends and seine nets had improved size-selectivity for haddock Melanogrammus aeglefinus, whiting Merlangius merlangus, and for seines Atlantic cod Gadus morhua, compared to the standard and smaller mesh sizes. Across both gear types, the length at which fish had a 50% chance of escape was greater with a 100 mm mesh (haddock: 19-32 cm, whiting: 23-39 cm, cod: 23-36 cm) than 90 mm mesh (haddock: 15-28 cm, whiting: 19-34 cm, cod: 19-30 cm) which in turn was greater than an 80 mm mesh (haddock: 10-24 cm, whiting: 14-29 cm, cod: 12-24 cm). No cod were caught in the trawl nets. Commercial trawl and seine nets were tested with 100 mm, 90 mm (the minimum mesh size in force in the commercial fishery), and 80 mm mesh codends. Three surveys were carried out on two commercial vessels in the North Sea in 1986–1988, two with a trawl net and one with a seine net. Covers were fitted over the codends to catch fish escaping through the meshes. Thirty-five trawl deployments were made with the 100 mm mesh and 36 with each of the 90 mm and 80 mm meshes. For the seine nets, a total of 94 (100 mm mesh), 113 (90 mm mesh) and 121 (80 mm mesh) deployments were made. All fish in the codends and covers were identified and their lengths measured.

    (Summarised by: Rosslyn McIntyre)

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