Study

Development and test of selective sorting grids used in the Norway lobster (Nephrops norvegicus) fishery

  • Published source details Madsen N., Holst R., Frandsen R. & Hansen K. (2017) Development and test of selective sorting grids used in the Norway lobster (Nephrops norvegicus) fishery. Fisheries Research, 185, 26-33.

Actions

This study is summarised as evidence for the following.

Action Category

Use a different design or configuration of size-sorting escape grid/system in trawl fishing gear (bottom and mid-water)

Action Link
Marine Fish Conservation

Fit a size-sorting escape grid (rigid or flexible) to a prawn/shrimp trawl net

Action Link
Marine Fish Conservation
  1. Use a different design or configuration of size-sorting escape grid/system in trawl fishing gear (bottom and mid-water)

    A replicated, paired study in 2010 in an area of seabed in the Kattegat and Skagerrak, Denmark (Madsen et al. 2017) found that the effect of using a different type of size-sorting escape grid system (three designs) in a prawn trawl net on the reduction of unwanted fish catch varied between species. The average percentage escape of small or undersized fish was higher for grids with horizontal and vertical bars than a grid with vertical bars and a guiding funnel, for three of five species (horizontal: 59–88%, vertical: 79–87%, vertical with panel: 35–55%). For one species escape of undersized fish was higher with the horizontal bar grid than either of the other two grids (horizontal: 85%, vertical: 67%, vertical with funnel: 48%) and for the other species escape rate of small individuals was not statistically different between all three grids (horizontal: 86%, vertical: 75%, vertical with funnel: 55%). In addition, escape of undersized individuals of the commercial target species Norway lobster Nephrops norvegicus was higher with the vertical bar grid (17%) than the other two grids (horizontal: 5%, vertical with funnel: 6%). There were also high losses of commercial sized catch in some cases (see paper for data). Sea trials took place in March 2010 on a commercial trawler rigged with a twin-trawl system. Thirty-four trawl deployments (2–4 h) were completed using three designs of flexible grid: horizontal bars (10 tows), vertical bars (12 tows) and vertical bars with a guiding funnel (14 tows). Both grids with vertical bars were fished simultaneously on each side of the trawl, while the horizontal grid was fished with a codend being used for another experiment. A small mesh cover installed over the escape openings collected individuals escaping from the grids. All grids were installed at a 45° angle with bar spacing of 45 mm (see original paper for gear specifications).

    (Summarised by: Chris Barrett)

  2. Fit a size-sorting escape grid (rigid or flexible) to a prawn/shrimp trawl net

    A replicated study in 2010 of a seabed area in the Kattegat and Skagerrak, North Sea, bordering Norway, Denmark and Sweden (Madsen et al. 2017) found that size-sorting escape grids of three designs fitted to prawn trawl nets all reduced the capture of unwanted small fish in a Norway lobster Nephrops norvegicus fishery. Overall, grids enabled 55–88% (225–6,766 fish) of undersized individuals of three of three roundfish (cod Gadus morhua, haddock Melanogrammus aeglefinus and whiting Merlangius merlangus) and 35–86% (337–3,463 fish) of undersized fish of two of two flatfish (lemon sole Microstomus kitt and plaice Pleuronectes platessa) to escape. In addition, grids reduced the catches of undersized individuals of the target Norway lobster by 5–17%, but there were losses above minimum landing size of 13–33%. Data were collected from 10–14 trawl deployments of 2–4 h at 42–71 m depth, for each of three grid systems in March 2010 using a twin-rigged trawler. Trawl nets of 90 mm mesh codend were fitted with grid systems of either: horizontal bars, vertical bars, or vertical bars and a mesh guiding panel. All grids were black in colour, 45 mm bar spacing, set at 45° angles and with a hole at the bottom part to stop debris (see paper for specifications). Small mesh covers attached over the grid escape opening collected fish escaping through the grid. Cover and codend catches were weighed and length measurements taken for all commercially important species.

    (Summarised by: Chris Barrett)

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