Study

Selectivity of large mesh trawl codends in the Gulf of Maine: I. Comparison of square and diamond mesh

  • Published source details He P. (2007) Selectivity of large mesh trawl codends in the Gulf of Maine: I. Comparison of square and diamond mesh. Fisheries Research, 83, 44-59.

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 2003 of a fished area of seabed in the northwest Atlantic Ocean off New Hampshire, USA (He 2007) found that the effect of using square mesh instead of diamond mesh codends on the size-selectivity of bottom trawl nets for five important commercial fish species depended on body shape (roundfish or flatfish), but for both square and diamond codends, selectivity increased with larger mesh sizes. For cod Gadus morhua and haddock Melanogrammus aeglefinus there were no differences in the selection length (the length at which 50% of fish are predicted to escape) between square and diamond mesh of the same mesh size (cod, square: 59–69 cm, diamond: 59–66 cm; haddock, square: 54–57 cm, diamond: 55–61 cm); but for three flatfish, the selection lengths were smaller in square mesh codends (long rough dab Hippoglossoides platessoides, square: 33–35 cm, diamond: 39–40 cm; yellowtail flounder Limanda ferruginea, square: 34–38 cm, diamond: 40–42; witch flounder Glyptocephalus cynoglossus, square: 36–40 cm, diamond: 43–46 cm). For all species, an increase in selection length was found with increasing mesh size in both diamond and square mesh (see paper for individual data by species and codend). Catch data was collected in May–July 2003 from 86 fishing vessel deployments in the western Gulf of Maine (33–142 m depths). Five codends were tested: two square mesh (165 and 178 mm), and three diamond mesh (152, 165 and 178 mm). The five codends were tested in a preselected random order, each for up to six consecutive tows, for between 14–20 hauls. A small mesh cover over each codend sampled escaped fish. Codend and cover catches were counted and weighed by species, and fish total lengths measured (sub-sampled where necessary).

    (Summarised by: Chris Barrett)

  2. Use a larger mesh size

    A replicated, randomized, controlled study in 2003 in an area of seabed in coastal waters in the Gulf of Maine, Atlantic Ocean, USA (He 2007) found that increasing the mesh size on trawl net codends, for both diamond and square mesh, typically improved the size-selectivity of five fish species compared to conventional smaller mesh sizes. For diamond and square meshes, the size at which fish had a 50% chance of escape typically increased with increasing mesh size for five of five species: cod Gadus morhua (diamond, large: 66 cm, intermediate: 59 cm small: 52 cm; square, large: 69 cm, small, 59 cm), haddock Melanogrammus aeglefinus (diamond, large: 61 cm, intermediate: 55 cm small: 50 cm; square, large: 57 cm, small, 54 cm), long rough dab Hippoglossoides platessoides (diamond, large: 39 cm, intermediate: 40 cm small: 36 cm; square, large: 35 cm, small, 33 cm), yellowtail flounder Limanda ferruginea (diamond, large: 42 cm, intermediate: 40 cm small: 38 cm; square, large: 38 cm, small, 34 cm), and witch flounder Glyptocephalus cynoglossus (diamond, large: 46 cm, intermediate: 43 cm small: 40 cm; square, large: 40 cm, small, 36 cm). For haddock and long rough dab, the difference between the large and intermediate diamond mesh sizes was not significant. Data were collected from 86 trawl deployments on a commercial fishing vessel in May–July 2003. Five codends were tested, three diamond mesh (178 mm, 165 mm and 152 mm mesh size) and two square mesh (178 mm and 165 mm mesh size). The existing minimum mesh was 165 mm for both diamond and square. The five codends were tested randomly in a preselected order with each being tested for up to six consecutive tows. Each codend had a small mesh cover to collect fish escaping through the meshes. Upon hauling, fish were sorted by species and individual lengths recorded.

    (Summarised by: Chris Barrett)

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