Study

On the influence of towing speed and gear size on the selective properties of bottom trawls

  • Published source details Dahm E., Wienbeck H., West C.W., Valdemarsen J.W. & O’Neill F.G. (2002) On the influence of towing speed and gear size on the selective properties of bottom trawls. Fisheries Research, 55, 103-119.

Actions

This study is summarised as evidence for the following.

Action Category

Change the towing speed of a trawl net

Action Link
Marine Fish Conservation

Change the size of the main body of a trawl net

Action Link
Marine Fish Conservation
  1. Change the towing speed of a trawl net

    A replicated, paired study in 1995 of one area of seabed in the North Sea off southern Norway (Dahm et al. 2002) found that changing the towing speed of a bottom trawl net did not improve the size selectivity of unwanted small cod Gadus morhua and haddock Melanogrammus aeglefinus. Across both vessels and net sizes, the length at which fish had a 50% chance of escaping was similar between two towing speed ranges for both cod (slower: 30–34 cm, faster: 30­–34 cm) and haddock (slower: 28–32, faster: 28–33). In April 1995, trawl towing speed was tested on two fishing vessels (20 deployments each), fishing at the same time on the same fishing grounds (exact location not reported). Vessels had different sizes of bottom trawl nets, one a standard size and one a scaled-down size but both had identical codends (see paper for specifications). For the larger net, catches were analysed for towing speeds above and below 3.0 m/s and for the smaller net <3.5 versus >3.5 m/s. Fish retained by the larger net were sampled in twin codends, one with the test net and one with a small mesh to sample all sizes of fish. On the smaller net a cover was attached to the codend to collect fish escaping through the meshes. All codend and cover catches were sorted and weighed by species and total lengths recorded.

    (Summarised by: Natasha Taylor)

  2. Change the size of the main body of a trawl net

    A replicated study in 1997 of an area of seabed in the North Sea off southern Norway (Dahm et al. 2002) found that reducing the size of a trawl net body did not affect the size-selectivity of Atlantic cod Gadus morhua and haddock Melanogrammus aeglefinus compared to a larger trawl body. The length at which fish had a 50% chance of escaping was similar between two trawl body sizes for both cod (small: 32.8 cm, large: 31.8 cm) and haddock (small: 27.0 cm, large: 27.1 cm). In addition, the size-selection ranges of retained fish of both species were similar between trawl sizes (small: 4.4–8.2 cm, large: 4.9–7.7 cm). In March/April 1997, two bottom trawl nets, one small (‘minihopper’) and one larger (‘codhopper’), of different body dimensions but identical codends, were tested in 23 trawl deployments by a research fishing vessel (see paper for specifications). The small trawl was a specially designed version of a larger trawl which was commonly used for stock assessments. Average measurements for the small trawl were 3.3 m (height) and 20.7 m (spread) and for the larger trawl 3.7 m (height) and 24.9 m (spread). Covers attached to the codend collected fish escaping through the net. All codend and cover catches were sorted and weighed by species and total lengths recorded.

    (Summarised by: Natasha Taylor)

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