Change the size of the main body of a trawl net

How is the evidence assessed?
  • Effectiveness
    not assessed
  • Certainty
    not assessed
  • Harms
    not assessed

Study locations

Key messages

  • One study examined the effects of changing the size of the main body of a trawl net to reduce unwanted catch on marine fish populations. The study was in the North Sea (Norway).





  • Improved size-selectivity of fishing gear (1 study): One replicated study in the North Sea found that reducing the size of the main body of a trawl net did not improve the size-selection of cod and haddock.

About key messages

Key messages provide a descriptive index to studies we have found that test this intervention.

Studies are not directly comparable or of equal value. When making decisions based on this evidence, you should consider factors such as study size, study design, reported metrics and relevance of the study to your situation, rather than simply counting the number of studies that support a particular interpretation.

Supporting evidence from individual studies

  1. 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.

    Study and other actions tested
Please cite as:

Taylor, N., Clarke, L.J., Alliji, K., Barrett, C., McIntyre, R., Smith, R.K., and Sutherland, W.J. (2021) Marine Fish Conservation: Global Evidence for the Effects of Selected Interventions. Synopses of Conservation Evidence Series. University of Cambridge, Cambridge, UK.

Where has this evidence come from?

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Marine Fish Conservation

This Action forms part of the Action Synopsis:

Marine Fish Conservation
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