Use a square mesh instead of a diamond mesh codend on trawl nets

Overall effectiveness category Unknown effectiveness (limited evidence)
Number of studies: 1
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How is the evidence assessed?

Effectiveness

50%
Certainty

20%
Harms

10%

Calculating overall effectiveness category

Background information and definitions

Trawling is a method of fishing that involves pulling a fishing net (trawl) through the water behind one or more boats. Trawl nets can catch a considerable number of unwanted organisms, including non-commercially targeted species and organisms under the legal-size limit. Nets are traditionally made of diamond-shaped mesh throughout, including at the codend. To reduce the amount of unwanted organisms caught, a codend made of square mesh can be used instead of diamond mesh, with the aim to allow smaller unwanted organisms to escape, while retaining the commercially targeted ones (Broadhurst et al. 2010).

Evidence related to the use of modified codend in combination with a mesh panel (“bycatch reduction device”) or a grid (“turtle excluder device”) are summarised under “Threat: Biological resource use – Fit one or more mesh escape panels/windows to trawl nets and use a square mesh instead of a diamond mesh codend” and “Fit one or more soft, semi-rigid, or rigid grids or frames to trawl nets and use a codend of different mesh geometry or size”. Evidence related to other codend modifications are summarised under “Threat: Biological resource use – Use a larger mesh codend”.

_{Broadhurst M.K., Millar R.B. & Brand C.P. (2010) Diamond-vs. square-mesh codend selectivity in southeastern Australian estuarine squid trawls. Fisheries Research, 102, 276–285.}

Study locations

Key messages

One study examined the effects of using a square mesh instead of a diamond mesh codend on trawl nets on unwanted catch of subtidal benthic invertebrate populations. The study was in the English Channel (UK).

COMMUNITY RESPONSE (0 STUDIES)

POPULATION RESPONSE (1 STUDY)

Unwanted catch abundance (1 study): One replicated, paired, controlled study in the English Channel found that a trawl net with a square mesh codend caught less non-commercial unwanted invertebrates in one of two areas, and similar amounts in the other area, compared to a standard diamond mesh codend.

OTHER (1 STUDY)

Commercial catch abundance (1 study): One replicated, paired, controlled study in the English Channel found that a trawl net with a square mesh codend caught similar amounts of commercially targeted fish species in two areas, and that in one of two areas it caught more commercially important shellfish, compared to a standard diamond mesh codend.

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

A replicated, paired, controlled study in 2007 in two areas of the English Channel, southwest England, UK (Wade et al. 2009) found that in one of two areas a trawl net fitted with a square mesh codend caught less non-commercial unwanted invertebrates (discard) compared to a standard trawl fitted with a diamond mesh codend. In one of two areas examined, using a square mesh codend instead of a standard diamond mesh codend reduced the biomass of invertebrate discard by 11% (square: 794 vs diamond: 889 kg). The square mesh codend caught similar amounts of commercially targeted species (megrim and anglerfish in that area) to the standard trawl and caught 26% more commercially important shellfish. In the other area, the square mesh codend caught similar biomass of invertebrate discard (1,746 kg) as the diamond mesh codend (1,842 kg), and similar amounts of commercially targeted species (Dover sole and plaice in that area). Two designs of trawl nets were towed simultaneously: a modified beam trawl with an 80 mm square mesh codend, and the industry standard beam trawl with an 80 mm diamond mesh codend. Twelve tows/area were conducted in July–August 2007. The catch was sorted into commercially important species (target and commercial unwanted catch) and discard species. Commercial organisms were counted, and discards were further sorted into benthic invertebrates and finfish and each were weighed.
Study and other actions tested
Referenced paper
Wade O., Revill A.S., Grant A. & Sharp M. (2009) Reducing the discards of finfish and benthic invertebrates of UK beam trawlers. Fisheries Research, 97, 140-147.

Please cite as:

Lemasson, A.J., Pettit, L.R., Smith, R.K. & Sutherland, W.J. (2020) Subtidal Benthic Invertebrate Conservation. Pages 635-732 in: W.J. Sutherland, L.V. Dicks, S.O. Petrovan & R.K. Smith (eds) What Works in Conservation 2020. Open Book Publishers, Cambridge, UK.

Where has this evidence come from?

List of journals searched by synopsis

All the journals searched for all synopses

This Action forms part of the Action Synopsis:

Subtidal Benthic Invertebrate Conservation

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Fit one or more soft, semi-rigid, or rigid grids or frames to trawl nets and use square mesh instead of a diamond mesh at the codend Action Link	Unknown effectiveness (limited evidence)	1	Synopsis Link
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