Use a larger codend mesh size on trawl nets

Overall effectiveness category Likely to be beneficial
Number of studies: 1
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How is the evidence assessed?

Effectiveness

65%
Certainty

42%
Harms

0%

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 larger size mesh can be used, with the aim to allow smaller unwanted organisms to escape, while retaining the commercially targeted ones (Burgos‐León et al. 2009).

Evidence related to other codend modifications are summarised under “Threat: Biological resource use – Use a square mesh instead of a diamond mesh codend on trawl nets”.

_{Burgos‐León A., Pérez‐Castañeda R. & Defeo O. (2009) Discards from the artisanal shrimp fishery in a tropical coastal lagoon of Mexico: spatio‐temporal patterns and fishing gear effects. Fisheries Management and Ecology, 16, 130–138.}

Study locations

Key messages

One study examined the effects of using a larger codend mesh size on trawl nets on unwanted catch of subtidal benthic invertebrate populations. The study was in the Gulf of Mexico (Mexico).

COMMUNITY RESPONSE (1 STUDY)

Unwanted catch species richness/diversity (1 study): One replicated, paired, controlled study in the Gulf of Mexico found that trawl nets fitted with a larger mesh codend caught fewer combined species of non-commercial unwanted invertebrates and fish compared to a traditional codend.

POPULATION RESPONSE (1 STUDY)

Unwanted catch abundance (1 study): One replicated, paired, controlled study in the Gulf of Mexico found that trawl nets fitted with a larger mesh codend caught lower combined biomass and abundance of non-commercial unwanted invertebrates and fish compared to a traditional codend.

OTHER (1 STUDY)

Commercial catch abundance (1 study): One replicated, paired, controlled study in the Gulf of Mexico found that trawl nets fitted with a larger mesh codend caught less biomass and abundance of commercially targeted shrimps compared to a traditional codend, but that the biomass ratios of commercially targeted to discard species was similar for both.

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 (year unspecified) of three estuarine sites in the Celestun Lagoon, Gulf of Mexico, Mexico (Burgos-León et al. 2009) found that trawl nets fitted with a 2.5 cm mesh codend instead of a traditional 1.3 cm mesh caught fewer combined non-commercial unwanted invertebrate and fish species (discard) and lower biomass and abundance of discarded organisms. On average, nets with the larger mesh codend caught fewer discard species (3–12) than nets with the traditional codend (12–20). Biomass and abundance of discards were on average lower with the larger mesh codend (biomass: 2–10 g/245 m²; abundance: 0.7 individuals/245 m²) than with the traditional codend (biomass: 22–57; abundance: 37). Nets with the larger mesh codend also caught less biomass and abundance of commercially targeted shrimps (biomass: 3–15 g/245 m²; abundance: 1 individual/245 m²) than nets with the traditional codend (biomass: 15–39; abundance: 20). This however led to similar biomass ratios of commercially targeted to discard species for both mesh sizes (1:1). On three occasions at each of three sites, a vessel towed two identical bottom-nets simultaneously over 100 m during paired deployments, one fitted with the traditional codend, the other with the larger mesh codend. For each deployment, discarded organisms were identified and their combined weight and counts recorded. Weights of commercially targeted shrimps were also recorded.
Study and other actions tested
Referenced paper
Burgos-León A., Pérez-Castañeda R. & Defeo O. (2009) Discards from the artisanal shrimp fishery in a tropical coastal lagoon of Mexico: spatio-temporal patterns and fishing gear effects. Fisheries Management and Ecology, 16, 130-138.

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

Related Actions

Actions	Effectiveness	Studies	Category
Modify harvest methods of macroalgae Action Link	No evidence found (no assessment)	0	Synopsis Link
Reduce the number or modify the arrangement of tickler chains/chain mats on trawl nets Action Link	Unknown effectiveness (limited evidence)	2	Synopsis Link
Use a square mesh instead of a diamond mesh codend on trawl nets Action Link	Unknown effectiveness (limited evidence)	1	Synopsis Link