Fit rigid (as opposed to mesh) escape panels/windows to a trawl net

Overall effectiveness category Awaiting assessment
Number of studies: 1
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Background information and definitions

Modifying fishing trawl nets is a common practice to improve selectivity, helping fishers to catch only the species and sizes they are commercially targeting. How trawl nets are modified depends largely on the sizes/shapes of the unwanted species and also behavioural characteristics. For example, the avoidance response of some species to trawl gear is to swim upwards, while some may attempt to flee in other directions. The use of panels or windows of modified netting (different orientation and/or mesh size) in trawl net codends has been assessed as one means to allow the escape of unwanted fish (see ‘Fishing gear modification – Fit mesh escape panels/windows to a trawl net’). Rigid escape windows are intended to work on the same principle as mesh escape windows but are specifically designed to enable the potential escape of flatfish species. Unlike mesh windows, rigid escape windows are made of steel and have a grid-like construction that creates well defined rectangular escape openings. They are located with the aim that unwanted flatfish (plus small or undersized roundfish species) can pass through the window in a natural swimming orientation and escape.

Study locations

Key messages

One study examined the effects of fitting rigid escape windows/panels to trawls for fish escape on marine fish populations. The study was in the Baltic Sea.

COMMUNITY RESPONSE (0 STUDIES)

POPULATION RESPONSE (0 STUDIES)

BEHAVIOUR (0 STUDIES)

OTHER (1 STUDY)

Reduction of unwanted catch (1 study): One replicated, paired, controlled study in the Baltic Sea found that fitting rigid escape windows in a section of trawl net reduced the catch of unwanted flatfish compared to a trawl net without escape windows.

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 2013 of an area of seabed in the western Baltic Sea (Santos et al. 2016) found that a trawl net modified with rigid escape windows in a section of net mounted in front of the codend, reduced the catches of unwanted flatfish compared to a trawl net without rigid escape windows. Total catches of plaice Pleuronectes platessa were 56% lower and flounder Platichthys flesus 62% lower in the modified trawl compared to standard trawl (windows: 1,033–1,310 fish, no windows: 2,354–3,437 fish). There was no significant difference in overall catches of the commercial target species, cod Gadus morhua, in the modified and standard trawls (windows: 1,602 fish, no windows: 1,824 fish) or of undersized cod (windows: 255 fish, no windows: 377). Catch comparison trials were done in March 2013 on a commercial twin trawler on cod fishing grounds west of the island of Bornholm. A total of 12 paired trawl deployments were completed using one modified trawl and one standard trawl net. Both nets were identical and fitted with a mandatory design of selective codend of large square mesh (“Bacoma”). In the modified trawl, rigid grid-like escape windows of 38 mm horizontal bar spacing were incorporated in the two side panels of a four-panel net extension piece in front of the codend (“FRESWIND” system - see paper for specifications). Catches from each haul were weighed by species, and the total length of all fish measured.
Study and other actions tested
Referenced paper
Santos J., Herrmann B., Mieske B., Stepputtis D., Krumme U. & Nilsson H. (2016) Reducing flatfish bycatch in roundfish fisheries. Fisheries Research, 184, 64-73.

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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This Action forms part of the Action Synopsis:

Marine Fish Conservation

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Rotate the orientation of diamond mesh in a trawl net Action Link	Awaiting assessment	6	Synopsis Link
Fit escape devices (panels/grids) to encircling nets Action Link	Awaiting assessment	3	Synopsis Link
Fit a size-sorting escape grid (rigid or flexible) to a prawn/shrimp trawl net Action Link	Awaiting assessment	30	Synopsis Link