Trawl Modification
WHAT IS A TRAWL MODIFICATION?
A trawl modification involves making an alteration to existing gear. Compared to switching to alternative gears, benefits of this approach can include avoiding the costs of completely replacing gear and the time and effort of learning to use new gear.
Around the world, many trawl fisheries have already modified their gear to improve selectivity and avoid catching undersized commercial fish species or other non-target commercial fish species. Less attention has been paid to trawl modifications in terms of their ability to reduce bycatch of sensitive marine species. The exception is the trawl excluder device, a mesh barrier or grid that is inserted into the trawl to enable escape of large animals such as seals and turtles without significantly affecting target catch rates. These excluder devices may also improve escape rates for some larger species of elasmobranch (sharks, skates, and rays). However, other trawl modifications hold potential to significantly reduce bycatch of smaller vulnerable elasmobranchs without negatively affecting fishing operations. Many of these are simple and/or cheap to implement, although the diversity of demersal and pelagic trawls means that a modification found to be suitable in one fishery may not be so suitable in others.
Diagram example of a demersal trawl. Source: Seafish (n/d)
CURRENT RESEARCH AND USE
As shown in the example above, while trawl design varies between regions and fisheries, the gear is typically formed of several distinct components including: trawl doors; the headline, foot rope, and ground gear; the trawl wings and body (formed of mesh); and the extension and cod-end (also formed of mesh). Bycatch mitigation research to date has focused on a subset of these components.
Ground gear
The flattened body shape of skates and rays can make it particularly challenging for even small individuals to escape the cod-ends of trawls, meaning that modifications focused on preventing these animals from entering trawls in the first place could have a significant positive impact. For example:
- At-sea trials with a research vessel in the Mediterranean were undertaken to see if bycatch reduction could be achieved by cutting the rigging twine connecting the fishing line and the footrope in the central part of the ground gear (a setup originally designed to prevent seabed debris entering the trawl). Bycatch of the endangered guitarfish and of the common stingray fell by 92% and 80% respectively; although that of the spiny butterfly ray remained the same, likely because of differences in behaviour. While catches of common sole were also significantly reduced, those for another five target fish species were unaffected by the modification. The study did not report on whether the modification led to the trawl picking up more seabed debris.
- Trawls often include “tickler chains”, designed to disturb fish from the seabed. Experimental removal of tickler chains in a demersal trawl fishery in Scottish waters led to a significant reduction in bycatch of sharks and skates. However, while catch rates of targeted haddock, whiting, and flatfish were unaffected, the modification did significantly reduce catches of commercially valuable anglerfish.
Mesh
Where elasmobranchs still enter trawls and are not large enough to escape via excluder devices or grids, mesh configuration can affect their chances of escaping from the cod-end. For example:
- A study of a bottom-trawl crustacean fishery in Portugal reported that switching from diamond-shaped to square-shaped mesh can increase the chance of escape for small deep-sea sharks. This is likely because tension on the net can cause diamond-shaped mesh to close, whereas square mesh stays open.
- A team of scientists and fishers in a South Australia prawn fishery experimented with retaining the diamond-shape of the mesh but adopting 58 mm polypropylene T90 mesh, which changes shape less under heavy loads than standard diamond meshes. This particular study also applied a trawl excluder device, which was highly effective at reducing elasmobranch bycatch, meaning that it is unclear in this case if the modified mesh might have been effective on its own with regards to elasmobranchs; however, diamond meshes that are more resistant to distortion could be useful for reducing bycatch of elasmobranchs if tried in other fisheries
This page was created on 03.07.25.
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