A benthic panel is a small panel of square mesh or large diamond mesh inserted in the bottom panel of a trawl, usually a beam trawl to allow the release of immature fish, benthos and general benthic debris.
The Benthic panel is fitted into the lower panel of a beam trawl in a similar manner and position to that of a square mesh panel in a demersal trawl. Most of the fish targeted by beam trawls do not had the natural escape stimulus of swimming upwards to escape danger. Small fish tend to head downwards towards the seabed to escape danger therefore an escape window in the bottom of the trawl is more effective for them to swim through. In this position it is also very effective at releasing much of the benthos (organisms that live on or close to the seabed) and benthic debris such as shells and small stones etc that can be scooped up by a beam trawl by simply ‘riddling ‘ them through the square meshes because they naturally sink to the seabed.
When using large square mesh (200mm) the weight of benthos was reduced by as much as 83% and weight of benthic debris by 56% however with tis mesh of mesh the reduction in valuable commercial species was as much as 45% which is unacceptable. With 150mm mesh these figures dropped to 70% and 34% with a reduction in marketable catch in the region of 18%. Using 120mm square mesh there was slightly less release of benthos and benthic debris but no appreciable loss of fish species, in fact in the short trials there was actually an increase in some of the fish species when 120mm mesh was used. Whenever any selective device such as square mesh is fitted it will cause changes the water flow inside the trawl, this can change how the fish react to the water flow as they pass down the trawl that in turn can create unexpected differences in catch composition in the cod'end.
As with many other selective gears by reducing the amount of by'catch in the cod'end and in this case the amount of benthic debris the retained catch will be in a better condition (better fish quality) when it’s taken on'board. This should lead to better market prices and it will also improve the fish welfare when in the gear that could result in better survivability of any fish that need to be discarded.
The use of Benthic panels or Benthos Release Panels (BRP’s) in beam trawls has been developed and tested in the Belgian beam trawl fishery by the research unit at the Institute for Agriculture and Fisheries Research (ILVO). As beam trawls are designed to catch fish that live tight down on the seabed in some fisheries there can be a by'catch of small fish and benthic species that are found close to, or on the seabed. By fitting a section of square mesh (benthic panel) of an appropriate mesh size in the lower panel of a beam trawl ahead of the cod'end much of this unwanted catch can be riddled out of the trawl with very little if any loss to marketable species.
In a normal trawl the diamond meshes have a tendency to close up, if the netting is turned through 45 degrees to make it square mesh the meshes will remain open allowing easy release of small fish and benthic debris. The Benthic panel (square mesh panel) used in the bottom panel of a beam trawls is approximately 1.8 metres long by 1.2 metres wide giving an escape window for small fish of just over 2 square metres. It is fitted just ahead of the cod end to release the benthos and small fish before they enter the cod'end. The size of mesh used for the benthic panel has a big influence on its efficiency at releasing the benthos and small fish in relation any loss in marketable species. The larger the mesh the more efficient the panel is at releasing benthos etc but if it’s too large there may be an unacceptable loss of valuable marketable species. This is a fairly simple adaption to make to a beam trawl so long as the netting used is of suitable thickness and the square mesh is fitted correctly in relation to the diamond mesh. This is done in a similar manner to that of a square mesh panel in the top panel of a demersal trawl. (See fitting a square mesh panel in the technical section for advice on this.)
Caveat-All dimensions and mesh counts and cuts are for generic advice and information. Before constructing and fitting this gear into a trawl you should consult with your local fishery officers or legislators such as MMO, Marine Scotland and Daera in the UK. This should ensure that your gear meets the requirements of the area that your vessel will be fishing in.