A separator panel is a horizontal panel of netting designed to divide the trawl into two sections one above the other. The idea is to be able to separate certain species that swim higher in the water column or rise ahead of the trawl into the upper chamber and the fish and shellfish that live close to the seabed into the lower chamber. This gives the possibility to then have different selective devices in each chamber each one better suited to those species in that particular chamber.
Very efficient species selectivity can be achieved using a separator panel once the height of the panel has been tuned to suit the area that is being fished, the trawl that the panel is in and the mix of target species being caught. Almost all the haddock, whiting, saithe etc will end up in the upper chamber of the trawl. Cod, flats, monkfish and any crustacean and shellfish will be in the lower chamber. The main variance to these guidelines are when a lot of juvenile haddock and whiting are in the trawl. Fish of these species below about 150mm tend to stay low in a trawl, having not yet developed the stimulus to rise ahead of the trawl footrope so have a tendency to enter the lower chamber of the trawl. However a separator trawl can be a very efficient tool at separating fish from nephrops. Often showing a slight increase in the nephrops catches in the lower cod end. This is probably due to there being much less fish in the lower cod-end therefore the meshes stay more closed up helping to retain more nephrops.
Over the years there has been many experiments with separator panels fitted into trawls but so far with little uptake in the UK catching sector. This may well be down to the added cost and handling difficulties from the introduction of an extra panel of netting and a second cod-end and extension piece. The separator panel works on the general understanding of fish behaviour in the water column. Certain species like shell fish, flat fish, monkfish and cod are known to stay close to the seabed ahead of a trawl and will generally pass into the lower chamber of a separator trawl. Other species such as whiting, haddock, hake, saithe etc. naturally swim slightly higher above the seabed and will usually display an escape stimulus of swimming forwards and upwards as they become aware of the advancing trawl net, eventually falling back into the upper chamber of the separator trawl.
Separation of species can be as efficient as 95% in some fisheries. The actual height of the leading edge of the separator panel above the seabed is important as a few centimetres difference can make a considerable change in the selection. For this reason it is normal to have two or three short ropes between the panel and the ground gear of the trawl setting the height of the panel above the seabed. If these ropes are not fitted the natural water flow will push the separator panel upwards towards the top panel of the trawl.
Juvenile round fish have been found not to have the same escape stimulus as the larger fish of the same species, often not showing any attempt to rise up ahead of the trawl mouth, preferring to stay low in the trawl and pass over the footrope below the separator panels. The separator panel does not show such an effective separation for small fish below approximately 150mm as it does for fish above this size.
The big advantage of the separator trawl is that having the fish divided into two separate compartments and cod-ends, one above the other, in the trawl allows for the use of different selective devices to be used in the two different cod-ends. The simplest version of this would be to retain the small mesh (usually 80mm) in the lower cod-end in a prawn / scraper trawl to retain the prawns but to have a larger mesh in the upper cod-end to release all the fish below minimum conservation reference size. Any suitable secondary selective device, such as a square mesh panel, could be fitted into either the top or lower chamber to improve selectivity and minimise unnecessary discarding.
By separating the relatively soft fleshed white fish in the top cod-end from the nephrops and bottom living species in the bottom, helps to minimise damage and abrasion to the fish in upper cod-end thereby helping maintain quality of the captured fish. With less fish in the lower cod-end they too are found to be of better quality than when landed from a single cod-end net.
To work efficiently the separator panel has to be designed to suit each trawl. A correctly designed separator panel should not make any difference to the shape of the trawl it’s fitted in. If the separator panel is made too narrow it will pull the sides of the trawl inwards and change the shape and fishing ability of the trawl. Similarly if it’s too short it will distort then trawl. If it’s made too wide or too long there will be excessive netting flapping about in the trawl and this could well scare fish away from the mouth of the trawl and they would be lost from capture.
There will be only minimal increase in drag from the extra panel and cod-end because the panel is orientated flat along the line of towing the gear so a very small frontal area exposed to the water flow, this will minimise drag. As for the cod-ends there will be minimal increase of drag with the addition of the second cod-end, as the water entering the mouth of the net now has two cod-ends to be filtered out through rather than it all flowing down into one cod-end. This can also help retain quality as the fish in both cod-ends, are in a more gentle water flow while in the cod-ends, enabling them to continue swimming into the flow without getting excessive stress and also avoid contact with other fish that may cause damage to the flesh and reduce the quality of the fillets from the fish.
There is a basic formula for the design and rigging of a separator panel included in the technical section of this website. Both these can be quite intricate but there are a few basic guide lines included the technical section. It is important that the separator panel is designed to suit the net it is to be fitted into to ensure optimum efficiency at separating the different species of fish.
Using a horizontal separator panel to separate by-catch from the catch in the beam trawl fishery
Evaluation of by-catch reduction devices in UK Nephrops fisheries - The use of separator trawls in the Irish Sea
Separating fish from prawns in bottom trawls
Further investigations into selectivity and separator trawls
A shrimp separator trawl for the Southeast Fisheries
Using a horizontal separation panel to improve selection in a pulse beam trawl
Separator trawls as a tool for improving selectivity for cod