SELTRA Box - Diamond mesh
The Seltra box is a four panel extension section with a top panel of either a large diamond mesh or large square mesh to encourage small fish to escape from a trawl before entering the cod-end.
The Seltra box with the large diamond or square mesh in the top panel is designed to provide an escape area for fish that have an escape stimulus to swim upwards such as whiting and haddock. In trials this has been shown to work in many areas with a reduction of by-catch of these species in the region of 60 - 70%. In the initial trials there was a loss of the smaller nephrops of about 10%. It is expected that this will be reduced as the skippers get experience using a Seltra box. One of the advantages of the Seltra with its 4 panel construction when compared to a standard square mesh panel, is that this section the net takes up a more open shape in the water with smoother water flow through it. This allows the fish time to swim for longer in the area beneath the large mesh top panel giving them time to recover and find the escape area. As with a square mesh panel the Seltra is not that efficient at reducing the by-catch of the smaller fish (below 160mm). This is probably due to the fact that they are not passing close to the top panel of the net, small fish tend to stay low when in a trawl, close to the bottom panel of netting or just above it clear of any sand cloud passing down the trawl.
The Seltra box is a selective device that originates from Denmark and Sweden where the standard two panel trawl is transformed into a four panel ‘box’ section in the extension of the net. It is transformed to four panel because this configuration is found that the net maintains a more open cross section. This helps to smooth out the water flow into the codend and allows the captured fish swim leisurely for a longer before falling back into the codend. Giving them a greater time to find an escape route. For many fish such as haddock and whiting the escape route will be upwards, for this reason the top panel of the Seltra is made from either larger diamond mesh 270mm in some fisheries or large square mesh , anything from 140mm to 300mm square mesh is commonly used. This device can be used with the standard two panel codend or the four panels can be continued to form a four panel codend. The four panel is probably better as it helps to maintain the open shape of the extension right down into the codend. This allows the fish space with in the codend to keep swimming in the relatively slow water flow in this area and helps to prevent the catch from getting packed into the codend where they are susceptible to damage through contact with other fish and shellfish in the codend
As most demersal trawl nets nowadays are made using two panels laced together at the sides, to use a Seltra device there has to be some way of converting this to four panels. The four panel section can be laced straight onto the end of a two panel net of the same circumference. There is a tendency for a two panel section to take up a cross sectional shape of a flattened oval whereas a four panel section keeps more open in a squared of circular shape giving the fish more space to swim in. If the four panel is laced straight onto the two panel net it is likely that the two panel section will influence the opening of the four panel section. To maximise the benefits of the Seltra it is better to fit an adapter section by re-cutting the last tapered section of the trawl and fitting two triangular side panels. This helps to let the Seltra stay more open. Similar is true at the aft end of the Seltra, a four panel codend will help to keep the section more open compared to a two panel. If a two panel codend is the be used it is better to fit a short adapter section with triangular side panels to transfer the net back to two panels. See net drawings of the Seltra in the technical section.
The Seltra is a fairly easy modification to make to a trawl to improve selectivity and should cause no problems with handling the trawl during shooting and hauling.
Caveat: All dimensions, mesh counts and cutting rates 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.