A selective box trawl is a trawl that is made up of 4 panels of netting, a top panel, a lower panel and 2 side panels with these 4 panels carried right down to a 4 panel cod-end. A 4 panel net will naturally take up a more open cross sectional shape compared to a 2 panel net and cod-end. This allows the catch more space to swim freely in thereby more time for the smaller ones to escape through the diamond meshes. It also gives more opportunity to include other selective devices to further improve either size species selectivity.
In a standard diamond mesh trawl the selectivity is dependant on the mesh sized used. This is also the same for a box or a 4 panel trawl. However, by fitting different panels with varying tensions it is possible to open the meshes more in certain areas and allow small fish to escape.
The main criteria for this being included as a selective device is the way that the after end of the trawl does not close up to the same degree as a 2 panel trawl as the drag in the cod-end increases. This helps to give fish more time to escape through the diamond meshes.
The reason that a box trawl is included as a selective device, is that due to the 4 panel construction there are many more opportunities to include secondary selective devices. The 4 panels can be carried right back into a 4 panel cod-end and extension to improve water flow. With a 'box' cross sectional shape it lends itself to the fitting of a rectangular rigid or flexible grid without any distortion of the panels of netting. Also, with 4 panels different selective panels can be fitted into the sides, top or bottom panels to target different species of fish depending on their natural escape stimulus.