A selective box trawl is a trawl that is made up of 4 panels of netting, a top panel, a lower panel and two side panels with these 4 panels carried right down to a 4 panel cod-end. A 4 panel net will natural 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 the same in a box or four 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 two panel trawl as the drag in the codend 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 four panel construction there are many more opportunities to include secondary selective devices. The 4 panels can be carried right back into a four panel cod-end and extension to improve water flow etc. With a 'box' cross sectional shape it lend itself to the fitting of a rectangular rigid or flexible grid with out any distortion of the panels of netting. Also, with four 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.