Target
A wide variety of benthic and demersal species (those that spend most of their lives on or near the seabed), including cods, haddock, flatfish and shrimp.
How they work
Bottom gillnets or 'set nets' are fine-filament nets, the lower edge of which touch the seabed, and are held in place by numerous floats, weights and anchors. If a fish's head goes through the net but its body can't follow, it is 'gilled' or entangled in the netting when it tries to get out. Gillnets are used either alone or in large numbers placed in a row. Trammel nets, a type of gillnet, are bottom-set nets made with three layers of netting, the two outer walls being of a larger mesh size than the loosely hung inner panel. The fish get entangled in the inner small-meshed panel after passing through one of the outer walls.
Advantages
When set correctly and with the right mesh size, these nets can be highly size selective: small fish can swim right throught the net while larger fish will not get their heads stuck. Bottom gillnets have a much lower impact on the seabed than other bottom gears such as trawls.
Problems
Poorly set gillnets or a bad choice of mesh size can result in higher levels of bycatch. Lost or abandoned nets continue to entangle marine creatures - known as ghost netting. Gillnets are associated with bycatch of marine mammals. Acoustic deterrents or 'pingers' attached to fishing nets may discourage some marine mammals from swimming too close and getting caught; however, trials of these have shown very mixed results for effectiveness, depending on the fishing area and the mammal species.