Schooling pelagic species (those that spend most of their lives in the mid-water, with little contact with the seabed) such as herring, mackerel, and tuna, and fish that gather to spawn such as squid.
How they work
Fish are encircled by a large 'wall' of net, which is then brought together to retain the fish by using a line at the bottom that enables the net to be closed like a purse.
This method can be highly specific, with little bycatch, when targeting adult schools of one species.
Some tuna fisheries set their nets on floating debris or on man-made 'fish aggregating devices' (FADs). These attract a range of fish, including tuna, sharks, turtles and marine mammals which come to feed and shelter at the FAD. When nets are set on these FADs, the resulting bycatch of juvenile tuna and other marine life is high. In the Eastern Tropical Pacific fishery where, unusually, schools of large yellowfin tuna associate with dolphins, purse seiners maximise their catches of yellowfin tuna by chasing and setting their nets on herds of dolphins. Prior to 1990, millions of dolphins were killed by this practice, but it has now been adapted to allow dolphins to escape alive, and has a very low bycatch of other marine life. However, the chasing and netting of dolphins can result in unseen dolphin deaths due to physiological stress, injuries from nets, and deaths of young calves separated from their mothers.