Purse Seine Fishing
A purse seine, which is attached to a boat, is a weighted net that hangs vertically in the water with floats on the top and rings on the bottom with a rope running through them. When a fishing vessel encircles a school of fish, the rope is tightened preventing the fish from escaping. This fishing method is commonly used to capture fish species that travel in schools close to the surface such as certain species of tuna, sardines, herring and salmon.
In the 1950's, fisherman in the eastern tropical Pacific Ocean discovered that large yellowfin tuna congregate under pods of pantropical spotted, spinner, and common dolphins. Fishermen began to seek out dolphins to guide them towards the tuna. After herding the dolphins into a tight group using speed boats, they surround them with a purse seine net, which encloses the dolphins and the tuna below when pulled tight. This chase, which can take up to two hours, traumatizes and confuses the dolphins so that even if they are given the opportunity to escape, they are often unable to do so. For those who survive, there is no way of knowing how this disturbance affects them. The dolphins are hauled aboard with the tuna and are usually discarded as bycatch. Survivors may endure this unpleasant capture and release many times throughout their lives.
The tuna industry in the Eastern Tropical Pacific alone is believed responsible for over seven million dolphin deaths since the advent of purse seine fishing there in 1959. Attempts by the U.S. government to ban import of tuna caught via purse seines that harm dolphins, and alternately to create a robust “dolphin safe” tuna label so consumers could avoid tuna caught using such methods, have been challenged as “restrictions on free trade” through the World Trade Organization and other international trade forums.