Purse seines are used to catch entire schools of fish. Fishers can identify schools by keeping watch for associated whitewater, dolphins, or seabirds at the surface, while scanning the depths with echo-sounders and sonar. Larger operations send spotter planes or helicopters to look for schools and notify the main fishing boat.
When a school is found, the main vessel and assisting skiff encircle the fish with a net. The bottom is pulled closed around the fish with a drawstring known as a purse cable. The net is then pulled out of the water by the main vessel like a giant "purse" full of fish. More tuna is caught by purse seines than by any other fishery (Hall, IATTC).
Purse seiners can also set their nets on passing debris (known as "logs") or fish aggregating devices (FADs), since fish are attracted to floating objects. At night, fish can be attracted to the net by shining light into the water. Different methods for targeting schools of tuna are known as "fishing on schools," "fishing on dolphins," or "fishing on logs," when whitewater, dolphins, or floating objects are used. (Hall, IATTC)
Fishing on dolphins yields larger yellowfin tuna than other purse seine sets, but can lead to incidental catch of dolphins. Helicopters may be used to spot dolphins, leading to their pursuit and capture. Specific methods for releasing dolphins from the net have been developed, allowing bycatch to be reduced in some regions.
Fishing on schools is sometimes less successful than other methods because the school of fish can disperse before the net is drawn. School sets are often coastal, and are especially common along the west coast of Baja California and the mouth of the Gulf of California (Mexico), the Gulf of Tehuantepec (Mexico) and the Gulf of Guayaquil (Ecuador). (Hall, IATTC)
Purse seines can also be set around any object floating at or near the surface, from tree trunks to sea turtles, whales, or discarded fishing gear. In coastal areas of Central America, fishing on tree trunks is concentrated where rivers carry debris from forested areas (Hall, IATTC). Many other kinds of fish, sharks, and birds also associate with floating objects, leading to a wider range of bycatch than for other purse seine strategies. (Hall, IATTC)
Kinds of purse seines
Schooling fish of all sizes, including anchovy, herring, mackerel, pilchards, salmon, sardines, tuna
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