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Purse-seine nets are set vertically in the water, with floats attached to the upper edge, while along the lower edge is a chain, for weight, and a series of rings, through which the pursing cable passes. Purse-seine nets can be as long as 1.5 km and more than 150 m deep. On sighting a school of tuna, a large skiff with the end of the net attached is released from the stern of the fishing vessel. The vessel encircles the school with the net. The cable is hauled aboard the vessel, causing the bottom of the net to close, and the fish are trapped inside the pursed net. Most of the net is then pulled aboard the vessel, confining the fish in a “sack,” from which they are transferred to the deck of the vessel. Tuna purse-seiners vary in length between about 30 to 115 meters, and can pack on board up to 4 000 tonnes of frozen fish. However, most high-seas tuna seiners average about 70 to 80 meters in length and can carry about 1 000 to 1 500 tonnes of frozen tuna. Such vessels can fish throughout the oceans of the world, and make trips that last up to several months before returning to port. Many carry helicopters to improve their efficiency in finding and catching fish.
Purse seiners target mostly yellowfin tuna and skipjack, and on a world scale account for roughly 60% of all the tuna landed. In recent years the purse-seine catch of bigeye tuna has been increasing rapidly, mostly due to the increased use of FADs.