How was your tuna caught?

Purse seines are large nets that can measure over 2 km long and 200 metres deep. They are deployed in a circular form around a school of tuna, hanging vertically in the water column. Once the fish are completely encircled by the net, it is drawn tight at the bottom, like a purse, to prevent the fish from escaping below. It is then brought alongside the fishing vessel, hoisted out of the water, and the fish are brought on board. Purse seines are used to target mostly yellowfin tuna and skipjack, and on a world scale account for roughly 60% of all the tuna landed. They are the preferred fishing gear of the French and Spanish fleets.

Purse-seine vessels often use fish aggregation devices (FAD’s) - large floating platforms, often trailing streamers which attract large shoals of pelagic fish, which in turn attract larger predatory fish such as tuna. Over one million tonnes of tuna are caught in the vicinity of FAD’s each year. They often have satellite transmitters, allowing fishers to find them more easily, ever increasing the ruthless efficiency of tuna fishing operations.

This type of gear involves the use of a main line of up to 150 km in length from which as many as 3,000 shorter branch lines, each with a baited hook, are dangled in the water column. The mainline is kept afloat by a series of buoys attached at intervals. The gear is passive, in that it captures whatever fish happen to take the bait. Longlines operate mostly at depths between 100 and 150 metres, but can be set as deep as 300 metres when targeting bigeye.

Longlines are used to catch the high-value fish that are marketed as sashimi, historically in the Japanese market but also increasingly in North America and Europe. Since very high quality fish is needed for sashimi, most vessels are equipped with "flash freezers" to freeze the fish to -60oC almost immediately. Taiwan and Japan are specialists in longlining, targeting primarily bigeye, with some yellowfin and albacore.

Pole and line fishing has been practised for centuries in several different parts of the world. The method involves attracting a school of tuna to the side of a "bait-boat" by throwing live fish (caught previously) overboard. This creates a tuna "feeding frenzy" and fish are hauled out of the water, one-by-one, using pole and line. The size of the tuna caught this way is small, mostly consisting of skipjack, but also some yellowfin and bigeye. Many countries use this technology but the most important fleet of industrialized baitboats is based in Ghana.

Most fisheries accidentally catch unwanted animals along with their target catch. This non-target catch, known as "bycatch", is normally thrown back into the ocean, dead or dying. Tuna fishing is no exception to this rule. Longlines, for instance, can catch sharks, rays, sea turtles, seabirds and many species of fish. Globally, it has been estimated that 200,000 loggerheads and 50,000 leatherback sea turtles are hooked by longlines every year.

Purse seines are no better, with bycatch consisting of a diverse array of marine life, including dolphin fish, billfish, wahoo, triggerfish, barracuda, rainbow runners, sharks and sea turtles, especially when used in conjunction with floating FADs. Around100,000 tonnes of bycatch, such as sharks and turtles are caught in the vicinity of FAD’s each year. The use of FADs is also associated with the capture of very high levels of juvenile tuna, especially bigeye, which end up as bycatch due to their small size. In fact, one study reported that almost 20% of the tuna catch is liable to be discarded from these fisheries because it is below the minimum market size. Obviously, the capture of large numbers of juveniles is a serious concern for the health of the stock as it depletes the source of future adult fish.

There is virtually no bycatch associated with the pole and line technique, which is regarded as the least bad fishing method for tuna, a fact worth remembering if you buy. In most cases, however, labelling does not include information on the catch method. If it is absent, assume the worst.