Commercial Ahi Fishing
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Purse seining is the practice of catching fish with a large, surrounding net, and is commonly applied to fishing Ahi or Yellowfin Tuna.
This method overtook fishing industries in the 1960s and 1970s and accounts for most of the commercial fishing catch than any other method. It is most dominantly employed in the Pacific Ocean specifically in the easter Pacific in the historic tuna grounds of San Diego, and in the islands of the western Pacific. Significant purse-seine catches also occur in the tropical Atlantic Ocean, especially in the Gulf of Guinea (by Spanish and French fishermen) and in the Indian Ocean.
To locate tuna, purse seine ships use onboard lookouts as well as sophisticated electronics, sea-surface temperature indicators and satellite observances. Overhead helicopters also assist the locating of tuna schools. Once the school is located, the net is set around it. A single set of fish may yeild 100 tonnes. Modern vessels have a capacity of up to 2,000 metric tons, can reach speeds over 17 knots, and carry multiple helicopters.
Rod and Line Fishing
Historically, most of the commercial tuna catch was made with pole and lines and the use of live bait to attract schools close to the ships. Once close, the fish were caught with baited jigs on sturdy bamboo or fiberglass poles, or on handlines. This fishery practice reached its heyday between WWI and 1950, before declining.
Today, pole and line fishing is still used in Maldives, Ghana, the Canary Islands, Maderia, and the Azores. However, few pole and line boats target yellowfin. Usually skipjack tuna and juvenile yellowfin (much smaller) are associated with the rod and line fishing method.
The catching of yellowfin with purse seining way became highly controversial in the '70s when other oceanic life that accompanies the fish became impacted by fishing. Marine life such as dolphins and porpoises became caught in the nets, generating public outcry. This unintended catch is known as bycatch, and affects many marine species, sometimes to critical species populations.
Dolphin-friendly labeling was introduced, spurring purse seine sets to be made on 'free schools' of tuna, unassociated with dolphins. Additionally they are unassociated with floating objects -- another association between tuna locations and unintended bycatch. Floating objects often house manta rays, sea turtles, sharks, billfish, and other threatened marine species, thus the impact of bycatch can be major, ecologically.
Tuna caught in 'dolphin-friendly' means are often significantly smaller than larger, adult tuna that roams with dolphins. While the conservation efforts cannot be discounted and significantly aid dolphins, other by catch species are not addressed with 'dolphin-friendly' labeling, and it remains to be seen what the consequences of harvesting fish that have yet to reach breeding age may be.
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