Longtail tuna, Thunnus tonggol, lives in the tropical and subtropical coastal waters of the Western Indian Ocean to the Western Pacific Ocean. It is not found in other oceans. Longtail tuna is found in water temperatures between 16℃ and 31℃. Juveniles and young adults prefer warmer waters and larger adults cooler waters. It lives in shallow waters less than 200 m depth and is most common at less than 50 m. It avoids water with low salinity and high turbidity. Unlike other Thunnus species, longtail tuna is not considered a highly migratory species.
Large and continuing increases in landings throughout its range have motivated interest in the longtail tuna population stock structure and stock assessment for management. From several initial genetic studies, two stocks appear to occur: one along coastal northwest India and a separate, single stock across the South China Sea. Earlier genetic studies suggested a single stock along the coast of Australia and Papua New Guinea. Limited tagging data indicates some fish are capable of swimming hundreds of km in several weeks, yet most tagged longtail tuna maintain small ranges of only a few km over several years.
Longtail tuna is a small to medium-size tuna species, reaching maximum length of 136 cm fork length, maximum weight of 36 kg, and can live for at least 18 years, although most fish harvested are much younger. Longtail tuna in the northern hemisphere appear to grow faster than those in the southern hemisphere.. Longtail tuna feeds on marine crustaceans, fish, and cephalopods, and has a relatively slower growth rate than other tuna of comparable size.
FISHERIES
Historically longtail tuna has not been valued as highly as other tuna species such as the oceanic skipjack, yellowfin, bigeye, albacore and bluefin tuna. Despite currently supporting a large fishery, management agencies do not consider it a major commercial species. Because longtail tuna occurs in shallow coastal waters, it has local importance to smaller fishers for whom it provides income and food.
The Western and Central Pacific and Indian Ocean produce a total catch of approximately 250,000 tonnes per year. Between 1960 and 2014, reported landings have ballooned from 0 to 155,000 tonnes. In 2008, catches peaked at nearly 300,000 tonnes. The majority of reported longtail tuna landings (99%) are in the coastal waters of the following seven developing countries, namely (in order of 2015 landings), Iran, Indonesia, Pakistan, India, Oman, Malaysia, and Thailand.
Longtail tuna is primarily fished using gillnets, purse seines and troll lines. Gillnets target larger, sexually mature fish whereas purse seines capture smaller size classes, including juveniles. Although multiple gear types are used to target longtail tuna, gillnets are more commonly used in Pakistan, India, and Iran, purse seines used in Malaysia and Thailand, and trolls used in Indonesia, Yemen, and Oman, with gear type data incomplete for some countries. Free schools are targeted by the above gear. Juvenile longtail tuna may also be caught by fishers targeting other species associated with FADs.
SUSTAINABILITY AND MANAGEMENT
Recent assessments consider the Indian Ocean stock to be both overfished and subject to overfishing. However, there remains considerable uncertainty over the stock structure and total catches in the Indian Ocean. Research on longtail tuna in the Western Pacific Ocean is not as advanced and the number and status of stocks is unclear. No longtail tuna fishery has received certification of sustainability.
Management measures specific to longtail tuna fisheries are limited or absent. In the Indian Ocean, longtail tuna assessment and research is overseen by the Indian Ocean Tuna Commission. Coordinated monitoring of Western and Central Pacific longtail tuna is currently not in place. Longtail tuna stock assessments have and are being conducted in certain nation states, and efforts are underway, notably by SEAFDEC (Southeast Asian Fisheries Development Centre), to coordinate future research and management efforts for part of the range. Presently, longtail tuna is not covered in the mandate of the Western and Central Pacific Fisheries Commission.
The growth of fishing pressure on longtail tuna warrants research and management considerations, but existing tools that are effective for oceanic tuna fisheries may not be appropriate for neritic species like longtail tuna. Further, longtail tuna are less likely to receive international attention and management given that the species stays in near-shore, shallow water and avoids deeper, international waters. Since longtail tuna is commonly caught in multispecies fisheries, general fishing effort controls may be required to sustain longtail tuna across the Indo-West Pacific.
VALUE CHAINS
Longtail tuna is sold canned, smoked and fresh, including as sashimi, in domestic Indo-West Pacific markets and on the global market. Longtail tuna canning facilities are located in India, Indonesia, Iran, and Thailand and are supplied with fish captured primarily from purse seines. As a white meat tuna, canned longtail tuna is popular in markets such as in the Middle East, that sell other white meat species such as albacore tuna. In Indonesia, longtail tuna is cured by smoking and sold as a smoked fish. In Japan, longtail tuna is used fresh in sashimi.
FOOD
Information on the complete nutritional value of longtail tuna is not available. Comparisons among tuna indicate longtail tuna has lower fat and protein content than skipjack and yellowfin tuna.
ECOSYSTEM AND CLIMATE
Fishing affects attributed specifically to longtail tuna are difficult to assess because the species is commonly caught in multispecies fisheries, and landing reports across the Indo-West Pacific region are variable or incomplete. The carbon footprint of fishing for longtail tuna is likely small given fishing gear is primarily passive and deployed from small vessels with small engines and no refrigeration. In contrast, the ecological footprint of fishing longtail tuna is likely significant due to the use of large drifting gillnets that indiscriminately catch multiple tuna as well as species of conservation significance, including cetaceans and sea turtles. Ghost fishing by lost or damaged gillnets may also contribute to the unintended, yet significant loss of pelagic and benthic animals.
Longtail tuna catch processing likely creates a significant ecological impact. As for other fish species, canning and fresh fish processing use large volumes of water; large scale production and dumping of wastewater can contain high levels of organic, inorganic and particulate matter that pollutes local waterways.
Global climate change impacts on longtail tuna are not known, but from available biological and ecological knowledge, climate change is predicted to impact abundance and distribution through water temperature and prey changes. In decades to come, longtail tuna fisheries may become less productive in the tropics and grow in the subtropical and temperate regions.