Skipjack Tuna and Bonito | Sea Life, Islands and Oceania

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SKIPJACK TUNA

Commonly used in canned tuna, skipjack tuna reach lengths of two and three feet. They live in tropical and subtropical waters. One of the most heavily fished of all fishes, they are found in huge schools near the surface that are scooped up with purse seine nets and caught with hooks and lines. Skipjack tuna stock are still healthy. Skipjack tuna closely resemble bonitos. Skipjack tuna are often called a bonitos, especially in Japanese contexts. Bonitos technically are different group of tuna-like fish but are not tuna (Bonitos, See Below). [Source: NOAA, Wikipedia]

Skipjack tuna (Scientific name: Katsuwonus pelamis) grow fast, like other tropical tunas, up to nearly 1.3 meters 4 feet and more than 30 interesting (70 pounds.) They have a short life span compared to other temperate tunas, around 8 to 12 years. They are opportunistic feeders, preying on a variety of fish, crustaceans, cephalopods, mollusks, and sometimes other skipjack tunas. Large pelagic fishes such as billfish, sharks, and other large tunas prey on skipjack tuna..

Also known as ocean bonito, lesser tuna, aku and katsuo, skipjack tuna do not have scales except on the corselet and the lateral line. The corselet is a band of large, thick scales forming a circle around the body behind the head and extending backward along the lateral line. The lateral line is a faint line running lengthwise down each side of the fish. Their back is dark purplish-blue, and their lower sides and belly are silvery with four to six conspicuous dark bands that run from behind the head to the tail, which may look like a series of dark blotches.

In the Pacific, skipjack are able to reproduce when they reach about 40 centimeters (1.3 feet) in length. They spawn throughout the year in tropical waters and seasonally (spring to early fall) in subtropical waters. Skipjack spawn more than once a season, and some spawn almost every day. Depending on their size, females produce between 100,000 and 2 million eggs each time they spawn. Once fertilized, the eggs hatch in about 1 day, depending on temperature.

Skipjack tuna are found in tropical and subtropical oceans around the world, including the waters around the U.S. Pacific Islands and the U.S. West Coast. They are a pelagic species — they mostly live in the open ocean, although they may spend part of their life in nearshore waters. They can be found in large schools swimming in warm, well-mixed surface waters and to depths of 850 feet during the day. They generally stay near the surface at night.. Skipjack tuna is a highly migratory species, swimming long distances to feed and reproduce.

Websites and Resources: “Netting Billions 2020: A Global Tuna Valuation, Pew Charitable Trusts”, October 6, 2020 pewtrusts.org; Animal Diversity Web (ADW) animaldiversity.org; National Oceanic and Atmospheric Administration (NOAA) noaa.gov; Fishbase fishbase.se ; Encyclopedia of Life eol.org ; Smithsonian Oceans Portal ocean.si.edu/ocean-life-ecosystems

Skipjack Tuna Fishing

2,795,339 tonnes (1000 kilograms) of skipjack tuna was caught globally in 2012, the third most among wild-caught fish according to the Food and Agriculture Organization of the United Nations (FAO). Most of the global harvest of Pacific skipjack tuna comes from the Western and Central Pacific Ocean (WCPO). U.S. fisheries typically account for less than 10 percent of harvest from the WCPO. In 2021, the commercial landings in the U.S. of Pacific skipjack tuna totaled 131,000 kilograms (290,000) pounds and were valued at $570,000, according to the NOAA Fisheries commercial fishing landings database (these landings do not include skipjack tuna landed outside the U.S.). The majority of landings of non-purse seine caught skipjack tuna are in Hawaii, with some landings in American Samoa, the Commonwealth of the Northern Mariana Islands, and Guam. [Source: NOAA]

Purse seines are primarily used to catch Pacific skipjack tuna. Skipjack tuna is also caught in longline, pole-and-line, and troll fisheries. Fishing gear used to catch Pacific skipjack tuna rarely contacts the seafloor so habitat impacts are minimal. Interactions with protected species such as sea turtles, marine mammals, and seabirds in these fisheries are rare and survival rates are estimated to be high for all gear types.

Atlantic Skipjack

Many purse seine fishermen use fish aggregating devices (FADs) to target tunas. FADs can be drifting, floating or submerged objects deployed and tracked by vessels, including through the use of radio or satellite buoys, for the purpose of aggregating target tuna species. FADs are also known to attract non-target and bycatch species, including juvenile tunas, sharks, other fish, and occasionally protected species. Use of FADS is prohibited in certain areas at certain times of the year to regulate fishing effort and reduce bycatch of juvenile tunas. Longline and purse seine fishermen are trained in safe handling and release techniques for sea turtles, seabirds, and marine mammals, and they carry and use specific equipment for handling and releasing these animals. Scientists and managers continue to monitor bycatch in these fisheries through logbooks and fishery observer programs.

Recreational anglers fish for skipjack tuna with troll, rod-and-reel, and handline gear and sometimes by free-diving with spear guns. Off California, anglers must be licensed and must follow daily bag limits. Recreational charter boats must keep logbooks of their catch. There are no federal regulations for recreational fishing off Hawaii and U.S. Pacific Island territories, but local rules may apply. In 2019, recreational anglers landed more than 1.2 million kilograms (2.7 million pounds) of skipjack tuna, according to the NOAA Fisheries recreational fishing landings database. These figures may not match other agency sources of data due to confidential information.

Skipjack Tuna Fishing Management

Two international organizations, Inter-American Tropical Tuna Commission (IATTC) and the Western and Central Pacific Fisheries Commission (WCPFC), manage this fishery internationally. Working with the U.S. Department of State, NOAA Fisheries domestically implements the IATTC and WCPFC conservation and management measures. The United States has implemented conservation and management measures adopted by the IATTC to control effort in the tuna purse seine fishery and reduce impacts to other species such as sea turtles, sharks, seabirds, and juvenile tunas. They include 1) time-area closures to reduce the catch of juvenile tunas; and 2) Required retention of tuna caught in the purse seine fishery. [Source: NOAA]

U.S. wild-caught Pacific skipjack tuna is a smart seafood choice because it is sustainably managed and responsibly harvested under U.S. regulations. Populations are assumed to be above target population levels in the Eastern Pacific. Near target levels and fishing rate promotes population growth in the Western and Central Pacific. The fishing rate is at recommended levels. Fishing gear used to catch skipjack tuna rarely contacts the ocean floor so habitat impacts are minimal. Regulations are in place to minimize bycatch.

There are two stocks of skipjack tuna: the Eastern Pacific stock and the Western and Central Pacific stock. According to the most recent stock assessments they are not overfished and not subject to overfishing (2004 and 2019 stock assessment).

NOAA Fisheries and the Pacific Fishery Management Council manage this fishery on the West Coast. The species are managed under the Fishery Management Plan for U.S. West Coast Fisheries for Highly Migratory Species, which says: 1) Fishermen must have a permit to harvest tuna and must keep logbooks documenting their catch. 2) Gear restrictions are in place to minimize bycatch. 3) Large purse seine vessels that fish for tuna in the eastern Pacific Ocean are required to have 100 percent observer coverage. 4) All other purse seine vessels must carry a fishery observer, if requested by NOAA Fisheries.

Skipjack tuna shoal

NOAA Fisheries and Western Pacific Fishery Management Council manage this fishery in the Pacific Islands. The species are managed under the Fishery Ecosystem Plan for the Pelagic Fisheries of the Western Pacific, which requires fishermen to have a permit to harvest tuna and must keep logbooks documenting their catch. Gear restrictions and operational requirements minimize bycatch. A limit on the number of permits for Hawaii and American Samoa longline fisheries controls participation in the fishery.

Management of highly migratory species, like Pacific skipjack tuna, is complicated because the species migrate thousands of miles across international boundaries and are fished by many nations. Effective conservation and management of this resource requires international cooperation as well as strong domestic management.

The United States has also implemented conservation and management measures adopted by the WCPFC to control juvenile tuna catch in the purse seine fishery targeting skipjack and to minimize impacts to non-target species such as sea turtles and sharks. 1) Limits on the number of days purse seiners can spend fishing in certain areasl 2) A seasonal prohibition on the use of fish aggregating devices by purse seine vessels.; 3) Closure of specific high seas areas in the Western and Central Pacific to purse seine vessels; 4) A requirement for purse seine vessels to retain certain tuna species.

Purse seiners fishing in the WCPFC’s management area must also carry a fishery observer and must follow specific handling requirements in case they accidentally catch a sea turtle. Under the South Pacific Tuna Treaty, U.S. purse seine vessels operating throughout the Western and Central Pacific Ocean must be registered and are monitored through logbooks, cannery landing receipts, national surveillance activities, observers, and port sampling. Purse seiners in the Eastern Pacific also operate under the International Dolphin Conservation Program, a multilateral agreement aimed at reducing and minimizing bycatch of dolphins and undersize tuna. In 2000, the United States established the Dolphin-Safe Tuna Tracking and Verification Program to monitor the domestic production and importation of all frozen and processed tuna products nationwide and to authenticate any associated dolphin-safe claim.

Skipjack tuna is a notoriously difficult species to assess. Due to skipjack’s high and variable productivity, it’s difficult to determine the effect of fishing on the population using standard fisheries data and stock assessment methods. Assessments are particularly difficult for the Eastern Tropical Pacific stock due to limited data.

Bonito

Bonito is a name given to various species of medium-sized, predatory fish in the Scombridae family. 1) Bonito most commonly refers to species in the genus Sarda, including the Atlantic bonito (Sarda sarda) and the Pacific bonito (Sarda chiliensis lineolata). 2) In Japanese cuisine, bonito refers to the skipjack tuna (Katsuwonus pelamis), which, in Japan, is called by its local name, katsuo. 3) Bonito can generally refer to any of various scombroid fish related to, but smaller than, tuna. [Source: Wikipedia]

Atlantic bonito

Katsuo is used extensively in Japanese cusine. Aside from its prevalance as in raw preparation (such as sushi and sashimi), it is also smoked and dried to make katsuobushi, an important ingredient in dashi (a type of common Japanese fish stock). It is also a key ingredient in shiokara.

Pacific and Atlantic bonito meat has a firm texture and a darkish color. The bonito has a moderate fat content. The meat of young or small bonito can be of lighter color, close to that of skipjack tuna, and is sometimes used as a cheaper substitute of skipjack, especially for canning purposes. Bonito may not be marketed as tuna in all countries, however. In Spain, tuna (Thunnus alalunga) is sometimes referred as Bonito del Norte. The Atlantic bonito is also found in the Black Sea. Called palamut in Turkish, it grows to a size of 65 centimeters, and is a popular seasonal choice.

The Maldives are known for "Maldives fish," bonito or tuna that has been boiled, dried and smoked. Maldive fish is popular in India and Sri Lanka. Traditionally produced in Maldives it is made from lightly boiled, smoked and dried tuna or bonito. It is a staple of the Maldivian and Sri Lankan cuisine, as well as the cuisine of the Southern Indian states and territories of Lakshadweep, Kerala and Tamil Nadu. In the past it was one of the main exports from Maldives to Sri Lanka, where it is known as umbalaka. [Source: Wikipedia]

Fish caught in the Indian Ocean around the the Maldives including yellowfin tuna, skipjack, little tunny (known locally as la i) and frigate mackerel. All these fishes have been traditionally processed into Maldive Fish although bonito (related to mackerel and tuna) is most commonly used. On a good night, fishermen using simple bamboo poles, can catch 600 to 1,000 fish in two to three hours.

Dogtooth Tuna — a Kind of Bonito

By one reckoning there are nine bonito species. One of them, the dogtooth tuna is prized for its meat, which is whiter than that of other species of tuna, so they are widely sought after and highly prized both by commercial and sport fishermen. Dogtooth tuna (Gymnosarda unicolor) are found in the West Pacific and Pacific Oceans, from the Great Barrier Reef in Australia to East Africa and the Red Sea, and in the waters off the coasts of Japan, Philippines, New Guinea and French Polynesia. [Source: Kevin Samuels, Animal Diversity Web (ADW) /=]

dogtooth tuna

Dogtooth tuna feed upon shoaling fishes like herring, sprats, mackerel, whiting, cuttlefish and sometimes squid. They are primarily a pelagic (open ocean) fish, but occasionally come close to shore and are found around coral reefs and atolls at depths from 15 meters (50 foot) to 45 meters (150 foot). They prefer water temperatures between 21° and 26°C (70° and 80°F). They are migratory and their movements are linked to water temperatures and the fish they feed upon.

Dogtooth tuna are popular game fish. Many charter-fishing boats operate out of Australia and other parts of the South Pacific providing a very lucrative business for their owners. They are also marketed commercially either canned or frozen. Dogtooth tuna are susceptible to overfishing, with their biggest threat coming from and commercial net fishing. They have no special status according to the International Union for Conservation of Nature (IUCN) Red List and the Convention on the International Trade in Endangered Species (CITES).

Bonito Characteristics and Behavior

Members of the tuna and mackerel family, dogtooth tuna weigh up to 131 kilograms (288.55 pounds), with their average weight being 15-20 kilograms (33 to 44 pounds). Both sexes are roughly equal in size and look similar. Bonitos in general are endothermic (use their metabolism to generate heat and regulate body temperature independent of the temperatures around them) and cold blooded (ectothermic, use heat from the environment and adapt their behavior to regulate body temperature). Some are or have hints of being warm-blooded (homoiothermic, having a constant body temperature, usually higher than the temperature of their surroundings).

According to Animal Diversity Web: Distinguishing features include a streamlined body with a large head and a mouth that contains twenty sharp dog-like teeth per jaw. They have two dorsal fins; the first is spiny and large, and the second, right behind it is soft-rayed. The ventral fin is similar in size and shaped like the second dorsal. spiny finlets stretch down the upper and lower tail section toward its crescent shaped tailfin. This species exhibits counter shading and has no scales. The dorsal surface is blue green, the sides are silver, and the belly is white. They swim constantly with their mouth open to force water through the gills because of a high oxygen requirement and great muscular activity. An unusual vessel system in the liver and tail provides counter-current temperature exchange, raising the body temperature 6° to 12°C higher than the water temperature. They can reach speeds of up to 80 kph (50 miles per hour). The spear fishing record is 55 kilograms and the all-tackle reco pounds). (Fishbase, 2007; Grizimek and Ladiges, 1974; Maas, 1997) /=\

Dogtooth tuna are diurnal (active mainly during the daytime), motile (move around as opposed to being stationary), nomadic (move from place to place, generally within a well-defined range), and social (associates with others of its species; forms social groups). They sense using touch and chemicals usually detected with smell and usually form schools of individuals of the same relative size. Larger fishes are independent, but sometimes swim with grey reef sharks. When they encounter a school of food fishes in the open ocean, feeding frenzies are not uncommon. Often these frenzies result in injury to other dogtooth tunas. (Maas, 1997) /=\

Dogtooth tuna are oviparous (young are hatched from eggs) and iteroparous (offspring are produced in groups). They engage in seasonal breeding. Reproduction is external, meaning the male’s sperm fertilizes the female’s egg outside her body. Spawning takes place from December to February. Dogtooth Tuna are categorized as open water substratum egg scatterers. The eggs are small and float near the surface, hatching within two days. Larvae are .635 centimeters (0.25 inch) long and grow very quickly. There is no parental care.

Bonito, Japan and Bonito Fishing

dogtooth tuna in the Maldives

In November 2010, the Yomiuri Shimbun reported that the Japanese government is considering proposing a limit to the number of fishing boats that use encircling nets to catch bonitos, whose stocks are regulated by the Western and Central Pacific Fisheries Commission (WCPFC). Tokyo is mulling the proposal because the catch of bonitos that swim north to Japanese waters has been on the decline as the number of fishing boats with encircling nets, or seines, are increasing rapidly near the equator in the Pacific Ocean. Many such boats from Taiwan, China and other countries are working in the area to capitalize on a growing bonito boom in the United States and European countries, where the fish had not been consumed on a major scale in the past. [Source: Yomiuri Shimbun, November 2, 2010]

Bonito is a familiar part of the Japanese diet in dishes such as sashimi and the lightly seared fish meat called tataki. Bonitos come to Japan from areas near the equator, riding on the Kuroshio current. In Japan, bonitos have been caught by a traditional single-hook fishing method since the Edo period (1603-1867). The bonito catch using this method off Japanese shores exceeded 100,000 tons in 1991, but dropped to 30,475 tons in 2009. The Fisheries Agency attributes the fall to the sharp increase of fishing boats with seines that sweep up very young fish along with mature ones. According to the agency, the number of fishing boats with such nets near the equator increased by 64 to 221 in the past 10 years. Nearly 70 percent of the new boats are from Taiwan and China, the agency said.

According to the agency, the increase in fishing boats with encircling nets is partly due to the economic strategies of Fiji and other Pacific island nations. Since the 1980s, the island nations have encouraged other countries' fishing boats to operate in their exclusive economic zones. In exchange, the countries earned foreign currency through fishing fees and registration fees. In the United States and European countries, canned bonito is increasingly popular among a health-conscious public. Therefore, fishermen from Taiwan, China and other countries have flocked to equatorial fishing grounds to catch bonitos for export.

Though Japanese fishing boats with encircling nets have also entered those areas, the government has limited the number of Japanese boats to 35 for more than 10 years to prevent the bonito resources from running out. A long-term decline in the bonito catch has dealt a severe blow to domestic fishery operators. Hideo Kamimaki, 58, who runs a single-hook fishing operation in waters off Kesennuma, Miyagi Prefecture, said: "Twenty years ago, we saw a large school of bonitos swimming as if they were a river. But now, the volume looks like only one-tenth of what they were." According to the Fisheries Agency, the number of single-hook fishing operators in Japan decreased from 105 in 1998 to 58 in 2009 year because of financial difficulties. A 71-year-old fish wholesaler in Tokyo's Tsukiji market said: "Bonitos caught by single-hook fishing are very good in quality as they are free of blemishes on the body. But we've seen falling volume of shipments lately."

At the 10th meeting of the Conference of the Parties to the Convention on Biological Diversity, or COP10, in 2010, Ajinomoto Co., an umami seasoning manufacturer, demanded that bonito resources be preserved. Ajinomoto produces umami broth made from bonito. Aiko Yamauchi, WWF Japan fisheries officer, said international management systems to check the haul of bonito, like those to monitor bluefin tuna, should be introduced before bonito is overfished.

Image Source: Wikimedia Commons, NOAA, graphics from The Pew Charitable Trusts (“Netting Billions 2020: A Global Tuna Valuation, October 6, 2020 pewtrusts.org)

Text Sources: Animal Diversity Web (ADW) animaldiversity.org; National Oceanic and Atmospheric Administration (NOAA) noaa.gov; Wikipedia, National Geographic, Live Science, BBC, Smithsonian, New York Times, Washington Post, Los Angeles Times, The New Yorker, Reuters, Associated Press, Lonely Planet Guides and various books and other publications.

Last Updated April 2023