Pacific Skipjack Tuna (Katsuwonus pelamis)

• Population level estimates for Pacific skipjack tuna are high and no overfishing is occurring.
• Scientists have concluded that while current populations are healthy, there is high variability in population size, which is a very important consideration for international and national managers.
• Skipjack is an excellent source of low-fat protein. For more information, see Nutrition Facts. (USDA)
• Skipjack are primarily sold to the canning market.

Sustainability Status

Biomass: Unknown
Overfishing: No
Overfished: No
Fishing and habitat: In general, highly migratory species (HMS) fishing gear is not known to directly alter water column habitat because the approved gears that are used in the HMS fisheries do not contact the bottom substrate, but habitat can be affected by inadvertent loss of gear that is left to "ghost fish," and create marine debris that can cause harm to other species in the pelagic environment.
Bycatch:Skipjack tuna are mostly caught with purse seines (dominant gear), but are also sometimes caught on pole-and-line gear (baitboats) and longlines. Bycatch in the purse seine fishery is a contested issue and is managed by international and high seas treaties. Longline vessels encounter endangered and threatened species of sea turtles and marine mammals while targeting highly migratory species (HMS). Longline vessels also encounter a number of sea birds, including the endangered short-tailed albatross. Measures have been adopted to minimize interactions of gears with protected species and to ensure that the fisheries are operating consistent with Federal law. An array of time and area closures and gear specifications were adopted into the HMS FMP in order to protect marine mammals and endangered sea turtles in the Eastern Pacific Ocean. Measures have also been implemented in the Western and Central Pacific to reduce interactions with sea turtles.
Aquaculture: There is currently no commercial aquaculture of skipjack tuna in the United States.

Science and Management

In the Pacific Ocean, both the Pacific Fishery Management Council and the Western Pacific Fishery Management Council have jurisdiction over skipjack tuna. Specifically, skipjack is managed under both the Fishery Management Plan (FMP) for West Coast Highly Migratory Species and the Western Pacific Pelagics FMP in the Eastern Pacific Ocean. In the Western and Central Pacific Ocean, skipjack is only managed under the Western Pacific Pelagics FMP.

Because highly migratory species move throughout large areas of the Pacific and are fished by many nations and gear types, management by the United States alone is not enough to ensure that harvests are sustainable in the long term. A new intergovernmental organization, called the Central and Western Pacific Fisheries Commission, was established in 2004 to coordinate management of HMS in the western and central Pacific. Although the United States is a signatory to the convention establishing this organization, until it is ratified we merit observer status only.

The United States has responsibilities under the United Nations Agreement on the Conservation and Management of Straddling Fish Stocks and Highly Migratory Fish Stocks (known as the UNFSA). The United States is also a member of the Food and Agriculture Organization of the United Nations (FAO), which has implications for HMS management. In 1995 the FAO's Committee on Fisheries developed a Code of Conduct for Responsible Fisheries, adopted by more than 170 member countries, including the United States.

Researchers at NMFS Southwest and Pacific Islands Fisheries Science Centers contribute to research on Pacific tropical tunas including skipjack by monitoring U.S. fisheries, monitoring and conducting stock assessments, and participating in international forums for the assessment of these species.

Life History and Habitat

Life history, including information on the habitat, growth, feeding, and reproduction of a species, is important because it affects how a fishery is managed.

• Geographic range: Skipjack tuna form schools in the surface water of tropical, subtropical, and warm temperate waters of all oceans.
• Habitat: Skipjack tuna are a pelagic species, which means they mostly live in the open ocean, although they may spend part of their life cycle in nearshore waters. They are found in surface waters and to depths of 850 feet during the day, but seem to stay near the surface at night. Skipjack tuna prefer warm, well-mixed surface waters and are often found in large surface swimming schools throughout the Pacific.
• Life span: Maximum age is estimated to be 8 to 12 years.
• Food: Skipjack tuna are opportunistic foragers. They feed on fishes, crustaceans, cephalopods and mollusks. Cannibalism is also common.
• Growth rate: Rapid. It has been estimated that a 1-year-old skipjack measures 0.83 to1.3 feet in length and can grow to 1.75 to 3 feet long by age 4.
• Maximum size: The maximum reported size of a skipjack tuna is about 3.3 feet, and the maximum weight is about 41 pounds.
• Reaches reproductive maturity: Skipjack mature early. The minimum length for females at maturity is 15.75 inches. Spawning usually occurs between 15.75 and 17.7 inches. Skipjack are estimated to be about 1 year old or less at this size.
• Reproduction: Females produce between 100,000 to 2 million eggs per spawning event, depending on the size of the fish. Once fertilized, eggs hatch in about 1 day (depending on temperature). Skipjack spawn more than once a season, as often as every 1.18 days.
• Spawning season: Skipjack tuna spawn throughout the year in tropical waters and seasonally, spring to early fall, in subtropical areas.
• Spawning grounds: In the tropics, eggs are released in several portions of the ocean.
• Migrations: Skipjack tuna is a highly migratory species, which means it has a wide geographic distribution. The species undertakes migrations of significant but variable distances across oceans for feeding and reproduction.
• Predators: Skipjack tuna are preyed upon by large pelagic fishes such as billfish, sharks, and other large tunas. Humans are also predators of skipjack tuna.
• Commercial or recreational interest: Both
• Distinguishing characteristics: Skipjack tuna do not have scales except on the corselet and the lateral line. Their back is dark purplish blue, and the lower sides and belly are silvery with 4 to 6 very conspicuous longitudinal dark bands, which may appear as continuous lines of dark blotches.

Role in the Ecosystem

The skipjack tuna life cycle progresses through four ecologically different phases. During spawning (in equatorial or warm tropical water) buoyant eggs are released and float at the surface where they become part of the zooplankton and food for the many organisms and small fish feeding in the equatorial surface waters. Within a day, the eggs hatch and the larval fish begin feeding on the same zooplankton that they are a part of. After 2-3 weeks, fully formed juveniles begin eating small fish, crustaceans, and mollusks. These juveniles also begin directed movement north and south of equatorial waters and are often preyed upon by larger tunas and billfish. After about one year, the adult skipjack has become an opportunistic predator with a highly varied diet and is now prey to not only larger tunas and billfish, but also to human fisheries activity.

Additional Information

Market name: Tuna
Vernacular names: Ocean Bonito, Lesser Tuna, Aku
Several other species are also marketed as Tuna.

Biomass

Biomass refers to the amount of skipjack tuna in the ocean. Scientists cannot collect and weigh every single fish to determine biomass, so they use models to estimate it instead. These biomass estimates can help determine if a stock is being fished too heavily or if it may be able to tolerate more fishing pressure. Managers can then make appropriate changes in the regulations of the fishery. While biomass estimates are currently unknown, skipjack tuna is believed to be exploited at sustainable levels.

Landings

Landings refer to the amount of catch that is brought to land. In the eastern Pacific, less than three vessels landed skipjack in 2006, compared to 10 vessels that landed 522 metric tons (1.15 million pounds) in 2005 with an ex-vessel revenue of $291,183. Annual landings have been declining following the historic high of 79,111 metric tons (179.4 million pounds) in 1980. Annual landings and ex-vessel revenues have been relatively flat since 1985, averaging 2,641 metric tons (5.8 million pounds) and $2.7 million.

In the western and central Pacific, estimated annual catch is over 800,000 metric tons (almost 1.8 billion pounds). The U.S. based fleets contribute about 1-2% of this total. Annual skipjack tuna landed by the U.S. in 2006 was 56,560 metric tons (123,458 pounds). Historically, in 1991, landings peaked at 183,000 metric tons (403,446 pounds) and subsequently declined to 61,400 metric tons (134,482 pounds) in 2007. Market sizes range from 1.5-13 kilograms (3-25 pounds).

Biomass and Landings

Are landings and biomass related? Landings are dependent on biomass, management measures in the fishery, and fishing effort.

Data sources:
Landings from NMFS Pacific Islands Fisheries Science Center

Important Dates

1987 – Western Pelagics Fishery Management Plan adopted
1992 – Western Pelagics FMP amended to include tunas (Amendment 6)
2004 – West Coast HMS FMP implemented

Notes and Links

General Information:
Pacific Fishery Management Council - Background on Highly Migratory Species

Managing Marine Fisheries of Hawaii and the U.S. Pacific Islands - Past, Present, and Future - Pelagic Fisheries

Fishery Management:
West Coast Highly Migratory Species FMP

Western Pacific Pelagics FMP

Stock Assessments:
Status of the U.S. West Coast Fisheries for Highly Migratory Species through 2006