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Skipjack tuna (Katsuwonus pelamis)

Also known as: Ocean bonito, lesser tuna, aku, katsuo.

Illustration: Sarah Rot

Morphology & Biology

Skipjack tuna are the smallest of the tuna species, growing to a maximum of 110 cm in length. They have a streamlined body that is mostly without scales, except on the lateral line and just behind the head where thick scales form a circle around their body. Hints of dark purple-blue are found on the skipjack’s back and their lower sides and bellies are silver with dark bands. Skipjack tuna lack a swim bladder and therefore need to keep swimming at all times to avoid sinking.

They are at the top of the food chain and feed on squid, smaller fish and crustaceans. Skipjack tuna reach sexual maturity at age 1-2 and can live up to 8 years. They are the most reproductive of the tuna, and reproduce all-year round. In tropical seas female skipjack tuna can spawn almost daily with their eggs hatching in as little as one day after fertilization (depending on water temperature). Skipjack tuna normally gather in large schools in surface waters. Schools are often associated with birds, sharks, whales or other tuna species. They will often be found swimming at or near the surface at night, but can dive to depths of over 250 m during daylight hours.

Despite being an apex predator and at the top of the food chain, skipjack tuna do sometimes find themselves in the situation of being prey to larger tuna species, sharks, billfish and even other skipjack tuna.

A skipjack tuna in the ocean ocean (Source: enviearth)

Distribution

Source: http://www.fao.org/figis/geoserver/factsheets/species.html?species=SKJ-m&prj=4326

The skipjack tuna lives in large shoals and is found in offshore tropical and warm-temperate waters. They occur in the Eastern Pacific from British Columbia to northern Chile, throughout the warm Atlantic, including the Caribbean and the Gulf of Mexico and in the Mediterranean. Skipjacks are thought to have a north to south migratory seasonal pattern but it is still unknown whether or not these tuna migrate with a specific purpose.

Threats

The greatest threat to skipjack tuna is fisheries: they make up 60% of the commercial tuna catch worldwide and are mostly used for canning. In some areas the use of fish aggregating devices (FADs) are killing large numbers of juvenile skipjack tuna, a trend that is threatening the survival of the population. Skipjack are used as a food source for people, however they are more than just seafood. Tuna are a top predator in the marine food chain, maintaining a balance in the ocean environment. Being a keystone species means that if their numbers continue to decline, devastating consequences would be found all across the ecosystem.

Additionally, the tuna fishery practices cause great threats to other species. The impact of tuna fisheries on dolphins is well known, and has been dealt with to some extent by a change in gear types to allow dolphins accidentally caught in a net to escape (assuming they’ve not been injured or killed during the process of being caught in the net). Despite the improvements made on dolphin mortality in the hunt for tuna, long liners targeting tuna regularly catch sharks, turtles and albatross as bycatch and in some regions it is thought that 4 sharks are caught for every one tuna.

Canned tuna at a supermarket commonly consists of skipjack tuna (Photo: Daniel Case).

Fisheries & Aquaculture

While skipjack tuna are currently classified as “Least Concern” in the IUCN’s Red list and many consider the landings to be sustainable (45% of landings meet the standards of the Seafood Watch’s “Good Alternative” rating and 7% of the global landings are certified as sustainable by the Marine Stewardship Council), the method of capture is extremely damaging to surrounding wildlife. With most of the landings coming from purse seine nets (often with the use of FADs), the resultant bycatch from the Skipjack fishery is thought to be extensive, with unregulated damage being inflicted on oceanic sharks, dolphins, turtles, and rays.

Source: http://www.fao.org/fishery/species/2494/en

Fun fact

Skipjack tuna is a high speed hunter capable of chasing down prey including fishes and squid but when they get hungry they are known to turn on others around them and chase down and eat smaller skipjack tuna.

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