Skipjack Tuna

The Basics

The skipjack tuna is a medium-sized predatory marine fish of the tuna family. It is common in tropical and subtropical waters throughout the world’s oceans and is a common target of commercial and sport fisheries.

Description

The skipjack tuna (Katsuwonus pelamis) is a member of the family Scombridae, which includes “true tuna” such as the yellowfin tuna and other members of the genus Thunnus. It is generally similar in appearance to other species of tuna, albeit significantly smaller.

The skipjack, which is also known as the arctic bonito, striped tuna, oceanic bonito, balaya, tongkol, and more, has a torpedo-like shape with a streamlined and muscular body and a cone-shaped head. It is silver on its ventral side and becomes dark-grey on its dorsal side. It has two dorsal fins, the most-forward of which is large and has a sail-like appearance, as well as two small anal fins.  Its pectoral fins are relatively small and it has 5-8 small finlets on each of the dorsal and ventral sides of its body between its posterior fins and large caudal or tail fin. Most individuals grow to about three feet in length and more than 20 pounds, although some large individuals have been landed, the largest being over 75 pounds.

Distribution and Habitat

The skipjack tuna is found throughout the world’s oceans in tropical and subtropical waters. They are generally pelagic, inhabiting offshore waters generally near the surface. They are known as a shoaling species, sometimes forming groups of up to 50,000 individuals. Although they tend to prefer surface waters, they are known to dive to depths of up to 900 feet, normally during the day, presumably in search of prey which also retreats to the darkness of the deep sea during the daylight hours.

Diet and Predators

Like other members of its family, the skipjack tuna is a predatory fish. It uses its strong, large muscles and efficient swimming stroke to ambush prey at high speeds. Often this prey may include other fish small enough for the skipjack to consume. They will also prey on other organisms such as various species of crustaceans and cephalopods.

In addition to being a voracious predator itself, the skipjack tuna also forms an important part of the diet of many other large predatory species. Large pelagic fishes such as marlin and other larger “true tuna” species will prey on the large shoals. In addition, larger predators such as dolphins and other toothed whales like orca will feed on skipjack, as will various species of sharks.

The skipjack tuna also forms important game and commercial fisheries and is one of the most common species of canned ‘tuna’. In 2009, for example, only the Peruvian anchoveta comprised a larger global catch of any one species. The skipjack is commonly fished in various parts of its range including the Maldives, western Europe, and Southeast Asia.

Reproduction

Like other members of its family, the skipjack tuna likely broadcast spawns, although their specific breeding behaviors remain poorly understood. Females release their eggs into the water column in batches throughout the spawning season, normally in rough synchronization with males. Near the equator, spawning occurs throughout the year, whereas in subtropical waters this behavior becomes more seasonal.

Like many other fish species, once external fertilization has occurred the embryo will develop into a larval fish and then a juvenile as part of the zooplankton. Once it is large enough, it will start to hunt other small animals. Juveniles will become sexually mature at approximately 2-4 years of age while the maximum life expectancy for most individuals is in the range of 8-12 years.

Conservation Status

The skipjack tuna is highly fecund, allowing it to reproduce efficiently and relatively rapidly in comparison with longer-lived species such as the bluefin tuna. Unlike other ‘fishery tuna’ species, the current skipjack fishery is considered sustainable. However, controversy still exists over the method by which this species is fished, where purse seine nets are used to round up large shoals all at once, often damaging habitat or catching a significant amount of bycatch in the process.

When purchasing canned skipjack, consumers can determine the method of fishing by which the fish was caught, with rod and reel fishing considered the most sustainable and environmentally friendly of all methods. Currently, the skipjack tuna is listed as Least Concern on the IUCN Red List of Threatened Species.

Fun Facts about Skipjack Tuna!

The skipjack tuna is known for having very few scales. Only a small part of its body has scales: its lateral line and its corselet, a band of large, thick scales that create a cross-sectional ring around the body just behind its head. This is but one of the many fun facts and interesting insights available to be explored through the skipjack tuna.

No Scales

The lateral line itself, present on the skipjack and other tuna species, is an interesting physiological feature. It is a sensory organ that allows fish the ability to detect minute movements, vibrations, and electrical impulses in their environments that wouldn’t otherwise be possible.

Externally, the lateral line appears as a line along each side of the animal. Internally, there is a ‘canal’-like structure that is filled with a fluid material known as endolymph. This is the same liquid that is in the inner ear of humans and other terrestrial animals. Organelles comprised of multiple hair cells closely bundled called neuromasts effectively connect this underlying sensory structure with the superficial structure. Armed with this unique sensory organ, fish including the skipjack tuna can sense mechanical changes in the water that surrounds them in ways that are difficult for humans to relate to.

Moderate Mercury

You may often hear that it is unwise to eat too much tuna due to potentially high levels of mercury. This is due to the phenomenon of bioaccumulation, in which contaminants are increasingly accumulated in the food chain as a result of their increasing presence in the environment. Ultimately, they aggregate in the flesh of the top predators in that food chain. In many cases, this includes tuna. As ambient levels of toxins such as mercury increase in natural environments, so does the risk of commercial species such as the skipjack of becoming contaminated.

Currently, the skipjack tuna is considered “Moderately Contaminated“, with mercury levels as low as 0.126 ppm, in contrast with the bigeye tuna which has levels more than 5 times higher. Because mercury is a potent neurotoxin that can have significant health impacts on humans, it is important to be aware of how likely it is for our foods to be contaminated, as well as how frequently we consume these foods.

Stay-at-Home Shoals

While other tuna species are generally described as schooling due to their largely migratory nature, large groups of skipjack are typically referred to as shoals. This is due to the nature of the group and its collective behavior itself.

In the case of schooling tuna species such as the yellowfin tuna, they will regularly migrate long distances throughout their range, often schooling with other similarly-sized fish along the way. Skipjack tuna likely do participate in some seasonal north-south migration in response to changes in water temperature and prey availability, but they are not considered to be a migratory species in the way that other ‘true tuna’ are. Because shoaling is a term reserved for a group of fish that don’t necessarily move in a particular direction in unison or with any collective ‘purpose’, groups of skipjack tuna are referred to as shoals.