Salmon shark

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Salmon shark
Lamna ditropis.jpg
Scientific classification OOjs UI icon edit-ltr.svg
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Chondrichthyes
Order: Lamniformes
Family: Lamnidae
Genus: Lamna
Species:
L. ditropis
Binomial name
Lamna ditropis
Lamna ditropis distmap.png
  Range of the salmon shark

The salmon shark (Lamna ditropis) is a species of mackerel shark found in the northern Pacific ocean. As an apex predator, the salmon shark feeds on salmon, squid, sablefish, and herring. [2] It is known for its ability to maintain stomach temperature (homeothermy), [3] which is unusual among fish. This shark has not been demonstrated to maintain a constant body temperature. It is also known for an unexplained variability in the sex ratio between eastern and western populations in the northern Pacific. [4]

Contents

Description

Jaws and some vertebrae of a salmon shark Lamna ditropis parts of the skeleton.jpg
Jaws and some vertebrae of a salmon shark

Adult salmon sharks are medium grey to black over most of the body, with a white underside with darker blotches. Juveniles are similar in appearance, but generally lack blotches. The snout is short and cone-shaped, and the overall appearance is similar to a small great white shark. The eyes are positioned well forward, enabling binocular vision to accurately locate prey. [5]

Comparison of the size of a salmon shark (top) and its relative the great white shark (bottom) Comparison of great white shark and salmon shark.png
Comparison of the size of a salmon shark (top) and its relative the great white shark (bottom)

The salmon shark generally grows to between 200 and 260 cm (6.6–8.6 ft) in length and weighs up to 220 kg (485 lb). [6] Males appear to reach a maximum size slightly smaller than females. Unconfirmed reports exist of salmon sharks reaching as much as 4.3 m (14.2 ft); however, the largest confirmed reports indicate a maximum total length of about 3.0 m (10 ft). [4] The claims of maximum reported weight over 450 kg (992 lb) are "unsubstantiated". [4] [6]

Biology

Reproduction

The salmon shark is ovoviviparous, with a litter size of two to six pups. [7] As with other Lamniformes shark species, the salmon shark is oophagous, with embryos feeding on the ova produced by the mother.

Females reach sexual maturity at eight to ten years, while males generally mature by age five. [8] Reproduction timing is not well understood, but is believed to be on a two-year cycle with mating occurring in the late summer or early autumn. [4] Gestation is around nine months. Some reports indicate the sex ratio at birth may be 2.2 males per female, but the prevalence of this is not known. [4]

Homeothermy

As with only a few other species of fish, salmon sharks have the ability to regulate their body temperature. [3] This is accomplished by vascular counter-current heat exchangers, known as retia mirabilia, Latin for "wonderful nets". Arteries and veins are in extremely close proximity to each other, resulting in heat exchange. Cold blood coming from the gills to the body is warmed by blood coming from the body. This results in blood coming from the body losing its heat so that by the time it interacts with cold water from the gills, it is about the same temperature, so no heat is lost from the body to the water. Blood coming towards the body regains its heat, allowing the shark to maintain its body temperature. This minimizes heat lost to the environment, allowing salmon sharks to thrive in cold waters.

Their homeothermy may also rely on SERCA2 and ryanodine receptor 2 protein expression, which may have a cardioprotective effect. [9]

Range and distribution

It is common in continental offshore waters, but ranges from inshore to just off beaches. It occurs singly, in feeding aggregations of several individuals, or in schools. Tagging has revealed a range which includes sub-Arctic to subtropical waters. [9]

The salmon shark occurs in the North Pacific Ocean, in both coastal waters and the open ocean. It is believed to range as far south as the Sea of Japan and as far north as 65°N in Alaska and in particular in Prince William Sound during the annual salmon run. Individuals have been observed diving as deep as 668 m (2,192 ft), [10] but they are believed to spend most of their time in epipelagic waters.

Regional differences

Age and sex composition differences have been observed between populations in the eastern and western North Pacific. Eastern populations are dominated by females, while the western populations are predominantly male. [6] Whether these distinctions stem from genetically distinct stocks, or if the segregation occurs as part of their growth and development, is not known. The population differences may be a result of Japanese fishermen harvesting the male population. Japanese herbalists use the fins of males as ingredients in many traditional medicines said to treat various forms of cancer. [11]

Human interactions

Currently, no commercial fishery for salmon shark exists, but they are occasionally caught as bycatch in commercial salmon gillnet fisheries, where they are usually discarded. Commercial fisheries regard salmon sharks as nuisances since they can damage fishing gear [7] and consume portions of the commercial catch. Fishermen deliberately injuring salmon sharks have been reported. [12]

Sport fishermen fish for salmon sharks in Alaska. [13] Alaskan fishing regulations limit the catch of salmon sharks to two per person per year. Sport fishermen are allowed one salmon shark per day from April 1 and ending the following March 31 in British Columbia. [14]

The flesh of the fish is used for human consumption, and in the Japanese city of Kesennuma, Miyagi, the heart is considered a delicacy for use in sashimi. [7]

Although salmon sharks are thought to be capable of injuring humans, few, if any, attacks on humans have been reported, but reports of divers encountering salmon sharks and salmon sharks bumping fishing vessels have been given. [12] These reports, however, may need positive identification of the shark species involved.

Declines in the abundance of economically important king salmon in the 2000s may be attributed to increased predation by salmon sharks, based on remote temperature readings from tagged salmon that indicate they have been swallowed by sharks. [15]

See also

Related Research Articles

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<span class="mw-page-title-main">Dall's porpoise</span> Species of porpoise endemic to the North Pacific

Dall's porpoise is a species of porpoise endemic to the North Pacific. It is the largest of porpoises and the only member of the genus Phocoenoides. The species is named after American naturalist W. H. Dall.

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<span class="mw-page-title-main">Porbeagle</span> Species of shark (Lamna nasus)

The porbeagle or porbeagle shark is a species of mackerel shark in the family Lamnidae, distributed widely in the cold and temperate marine waters of the North Atlantic and Southern Hemisphere. In the North Pacific, its ecological equivalent is the closely related salmon shark (L. ditropis). It typically reaches 2.5 m (8.2 ft) in length and a weight of 135 kg (298 lb); North Atlantic sharks grow larger than Southern Hemisphere sharks and differ in coloration and aspects of life history. Gray above and white below, the porbeagle has a very stout midsection that tapers towards the long, pointed snout and the narrow base of the tail. It has large pectoral and first dorsal fins, tiny pelvic, second dorsal, and anal fins, and a crescent-shaped caudal fin. The most distinctive features of this species are its three-cusped teeth, the white blotch at the aft base of its first dorsal fin, and the two pairs of lateral keels on its tail.

<span class="mw-page-title-main">Chinook salmon</span> Species of fish

The Chinook salmon is the largest and most valuable species of Pacific salmon. Its common name is derived from the Chinookan peoples. Other vernacular names for the species include king salmon, Quinnat salmon, Tsumen, spring salmon, chrome hog, Blackmouth, and Tyee salmon. The scientific species name is based on the Russian common name chavycha (чавыча).

<span class="mw-page-title-main">Sockeye salmon</span> Species of fish

The sockeye salmon, also called red salmon, kokanee salmon, blueback salmon, or simply sockeye, is an anadromous species of salmon found in the Northern Pacific Ocean and rivers discharging into it. This species is a Pacific salmon that is primarily red in hue during spawning. They can grow up to 84 cm in length and weigh 2.3 to 7 kg (5–15 lb). Juveniles remain in freshwater until they are ready to migrate to the ocean, over distances of up to 1,600 km (1,000 mi). Their diet consists primarily of zooplankton. Sockeye salmon are semelparous, dying after they spawn. Some populations, referred to as kokanee, do not migrate to the ocean and live their entire lives in fresh water.

<span class="mw-page-title-main">Spiny dogfish</span> Species of shark

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<span class="mw-page-title-main">Atlantic blue marlin</span> Species of fish

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The wildlife of Alaska is both diverse and abundant. The Alaskan Peninsula provides an important habitat for fish, mammals, reptiles, and birds. At the top of the food chain are the bears. Alaska contains about 70% of the total North American brown bear population and the majority of the grizzly bears, as well as black bears and Kodiak bears. In winter, polar bears can be found in the Kuskokwim Delta, St. Matthew Island, and at the southernmost portion of St. Lawrence Island. Other major mammals include moose and caribou, bison, wolves and wolverines, foxes, otters and beavers. Fish species are extensive, including: salmon, graylings, char, rainbow and lake trout, northern pike, halibut, pollock, and burbot. The bird population consists of hundreds of species, including: bald eagles, owls, falcons, ravens, ducks, geese, swans, and the passerines. Sea lions, seals, sea otters, and migratory whales are often found close to shore and in offshore waters. The Alaskan waters are home to two species of turtles, the leatherback sea turtle and the green sea turtle. Alaska has two species of frogs, the Columbia spotted frog and wood frog, plus two introduced species, the Pacific tree frog and the red-legged frog. The only species of toad in Alaska is the western toad. There are over 3,000 recorded species of marine macroinvertebrates inhabiting the marine waters, the most common being the various species of shrimp, crab, lobster, and sponge.

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References

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