The salmon shark (''Lamna ditropis'') is a species of

mackerel shark The Lamniformes (, from Greek ''lamna'' "fish of prey") are an order of sharks commonly known as mackerel sharks (which may also refer specifically to the family Lamnidae). It includes some of the most familiar species of sharks, such as the g ...

found in the northern

Pacific The Pacific Ocean is the largest and deepest of Earth's five oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean, or, depending on the definition, to Antarctica in the south, and is bounded by the cont ...

ocean. Similar to other mackerel sharks, salmon sharks have a thunniform body type. As an

apex predator An apex predator, also known as a top predator or superpredator, is a predator at the top of a food chain, without natural predators of its own. Apex predators are usually defined in terms of trophic dynamics, meaning that they occupy the hig ...

, the salmon shark feeds on

salmon Salmon (; : salmon) are any of several list of commercially important fish species, commercially important species of euryhaline ray-finned fish from the genera ''Salmo'' and ''Oncorhynchus'' of the family (biology), family Salmonidae, native ...

squid A squid (: squid) is a mollusc with an elongated soft body, large eyes, eight cephalopod limb, arms, and two tentacles in the orders Myopsida, Oegopsida, and Bathyteuthida (though many other molluscs within the broader Neocoleoidea are also ...

sablefish The sablefish (''Anoplopoma fimbria'') is one of two members of the fish family Anoplopomatidae and the only species in the genus ''Anoplopoma''. In English, common names for it include sable (US), butterfish (US), black cod (US, UK, Canada), ...

, birds, walleye pollock, and

herring Herring are various species of forage fish, belonging to the Order (biology), order Clupeiformes. Herring often move in large Shoaling and schooling, schools around fishing banks and near the coast, found particularly in shallow, temperate wate ...

. Salmon sharks get their name from their diet, which primarily consists of salmon. It is known for its ability to maintain stomach temperature (

homeothermy Homeothermy, homothermy, or homoiothermy () is thermoregulation that maintains a stable internal body temperature regardless of external influence. This internal body temperature is often, though not necessarily, higher than the immediate envir ...

), 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 A sex ratio is the ratio of males to females in a population. As explained by Fisher's principle, for evolutionary reasons this is typically about 1:1 in species which reproduce sexually. However, many species deviate from an even sex ratio, ei ...

between eastern and western populations in the northern Pacific.

Description

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 great white shark (''Carcharodon carcharias''), also known as the white shark, white pointer, or simply great white, is a species of large Lamniformes, mackerel shark which can be found in the coastal surface waters of all the major ocea ...

. The eyes are positioned well forward, enabling binocular vision to accurately locate prey. Comparison of great white shark and salmon shark

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). 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). The claims of maximum reported weight over 450 kg (992 lb) are "unsubstantiated". Salmon sharks have a wide, double keeled tail (a second, short ridge that runs along the upper part of the lower lobe of the tail.) The only other shark with a double keeled tail is the porbeagle shark, which the salmon shark is closely related to. Salmon sharks are voracious group hunters. Groups of 30 to 40 individuals have been spotted hunting salmon in Alaska. Salmon sharks can also hunt alone, however individuals congregate in large groups in coastal waters to hunt bony fish such as salmon. Studies on prey consumption show that salmon sharks have similar energetic requirements as sea lions and other piscivorous marine animals.

Biology

Reproduction

The salmon shark is

ovoviviparous Ovoviviparity, ovovivipary, ovivipary, or aplacental viviparity is a "bridging" form of reproduction between egg-laying oviparity, oviparous and live-bearing viviparity, viviparous reproduction. Ovoviviparous animals possess embryos that develo ...

, birthing a litter of two to six pups. As with other

lamniforme The Lamniformes (, from Greek ''lamna'' "fish of prey") are an order of sharks commonly known as mackerel sharks (which may also refer specifically to the family Lamnidae). It includes some of the most familiar species of sharks, such as the g ...

shark species, the salmon shark is also

oophagous Oophagy ( ) or ovophagy, literally "egg eating", is the practice of embryos feeding on eggs produced by the ovary while still inside the mother's uterus. The word oophagy is formed from the classical Greek (, "egg") and classical Greek (, "to ...

, with

embryo An embryo ( ) is the initial stage of development for a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male sp ...

s feeding on the

ova , abbreviated as OVA and sometimes as OAV (original animation video), are Japanese animated films and special episodes of a series made specially for release in home video formats without prior showings on television or in theaters, though the ...

produced by the mother. Females reach sexual maturity from eight to ten years; males generally mature by age five. Reproductive timing is not well understood, but it is believed the sharks are on a two-year cycle, with mating occurring in the late summer to early autumn.

Gestation Gestation is the period of development during the carrying of an embryo, and later fetus, inside viviparous animals (the embryo develops within the parent). It is typical for mammals, but also occurs for some non-mammals. Mammals during pregn ...

is around nine months. Some reports indicate the sex ratio at birth may be 2.2 (males to females), but the prevalence of this is not known. Once salmon sharks are born, they are completely independent. Pups are spawned along Baja California.

Homeothermy

As with only a few other species of fish, salmon sharks have the ability to regulate their body temperature. 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. Past studies have shown that the average surface body temperature for the Salmon shark is around 8°C, while the internal average body temperature was around 16–19°C relative to other sharks. Their homeothermy may also rely on SERCA2 and

ryanodine receptor 2 Ryanodine receptor 2 (RYR2) is one of a class of ryanodine receptors and a protein found primarily in cardiac muscle. In humans, it is encoded by the ''RYR2'' gene. In the process of cardiac calcium-induced calcium release, RYR2 is the major medi ...

protein expression, which may have a cardioprotective effect.

Range and distribution

North of the

equator The equator is the circle of latitude that divides Earth into the Northern Hemisphere, Northern and Southern Hemisphere, Southern Hemispheres of Earth, hemispheres. It is an imaginary line located at 0 degrees latitude, about in circumferen ...

, in the northern Pacific Ocean, the salmon shark is relatively common in continental offshore waters, where it ranges from inshore to just off the coast. Tagging has revealed a range which includes sub-Arctic to subtropical waters. The species is believed to range as far south as the

Sea of Japan The Sea of Japan is the marginal sea between the Japanese archipelago, Sakhalin, the Korean Peninsula, and the mainland of the Russian Far East. The Japanese archipelago separates the sea from the Pacific Ocean. Like the Mediterranean Sea, it ...

and as far north as 65°N in

Alaska Alaska ( ) is a non-contiguous U.S. state on the northwest extremity of North America. Part of the Western United States region, it is one of the two non-contiguous U.S. states, alongside Hawaii. Alaska is also considered to be the north ...

, notably appearing ''en masse'' in

Prince William Sound Prince William Sound ( Sugpiaq: ''Suungaaciq'') is a sound off the Gulf of Alaska on the south coast of the U.S. state of Alaska. It is located on the east side of the Kenai Peninsula. Its largest port is Valdez, at the southern terminus of the ...

during the annual

salmon run A salmon run is an annual fish migration event where many salmonid species, which are typically hatched in fresh water and live most of their adult life downstream in the ocean, swim back against the stream to the upper reaches of rivers to s ...

. Individuals have been observed diving as deep as 668 m (2,192 ft), but they are believed to spend most of their time in

epipelagic The photic zone (or euphotic zone, epipelagic zone, or sunlight zone) is the uppermost layer of a body of water that receives sunlight, allowing phytoplankton to perform photosynthesis. It undergoes a series of physical, chemical, and biological ...

waters. While salmon sharks primarily travel alone, or in feeding aggregations of several individuals, sometimes they may be seen in schools. According to a study in 2008, salmon sharks have been observed to use area restricted search behaviors in both southern and northern Pacific Ocean regions. Individuals that migrate through southern high productivity regions exhibit more of these behaviors than those who travel through low productivity regions. Salmon sharks travel fast during migratory patterns. Studies conducted on said migrations put the average speed of individual sharks at 33 km. Juvenile salmon sharks stick to more coastal regions along the west coast of North America. Juveniles prefer water temperatures between 12 and 16°C. Juveniles prey on a variety of mesopelagic (intermediate depth) and epipelagic (the upper layer of the water column where light penetrates for photosynthesis) offshore prey.

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. Whether these distinctions stem from genetically distinct

stocks Stocks are feet and hand restraining devices that were used as a form of corporal punishment and public humiliation. The use of stocks is seen as early as Ancient Greece, where they are described as being in use in Solon's law code. The law de ...

, 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 more of the male population; certain Japanese herbalists use salmon shark fins (of males, specifically) in some traditional soups and medicines said to treat various forms of

cancer Cancer is a group of diseases involving Cell growth#Disorders, abnormal cell growth with the potential to Invasion (cancer), invade or Metastasis, spread to other parts of the body. These contrast with benign tumors, which do not spread. Po ...

Human interactions

Currently, no commercial fishery for salmon shark exists, but they are occasionally caught as

bycatch Bycatch (or by-catch), in the fishing industry, is a fish or other marine species that is caught unintentionally while fishing for specific species or sizes of wildlife. Bycatch is either the wrong species, the wrong sex, or is undersized or juve ...

in commercial salmon gillnet fisheries, where they are usually discarded. Commercial fisheries regard salmon sharks as nuisances since they can damage fishing gear and consume portions of the commercial catch. Fishermen deliberately injuring salmon sharks have been reported. Sport fishermen fish for salmon sharks in Alaska. 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. The flesh of the fish is used for human consumption, and in the Japanese city of

Kesennuma, Miyagi is a Cities of Japan, city in Miyagi Prefecture, Japan. , the city had an estimated population of 59,803 and a population density of in 26,390 households. The total area of the city is . Large sections of the city were destroyed by the 2011 Tōh ...

, the heart is considered a delicacy for use in

sashimi is a Japanese cuisine, Japanese delicacy consisting of fresh raw fish or Raw meat, meat sliced into thin pieces and often eaten with soy sauce. Origin The word ''sashimi'' means 'pierced body', i.e., "wikt:刺身, 刺身" = ''sashimi'', whe ...

. 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. These reports, however, may need positive identification of the shark species involved. Salmon sharks have never been positively identified attacking humans and there is no confirmed evidence that salmon sharks have attacked humans. Declines in the abundance of economically important

Chinook salmon The Chinook salmon (''Oncorhynchus tshawytscha'') is the largest and most valuable species of Oncorhynchus, Pacific salmon. Its common name is derived from the Chinookan peoples. Other vernacular names for the species include king salmon, quinn ...

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.

Conservation Status

Similar to white sharks, salmon sharks have a slow growth and reproduction rate, making them susceptible to overfishing and overexploitation. They are considered least concern, however juvenile salmon sharks are extremely susceptible to temperature changes in coastal waters. This means that further ocean warming can severely impact the species. There are reports of declines in salmon shark populations from Alaskan fishermen.

References

External links

*
Salmon shark fact sheet

TOPP, Tagging of Pacific Predators
a research group that tags salmon sharks to learn more about their habits.
IMDB entry for Icy Killers
a wild-life documentary about salmon sharks. {{Authority control salmon shark Fish of the Bering Sea Fish of Japan Western North American coastal fauna salmon shark Apex predators