Found commonly in colder waters than other sharks, the salmon shark (Lamina ditropis) is a unique predator to its environment. They are part of the group Lamniformes (also known as mackerel sharks), which includes species such as the megamouth shark and great white sharks.  They appear very similar body shape to the great white shark but overall are much smaller and while they are still considered dangerous to humans, have never been the cause of an attack. The distribution of the salmon shark expands across the North Pacific in generally cooler waters than any of their relatives, with a range stretching from Japan to North California in typically coastal pelagic waters. Salmon sharks are found to be the apex predators of the environment which they exist, only on occasion being predated on by Orcas or by other larger shark species. In the most northerly part of it distribution only very few shark species can exist. Homeothermy, a trait not displayed by many fish species, is one adaptation it displays which has helped lead to its success of living in a colder water environment. This along with other factors allow the salmon shark to be a lethal and decisive hunter.

Salmon shark. Credit: NOAA

External morphology

Salmon sharks can grow to a considerable size. They may reach up to 10 feet long and up to a maximum of 700 lbs in weight. Like all sharks, have denticles on their skin. This reduces the overall drag of the shark as it swims and makes it a faster more effective predator. Found to be extremely fast swimming fish, they have been clocked swimming at over 50mph (80kmh). The fact it can swim fast increases its chance of catching prey.  The normal diet of the salmon shark will consist of predominantly of salmon, squid and various other fish species. A large mouth and a sharp array of teeth allow and it to consume any prey. Similarly to most sharks, the ventral side of the salmon shark is white. The dorsal coloration is a dark brackish color, with characteristic spots along the side.

Internal morphology

Salmon sharks on the whole have a very similar internal to structure to other sharks, including an enlarged liver for buoyancy and spiral valve to aid in digestion. Uniquely though, Lamniformes on the whole are referred to are as partially warm blooded. Salmon sharks maintain a body temperature of that which is roughly 7°C warmer than the marine environment. The increased elevation in body temperature is due to counter current heat exchangers, called the retia mirabilia, allowing heat made from metabolic processes to be retained. This provides them with an advantage by allowing them to hunt prey that other species physically cannot, reducing inter-specific competition. The fact they are warmer than the environment also allows the shark to swim faster, meaning that there is a greater variety of prey which it may consume.

Reproduction

The reproductive season occurs at the end of the summer when species move into shallower waters. Male salmon sharks reach sexual maturity at around 5 years of age, with females reach this stage later at around 8 years of age. Fertilization, as with all Lamniformes, occurs internally. The gestation period for a litter is up to 9 months, with a female carrying up to 6 pups at a given time. Only the strongest pups will survive the gestation period due to embryonic cannibalism taking place within the womb. Once the females have given birth, the young must fend for themselves with no parental care.

Threats

Currently marked on the IUCN Red List as a Least Concern (LC), there are no major threats to salmon shark populations. Some fishing is done for recreational purposes in Canada and Alaska, however this is not enough to seriously effect populations. Salmon sharks are viewed as pests by fishermen so may be killed when they come into contact with humans but this is not also in great numbers. However, salmon sharks can be landed heavily by Japanese long-lining vessels. Though they are not a direct fishing target if this continues populations may decline with negative implications on the whole.

Salmon shark caught by fishermen. Credit: NOAA-NMFS

Summary

Salmon sharks on the whole are highly adapted for living in their environment, with homeothermy playing a key role in the success and survival of the species. The warm blood of the salmon shark allows it to predate with extreme efficiency and its speed in comparison to it size makes it one of the fastest fish in the ocean. Embryonic cannibalism ensures that only the strong individuals of the species survive, meaning the population will thrive. There are no threats to the species as of yet but unless precautions are taken then there is the potential for this to change.

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