Salmon Shark: Lamna ditropis

Family: Lamnidae
Common name(s)

Salmon Shark.

Identification

A stocky, powerfully built shark with a very short, conical snout. First dorsal fin origin over or anterior to pectoral fin insertion. First dorsal apex rounded. Second dorsal fin very small; same size as anal fin. Prominent lateral keels on caudal peduncle support a large, powerful falcate, caudal fin. Shorter secondary caudal keels positioned below main keel. Upper caudal lobe slightly longer than lower.
Dorsal coloration slate grey, greyish brown, or black. Ventral surface white with irregular dark spots and blotches. Dorsal coloration may stop above or below the mouth.

Size

Maximum length 305cm. Size at birth 65-80cm.

Habitat

Cold, temperate, and sub-tropical oceanic and inshore environments. Found in 2-24ºC water but prefers water between 8-16ºC. From the surface to at least 1864m.

Distribution

Confined to the North Pacific Ocean. Males more common in the northwest, females in the northeast.
In the northeast Pacific, salmon sharks range from the Bering Sea to Baja California.

Conservation Status

LEAST CONCERN

The salmon shark has low biological productivity with small litters and a biennial reproductive cycle. It is caught as target and bycatch in coastal and pelagic commercial and small-scale longline, purse seine, and gillnet fisheries, and is often retained for its meat as well as fins. The population appeared to be stable at relatively high levels of abundance during the 1990s and early 2000s. Since cessation of an open ocean gillnet fishery in the early 1990s, the population may be rebuilding. There is no information available on population trends since that time, except an anecdotal note by recreational fishers of a decline in catch in Prince William Sound, Alaska that resulted in a self-imposed avoidance of this species. Other than this noted decline in Alaska, there is nothing to infer population decline and thus, the species is assessed as Least Concern. Improved catch monitoring, analysis of the population trend analysis, and precautionary fishing limits are recommended.

Citations and References
Rigby, C.L., Barreto, R., Carlson, J., Fernando, D., Fordham, S., Francis, M.P., Herman, K., Jabado, R.W., Liu, K.M., Marshall, A. & Romanov, E. 2019. Lamna ditropisThe IUCN Red List of Threatened Species 2019: e.T39342A124402990. https://dx.doi.org/10.2305/IUCN.UK.2019-3.RLTS.T39342A124402990.en. Downloaded on 15 December 2020.

Reproduction

An aplacental viviparous species with oophagous (egg eating) embryos. 3-4 embryos per litter.

Diet

Consumes a wide variety of bony fishes. Various salmon species constitute the bulk of the salmon shark’s diet when available.

Behavior

In North America, salmon sharks move into fjords from June to September to feed on spawning salmon, returning offshore in the winter.
In the northwestern Pacific, salmon sharks migrate south from the Kuril Islands to Japan in the winter.

In Prince William Sound, salmon sharks are frequently seen during the summer, slowly swimming in large circles at or near the surface with the tops of their dorsal fins exposed. The purpose of this behavior is unclear but it may be related to hunting for salmon.

Reaction to divers

Extremely shy. Difficult to approach on scuba or snorkel until fixated on a trawled bait, at which point the salmon shark becomes much bolder, making very close passes in its pursuit of the bait.

Diving logistics

The Gulf of Alaska is the only area where salmon sharks can be reliably encountered. Between June and August salmon sharks congregate in one spot dubbed ‘Shark Alley’ in Port Fidalgo. At this spot, snorkelers can observe the sharks lazily swimming in circles at the surface. Sometimes they are too shy to approach but the bolder ones will approach small boats if a shark wrangler attracts them with a small fish tied to a fishing line (without a hook). Once a shark takes interest, the snorkelers slip into the water but remain next to the skiff to avoid spooking the shark as it approaches.
Big Fish Expeditions runs a combined Salmon Shark Snorkeling and Alaskan Reef Diving Expedition each June/July.

Similar species

Shortfin Mako Shark Distinguished by more slender body, more acutely pointed snout, and lack of second caudal keel.

Great White Shark Distinguished by pointed first dorsal fin, lack of secondary caudal keel, and broadly triangular teeth with serrated edges.

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