Biology of the Salmon Shark
(Lamna ditropis)
| Field Marks | Body stocky, spindle-shaped; snout conical and short (distance from tip of snout to eye is about 40% distance from eye to first gill slit); teeth moderately large, blade smooth-edged with basal cusplets; gill slits long, extending partially onto top of head; first dorsal fin originating over axil ('armpit') of pectoral fins and with dark free rear tip; second dorsal and anal fins minute, with pivoting bases; strong keels on caudal peduncle (tail stalk), with a short secondary keel on each side of the caudal fin base; caudal fin lunate, with lower lobe almost as long as the upper. Color: dark bluish grey to bluish black above, underside of snout, lower jaw, and belly whitish with dark greyish blotches in individuals longer than about 5 ft (1.5 m). |
| Size | Pups are born at a length of about 31.5-34.3 in (80-87 cm); most individuals caught or encountered are 6-7.5 ft (1.8-2.3 m) in length; maximum recorded length for males is 6.2 ft (1.9 m) and for females is 7.0 ft (2.14 m), although the species reported to reach a length of up to 12.1 ft (3.7 m) and a weight of up to 1 000 lbs (454 kg). |
| Range | Widely distributed in the North Pacific north of 35°, though apparently not found in the East China Sea and Yellow Sea (as indicated by Compagno 1984). |
| Habitat | Common in coastal-littoral and epipelagic zones of boreal to cool temperate waters; found from the surface to a depth of at least 500 ft (152 m), with most catches at a depth of about 130 ft (40 m) in the Gulf of Alaska and 100-350 ft (92-107 m) elsewhere; while common offshore, this species sometimes comes close inshore, just off the breakers, and occasionally strands; partially endothermic, able to maintain its body temperature 14.5-20°F (8-11°C) above ambient; known water temperature range 38.3-72.5°F (3.5-22.5°C), with most catches recorded at about 57°F (14°C); known salinity range 31.5-34.25‰ (ppt). Catch data provide evidence of segregation by size and sex in the western North Pacific; usually solitary, but may aggregate at night and when feeding, sometimes forming schools of 30-40 individuals; adults highly migratory in some parts of the species' range, but juveniles do not appear to migrate. |
| Feeding | Primarily piscivorous, but also known to take squids, shrimps and benthic crabs; notorious as a predator on Pacific salmon (Oncorhynchus spp), although researchers differ on how significant these sharks are as a source of natural mortality; known non-salmonid fish prey in subarctic waters include daggereeth, lancetfish, Atka mackerel, and lumpsuckers in northern waters, and pollock, sauries, Spiny Dogfish (Squalus acanthias), lanternfishes, pomfret, and chub mackerel in southern. Non-fish prey includes oceanic squids as well as benthic crustaceans such as shrimps, and tanner crabs (Chionoecetes sp.). In summer and fall, large individuals migrate to the northern Sea of Japan to feed on pilchards; in Sea of Okhotsk and Bering Sea, feed on pollock during fall. |
| Reproduction | Ovoviviparous, featuring oophagy at late stage of development; litter size typically 4-5; in western Pacific, pups are born during spring (March-May) in offshore waters, northeast of Honshu and south of the Kuril Islands; juveniles 27-43 in (70-110 cm) long abundant north of the subarctic boundary, which may serve as a nursery area; mating apparently takes place during autumn (September-November); in eastern Pacific (off Alaska), pups are born in May after a gestation period of about 1 yr. |
| Age & Growth | Pups grow to a length of 47-55 in (120-140 cm) by the end of their first year, an increase of up to 75%; subsequent annual growth is 4-6 in (10-15 cm) per year up to age 4; for age 5 and older, females grow faster than males, attaining a length of 7.2 ft (2.2 m) at age 10, compared with 6.9 ft (2.1 m) for males of the same age; males mature at a length of about 6 ft (1.8 m) and an age of about 9-10 yrs, females at a length of about 7.2-7.5 ft (2.3-2.3 m) and an age of about 10-11 yrs; longevity for males is estimated to be 25 yrs, and for females 17 yrs. |
| Danger to Humans | Regarded as potentially dangerous due to its large size and relationship to species known to be dangerous; seldom implicated in attacks on humans - the SAF currently lists no attacks definitely attributable to this species (a few unsubstantiated attacks attributed to it, but these may be due to confusion with the White Shark [Carcharodon carcharias]). Recently, divers have encountered and photographed schools of this species underwater, with no aggressive or threatening behavior exhibited by the sharks. |
| Utilization | Fished in the North Pacific by Japanese coastal longliners and by sports anglers in Alaska and British Columbia, Canada, using rod and reel; commonly caught by Japanese, US, and Canadian offshore salmon gillnetters, but are usually discarded; occasionally trammel-netted by halibut fishermen off California and recently have begun showing up in gillnets set by thresher fishermen off the north-central part of that state, but are presently not considered marketable; most caught incidentally are considered a nuisance due to the damage they do to salmon nets. The flesh is used fresh for human consumption in Japan where it is processed into various fish products, and to a lesser extent in Alaska and California; its liver oil, skin (for leather) and fins (for shark-fin soup) are also utilized. |
| Remarks | A broad-spectrum predator that occupies a very high trophic level in subarctic waters of the North Pacific, and thus probably plays an important role in stabilizing the marine ecology where it occurs; recently, there has been a major attempt to launch a commercial fishery for this species off Alaska, where it does not appear to be migratory yet the basic life history parameters (population size and structure, recruitment and natural mortality rates, etc.) necessary for intelligent management of its stocks are poorly known; this seems a recipe for ecologic and economic disaster. |
Maintaining a warm body in cold seas is a constant battle. It is one of the fundamental facts about the Universe is that, unless substantial energy is expended to combat the trend, heat will always dissipate. Due to its high heat capacity, water can rob a warm body of heat some twenty times faster than can air at the same temperature. Most sea creatures therefore save considerable energy by simply taking on the temperature of their ambient environment. This strategy conserves calories that can be put to other uses, such as growth and reproduction, but it places strict limitations on where these cold-bodied critters can live. Yet a few types of sharks - including the Great White and a few of its lamnoid relatives - have developed mechanisms for retaining much of their body heat, joining a select number of warm-blooded marine animals that are free to puncture thermal barriers that cold-blooded creatures cannot.
The Salmon Shark is one of the warmest of the warm-bodied sharks. Measurements of its epaxial (upper back) muscles indicate that this species is able to maintain a body temperature 14 to 20 degrees Fahrenheit (8 to 11 degrees Celsius) above the ambient water temperature. It would be interesting to know what percentage of a Salmon Shark's total caloric budget goes toward stoking its internal fires and how that energy demand impacts its growth and reproductive rate. We do know, however, that the Salmon Shark is widely distributed in cold waters of the North Pacific and an active predator on fast-moving fishes.
The Salmon Shark's range extends from boreal to cool temperate seas of the North Pacific Ocean. In the western part of its range, the Salmon Shark occurs from the subarctic Bering Sea and Sea of Okhotsk south to the Sea of Japan, and in the eastern part from the Gulf of Alaska to southern California and possibly off the seaward side of the Baja Peninsula. Although it has been taken in waters as warm as 75 degrees Fahrenheit (24 degrees Celsius), the Salmon Shark seems to prefer colder temperatures between 36.5 and 46.5 degrees Fahrenheit (2.5 and 8 degrees Celsius). This species is most common in continental offshore waters, from the surface down to a depth of at least 500 feet (152 metres), but it has been known to come inshore - sometimes just beyond the breaker zone. In recent years, researchers have noticed that each spring a small number of Salmon Sharks beach themselves in central and southern California. This phenomenon is poorly understood and is being studied. However, the Salmon Shark's ability to maintain a warm body temperature has enabled it to make an active predatory living in even the chilliest waters of the North Pacific.
The Salmon Shark is reputed to be a voracious feeder, but little is known about how much it actually eats during a given period. Salmon Sharks are, of course, named for their dietary predilection — the flesh is often stained salmon red, just like that of its favorite food. Adult Salmon Sharks apparently migrate in loose groups of 30 to 40 individuals, following schools of Pacific salmon — including Sockeye (Oncorhynchus nerka), Pink (Oncorhynchus gorbusha), Chum (Oncorhynchus keta), and Coho (Oncorhynchus kisutch) — as they swim along the great arcs of current flowing off the coasts of northern Japan and the Kamchatka peninsula, the Aleutian Island chain, Alaska and British Columbia. No one, to my knowledge, has ever observed Salmon Sharks actually chase down and eat fish in the wild, but the image of a group of these stubby sharks picking off schools of salmon as they migrate around the North Pacific reminds one of a late-summer mob of children chasing an ice-cream truck. Other prey species in the northern part of the Salmon Shark's range include Pacific Tomcod (Microgadus proximus), Walleye Pollock (Theragra chalcogramma), tuna, Chub Mackerel (Scomber japonicus), herring, Atka 'Mackerel' (Pleurogrammus monopterygius —actually a pelagic greenling of the family Hexagrammidae), sculpins, lumpsuckers (identified as Eumicrotremus orbis, but may have been Aptocyclus ventricosus, which is common in the epipelagic layer of the Bering Sea), Longnose Lancetfish (Alepisaurus ferox), Daggertooth (Anotopterus pharao), and the Spiny Dogfish (Squalus acanthias); in southern parts of its range, the Salmon Shark feeds chiefly on squids, lanternfishes (Myctophidae), Pacific Pomfret (Brama japonica), and Pacific Saury (Cololabis saira).
The Salmon Shark is respectably large, growing to a maximum length of perhaps 12 feet (3.7 metres). Individuals longer than about 5 feet (1.5 metres) have the ventral surface coarsely blotched with black. Males mature at a length of about 6 feet (1.8 metres) and an age of about 9 to 10 years, females at a length of about 7.2 to 7.5 feet (2.3 to 2.3 m) and an age of about 10 to 11 years. Like other members of its family (Lamnidae), the Salmon Shark is ovoviviparous and the late-term embryos are oophagous. The gestation period in this species is believed to be somewhat less than a year, but details are lacking. Usually two to four pups are produced per litter, their length at birth is about 31.5 to 34 inches (80 to 87 centimetres). The young are born in selected nursery areas offering rich feeding and relatively few potential predators. Interestingly, young Salmon Sharks measuring between 43 to 47 inches (110 to 120 centimetres) long do not seem to migrate, possibly to save energy that can then be put toward growth and evasion of predators.
To many commercial salmon fisherman, the Salmon Shark is a hated rival. It has been estimated that, each year in the eastern North Pacific, Salmon Sharks take between 12 and 25 percent of the annual salmon run. In addition to competing with fishermen for salmon, each year Salmon Sharks cause considerable damage to salmon nets and gear. In response, many salmon fishermen do not merely kill the Salmon Sharks they capture incidentally, they torture them. Some clobber the sharks senseless with baseball bats or hack them to death with axes and knives; others exact revenge by slicing off the tail and dumping the still-living shark overboard, to be set upon by its fellows or slowly starve to death. The life-history of the Salmon Shark is better known than that of most other lamnids, due (in part) to recent investigations into the feasibility of launching a commercial fishery for this species off the coast of Alaska. The reasoning behind this fishery apparently being, "If you can't beat 'em, eat 'em!" I can attest that Salmon Shark meat is delicious, but given the species' low reproductive potential and the boom-and-bust history of shark fisheries in general, it seems unlikely that the Salmon Shark would prove better able to withstand the onslaught of a commercial fishery than other sharks.
Due to its large size, relationship to known dangerous species, and a few unsubstantiated attacks attributed to this species (possibly due to confusion with the White Shark), the Salmon Shark has traditionally been regarded as potentially dangerous. Recently, however, divers off California have observed and photographed groups of adult Salmon Sharks underwater, with no threatening or aggressive overtures on the part of the sharks. In August 1991, I enjoyed the good fortune of an underwater encounter with a lone, 7-foot (2.2-metre) Salmon Shark off Sandspit, in the Queen Charlotte Islands.
The shark had apparently been lured by our cleaning of pots and pans over the side of the boat. Realising that it was almost certainly a Salmon Shark, and still dressed in my wetsuit from that morning's dive, I quickly grabbed my mask, fins, and some salmon scraps left over from the previous night's dinner, then plunked overboard.
The water was bone-shatteringly cold, murky, and green. I swam a few strokes away from the boat, trying desperately to peer through the emerald haze. There! A flicker of movement in the diffuse half-light. The shark was incredibly beautiful! Faint webs of underwater sunlight danced along its spindle-shaped body and paddle-shaped fins. I studied the shark's proportions, to compare them with illustrations I had seen and confirm its identity. The dorsal fin seemed to be standing at attention, alert. The gills were quite long, extending almost the entire depth of the shark's throat. My temples throbbed from the cold, it was getting hard to think systematically, and my snorkel-breathing seemed incredibly, terrifyingly loud; I was deeply concerned that the noise might frighten away the shark. In profile, the shark's tail stalk seemed ridiculously slender to power its massive, crescentic and deeply forked caudal fin. At the other end, the shark's bullet-shaped snout was pocked with large, black, sensitive-seeming eyes. It was a Salmon Shark, all right, probably female.
My attention then shifted to the shark's behavior. Its movements were quick and jerky, as though conflicted about whether or not to approach the bait I offered. Try as I might, I could not lure the shark closer than about 25 feet (8 metres) — insufficient to confirm its sex. I had originally attributed the shark's stand-offish behavior to the caution typical of many predatory animals, perhaps compounded by proximity of the boat and the whoops and hollers of my companions back on deck. But, perhaps due to the mind-numbing cold, I couldn't help thinking that maybe somehow it knew I had recently eaten one of its brethren.
As a warm-bodied creature myself, a deep sense of self-preservation forced me to leave the water after being in for only about 8 minutes. I swam back to my companions' boat and half-climbed, was half-dragged, back on board. It was only then that I noticed that I was white as a parsnip and covered in goose-flesh. Shivering uncontrollably, I endured endless rounds of "fish food" jokes and being told, with an odd mixture of utter disbelief and absolute sincerity, that I was "f**king crazy". But in a young career already shimmering with scores of precious undersea adventures, I count the experience of swimming with a living Salmon Shark among the most wonderful 8-minutes of my life.
