North Pacific Salmon Sharks
Lamna Ditropis, Relative of the White Shark, is a Key Apex Predator
Feb 16, 2008
One of the North Pacific’s greatest predators is virtually unknown outside fishery circles. Lamna ditropis, the salmon shark, is a toothy, robust fish that averages around two meters in length and 200 kilograms. Morphologically it resembles white sharks, and is in fact a close evolutionary relative to white sharks, mako sharks, and porbeagles.
Morphology
Together, this group of sharks constitutes the family Lamnidae, or mackerel sharks—so named because their sharply curved, crescent shaped tails resemble those of tuna or mackerel. They are large, streamlined, fusiform fish, and are well adapted for a predatory lifestyle. They have a conical snout and long mouth harboring teeth designed for slicing and impaling prey. Large gills allow them to breathe efficiently as they swim at sustained high speeds. The first dorsal fin is large and triangular, while the second is small, and the caudal peduncle region just ahead of their tail is strengthened by prominent lateral keels, particularly in the salmon shark.
A Warm Blooded Shark
Most fish are exothermic: they lose metabolic heat through their bodies and gills and thus are the same temperature as the surrounding water. The salmon shark, however, is endothermic, and maintains its body temperature 7 to 11 degrees Celsius higher than surrounding water temperature. The higher temperature is achieved by an adaptation of highly developed, countercurrent networks of blood vessels that trade off heat like track runners passing a baton. These networks, called rete mirable, re-circulate heat throughout the muscles rather than letting it escape. Warm muscle out-powers cold muscle, giving the sharks an edge over their cold-blooded prey.
Distribution in the Eastern North Pacific
Along North America, salmon sharks range from California to the Bering Sea. Their distribution is due in large part to oceanographic conditions and the availability of food. Scientists suspect that salmon sharks follow seasonal migration routes in pursuit of prey such as salmon and herring. For example, the seasonal appearance of the sharks in inshore Southeast Alaskan waters is greatest in July through September, when they can be seen swimming at the surface in large aggregations. The sharks at this time are positioning themselves in places where oceanic salmon gather on the way back to their spawning streams.
Shark vs. Salmon
The salmon shark’s camouflage coloration (dark on top, light and blotchy underneath), its speed (the US Navy believes it to be one of the fastest fish in the sea), and sensory ability (well developed eyes, keen sense of smell, pressure sensitivity, and ability to detect the bioelectric current of other marine animals at close range), all combine to make it a formidable predator. The shark appears to catch its prey primarily through pursuit rather than ambush. Contests between shark and salmon are often tight maneuvering affairs, with the sharks even leaping clear of the water to pursue salmon fleeing into kelp beds.
An Uncertain Future
Ironically, as formidable as these sharks are at catching fish, they would themselves prove vulnerable to over-harvesting by humans if the species were ever commercially targeted. This is because shark species in general are slow to mature and have small brood sizes, making it difficult for their populations to recover when overfished. This is a growing problem worldwide, as industrial fishing decimate previously preferred species and must hunt in turn for new prey. Sharks have been increasingly targeted in recent decades, to the point that many species are now endangered.
For now, eastern North Pacific salmon shark populations remain relatively unfished. Fast moving and elusive, these sharks are one of the more mysterious denizens in northern waters. As researchers piece together their life history, it is clear the salmon shark is a highly evolved, warm-blooded predator, and is a critical player in the North Pacific ecosystem.
(For a look at a different yet equally fascinating group of predators, visit Jellyfish 101.)
References:
Anderson, Scot D., and Kenneth J. Goldman. 2001. Temperature Measurements from Salmon Sharks, Lamna ditropis, In Alaskan Waters. Copeia, 2001(3): 794-796.
Blagoderov, A.I. 1994. Seasonal Distribution and Some Notes on the Biology of Salmon Shark (Lamna ditropis) in the Northwestern Pacific Ocean. J. Ichthyol., 34(2): 115-121.
Nagasawa, Kazuya. 1998. Predation by Salmon Sharks (Lamna ditropis) on Pacific Salmon (Oncorhynchus spp.) in the North Pacific Ocean. N. Pac. Anadr. Fish Comm. Bull., 1: 419-433.
Paust, Brian, and Ronald Smith. 1986. Salmon Shark Manual: the Development of a Commercial Salmon Shark, Lamna ditropis, Fishery in the North Pacific. Alaska Sea Grant Report 86-01. University of Alaska, Fairbanks. 430 pp.
Urquhart, David L. 1981. North Pacific Salmon Shark. Sea Front., 27(6) [November-December 1981): 361-363.
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