Salmon Shark

Salmon sharks (Lamna ditropis) are large, powerful, warm-bodied (endothermic), and streamlined predators adapted for high-speed swimming (see figure to the right). Reports from the U.S. Navy have clocked salmon sharks exceeding 50 knots. This would make the salmon shark one of the fastest fish in the ocean. They are reported to reach 11.9 feet (3.6 m) in total length (Eschmeyer et al. 1983, Compagno 1984). Most of the salmon sharks encountered in Alaskan waters (the northeastern Pacific) are surprisingly uniform: over 93% are females ranging from 6 1/2 to 8 feet (2 - 2.5m) in length and roughly 300 pounds (136kg). Salmon sharks in the 700 pound range have been reported by sport fishermen in Alaska. The salmon shark is a member of the Lamnidae family of sharks which includes the great white shark (Carcharadon carcharias), makos, and the porbeagle shark (Lamna nasus).

Salmon sharks inhabit coastal and oceanic waters of the northern Pacific Ocean. They range from 350N to 650N in the Western Pacific, from 300N to 650N in the Eastern Pacific, and may occur as far south as Baja California, Mexico. In the Eastern Pacific the greatest densities appear to be found between 500N to 600N. They favor water temperatures ranging from 410F to 640F (50C to 180C) and occur from surface waters to over 500 feet (150m) and from coastal waters to offshore waters. There is annual north-south movement of salmon sharks in both the eastern and western Pacific. Salmon sharks inhabit Gulf of Alaska waters during all months of the year.

Salmon sharks are thought to live at least 25 years. Length at maturity in the northwestern Pacific occurs at 4 1/2 feet (140cm) pre caudal length and 5 years for males, and 5 1/2 to 6 feet (170-180cm) pre caudal length and 8-10 years for females. Salmon sharks produce eggs that hatch within the female's body, and they bear live young (ovoviviparous). They have an annual fecundity of up to 5 pups (Tanaka 1980). Based on mating occurring in the late summer and birth occurring in the spring, gestation would be around 9 months.

Studies have shown that salmon sharks have the highest body temperature of any shark. They are able to elevate their body temperature above that of the surrounding water by means of a counter-current heat exchange system called the rete mirabile. Smaller salmon sharks elevate their body temperatures from 140F to 180F (80C to 100C) above ambient while larger individuals elevate their body temperatures up to 24.50F (13.60C) above the temperature of the water. These adaptations enable salmon sharks to occupy sub-arctic waters and to swim at great speeds.

Salmon sharks are opportunistic predators that feed on a wide variety of prey. Their primary prey is Pacific salmon (Onchorhynchus spp.), Pacific herring (Clupea harengus), capelin (Mallotus villosus), pollock (Theragra chalcogramma), and sablefish (Anoplopoma fimbria). They have been seen taking other prey including sea otters (Enhydra lutris) and marine birds. In the photo to the left are researchers Barbara Block and Bruce Wright pumping the salmon shark's stomach to collect diet information.

Salmon sharks are seen as direct competitors for several commercially important species including salmon, sablefish, and herring. In 1989, the abundance of salmon sharks in the northwestern Pacific was estimated to be at least 2,000,000 fish. Of these, 595,000 salmon sharks were age 5 years or older and inhabitant sub-arctic waters. The author of the paper, Nagasawa, estimated these salmon sharks could consume between 76,000,000 and 146,000,000 salmon per year. That's about 12.6% to 25.2% of the total annual run of Pacific salmon (Nagasawa 1998).

Salmon sharks are highly migratory. The dramatic increase in salmon shark observations in some Alaska waters during the 1990's is unprecedented. The predation effect of so many salmon sharks may appear only as random salmon run failures. Nearly every year Alaska experiences unexpected run failures. Further research may reveal the importance of these predators and the part they play in the North Pacific Ocean.

Salmon sharks are well known to Alaska's commercial salmon fishermen who know them for damaging gillnets and for ripping fish and gear from trollers. The encounters can be costly to the fishermen and deadly for the shark.

Salmon sharks in Alaska are most commonly observed from July to September when they aggregate near streams where salmon are concentrated. The salmon are preparing to move up the streams to spawn. Salmon sharks have also been observed associated with the sac roe herring fishery in spring (April-May) and during the fall herring bait fishery (September-October). A few salmon sharks are taken in trawl gear during the winter pollock fishery in the Gulf of Alaska and Prince William Sound.

Reports of large aggregations of salmon sharks in Prince William Sound have caught the interest of commercial and sport fishermen who saw an opportunity to capitalize on an untapped and abundant resource. A Cordova fish processor identified a market for salmon shark flesh, and during the summer of 1996 a small commercial harvest of the sharks took place in Prince William Sound. Fishermen found sharks so abundant in places that they were able to capture as many as 40-50 at a time with purse seine nets. Handling so many sharks at a time proved to be a dangerous and a chaotic endeavor, so they opted to fish with surface longline gear. Fishing twenty hooks per set, they caught an average of three sharks per hour.

Several sport charter companies operating out of Seward and Cordova have begun to specialize in salmon shark angling. Salmon sharks are capable of high-speed runs and aerobatic leaps when hooked, and they are becoming the hot "new" Alaskan big game fish. This sudden interest, along with the lack of biological knowledge of the species, prompted the Alaska Board of Fisheries to close all commercial fishing for sharks and to heavily regulate the sport fishery in Alaska state waters in 1997. Closure of commercial shark fishing in federal waters is also being considered.

The primary objectives of the Alaska Shark Assessment Program is to understand the increase of sharks in the northeast Pacific, and what the increased shark population means for Alaska marine ecosystems. The reasons for these increases may be:

2. The ocean climate regime shift triggered a >250% increase in biomass of high trophic level groundfish (codfishes (Gadidae) and flatfish (Pleuronectidae)) which are important salmon shark prey.

To help understand the increased shark populations we investigated the movements of salmon sharks in the Pacific. We wanted to learn if salmon shark movements and seasonal residency in these waters is related to their high energy demands. We also wanted to determine if salmon sharks could exert a high predation rate in the Gulf of Alaska ecosystem.

Surface aggregations of salmon sharks, sometimes numbering in the thousands in a single bay, were frequently observed during summer. These shark aggregations were associated with pre-spawning migrations of Pacific salmon in Prince William Sound. We wanted to determine if the salmon shark residency in near-shore Gulf of Alaska waters was seasonal, tied to summer salmon spawning aggregations, or do they reside in Alaska waters during winter months as well? To address this question, a salmon shark tagging effort was initiated in 1999. About 230 sharks were tagged with conventional spaghetti (FLOY) tags and released. Only one of these sharks has been recaptured.

The Alaska Shark Assessment Program sometimes hosted others interested in sharks. The figure to the right is of a British Broadcasting Corporation (BBC) crew filming a salmon shark captured in the research vessel's net. After the filming, the shark was brought aboard the research vessel, measured and tagged, then released.

The research team deployed 18 satellite-linked tags on salmon sharks in Prince William Sound. Three PAT tags were deployed in 1999, 3 KiwiSat 101 tags were deployed in 2000, and 3 SPOT2 and 8 PAT tags were deployed in 2001.

The KiwiSat and SPOT2 are analogous tags that we bolted through the sharks' dorsal fin and transmit when the tag breaks the water's surface. These tags are referred to as position-only tags. Position is calculated by the Argos satellite system based on Doppler shift. The SPOT2 additionally transmits "time-at-temperature" histograms that report the relative time the study animal spent within user-defined temperature ranges during the time interval prior to transmission (we set this time interval at 12 hours). PAT tags are externally fastened by a tether with a stainless steel dart near the base of the first dorsal fin and are pre-programmed to detach and float to the surface on a specific date. Data on depth, temperature, and location were stored on the PAT tags and transmitted back to the lab via satellite after pop-up.

Some position-only tags successfully transmitted salmon shark locations, yielding many locations, while others quit transmitting suddenly. It is unknown whether the tags failed or were damaged, whether the sharks simply stopped frequenting surface waters, or whether the sharks died.

Two of three PAT tags performed as programmed in 1999. Of the 8 PAT tags deployed in 2001, two were programmed to release (pop-up) on November 1, 2001 and three each were programmed to release on February 1, 2002 and July 1, 2002. Two PAT tags (a February tag and a July tag) pre-released west of northern California and west of Oregon respectively. It is unknown why the tags pre-released. One of the two PAT tags scheduled to release on November 1 popped up near Pigot Bay in northwest Prince William Sound. The figure below shows the track of two salmon sharks that carried the position-only tags. The locations ended when the sharks stopped using surface waters.

Compagno, L.J.V. 1984. FAO species catologue. Vol. 4 Sharks of the World. An annotated and illustrated catalogue of shark species known to date. Part 1. Hexanchiformes to Lamniformes. FAO Fish. Synop., (125) Vol. 4 Pt. 1:249p.

Nagasawa, K. 1998. Predation by salmon sharks (Lamna ditropis) on Pacific salmon (Onchorhynchus spp.) in the North Pacific Ocean. Pac. Anadr. Fish Comm. Bull No. 1: 419-433.

Tanaka, S. 1980. Biological investigation of Lamna ditropis in the north-western waters of the North Pacific. In: Report of investigation on sharks as a new marine resource (1979). Published by: Japan Marine Fishery Resource Research Center, Tokyo [English abstract, translation by Nakaya].