Salmon Shark Migrations

Aug 17 • Predation • 156 Views • Comments Off on Salmon Shark Migrations

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:

  1. An ocean climate regime shift characterized by warmer ocean temperatures beginning in the late 1970’s
  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.
  3. North Pacific wild and hatchery salmon production increases following the ocean climate shift
  4. A moratorium on large-scale pelagic driftnet fisheries in the North Pacific in 1992 eliminated an important source of juvenile salmon shark removals on the high seas

Some other factor or a combination of factors may have influenced the increased shark population.
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.salmon_shark3Surface 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 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.
Satellite tags successfully transmitted salmon shark locations, yielding many locations (below).salmon_shark_tracking

We believe some of the movements of salmon sharks are related to the females seeking warmer or protected waters for birthing. Each year young salmon sharks wash up on California beaches, stranded for no apparent reason. In 2004, a young salmon shark washed up on an Oregon beach (Photos below by Marty Beyer).salmon_shark4

 

Salmon Shark Encounters in Alaska 

salmon_shark2¬†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.

 

References:
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.
Eschmeyer, W.N. and E.S. Herald 1983. A field guide to Pacific coast fishes of North America: from the Gulf of Alaska to Baja California. Boston, Mass: Houghton-Mifflin Co.

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].

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