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| Subject: SALMON SHARKS | |
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Author: ED |
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Date Posted: 928311599PDT  Posted by SALMON SHARK on June 01, 1999 at 22:05:40:at Allcoast Sharkboard Lamna ditropis Class Chondrichthyes Subclass Elasmobranchii Superorder Selachimorpha [Pleurotremata] Order Lamniformes Family Lamnidae (Muller & Henle 1838) – The Mackerel sharks Carcharodon C. carcharias (Linnaeus 1758) - Great white shark Isurus I. oxyrinchus (Rafinesque 1809) - Shortfin mako I. paucus (Guitart Manday 1966) - Longfin mako Lamna (Cuvier 1817) Salmon and Porbeagle sharks L. ditropis (Hubbs & Follett 1947) - Salmon shark L. nasus (Bonnaterre 1788) - Porbeagle Life History and Ecology Lamna ditropis, the salmon shark, is the predominant large predatory fish in the North Pacific. A member of the family Lamnidae, they are the sister taxon to the better known porbeagle (Lamna nasus) and are closely related to the white shark (Carcharadon carcharias), and the makos (Isurus spp.). Salmon sharks are large, powerful, warm bodied and streamlined predators adapted for high-speed swimming. Average size range appears to be between 2-2.5m (~6.5- 8ft) and they are reported to reach 3.6m (11.9ft) total length (Eschmeyer et al. 1983, Compagno 1984). Salmon sharks in Prince William Sound have been estimated to reach 12 feet and weigh up to 1000lbs (Luke Borer 1998 pers. comm.). Salmon sharks have been estimated to live at least 25 years. Length and age at maturity in the Northwestern Pacific has been estimated to occur at 140cm (4.6ft) pre caudal length and 5 years for males, and 170-180cm (5.6-5.9ft) pre caudal length and 8-10 years for females. Reproductive mode for L. ditropis is ovoviviparous (they produce eggs that hatch within the females body). They bear live young and "may" have an annual fecundity of up to 5 pups (Tanaka 1980). Gestation period is not documented. Based on mating occurring in the late summer and parturation occurring in the spring, gestation may be around 9 months (Goldman and Human in Press). The salmon shark is an opportunistic predator and likely posesses a high metabolic rate. Studies have shown that they may 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 (rete mirabile). Elevated temperatures from 8 to 110C above ambient have been reported for smaller specimens, while elevations up to 13.60C have been reported in larger specimens (Smith and Rhodes 1983, Goldman 1998 unpublished data). These adaptations enable them to occupy the highest trophic level in the food web of the sub-arctic Pacific Ocean. As apex predators, they feed opportunistically on a wide range of prey species (Ecopath). This free-ranging species frequents the temperate coasts of Asia and North America. Their distribution and abundance is often associated with aggregations of associated prey species, such as Pacific salmon (Onchorhynchus spp.), Pacific herring (Clupea harengus), capelin (Mallotus villosus), and juvenile pollock and sablefish (Anoplopoma fimbria). 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 million fish. Of these fish, 595,000 sharks age 5 years or older occurring in sub-arctic waters appear to have consumed between 76 million and 146 million salmonids, corresponding to 12.6-25.2% of the total annual run of Pacific salmon for that year (Nagasawa 1998). Salmon Shark Distribution 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 (Farquhar 1963, Compagno 1984, Croker 1942, Strasburg 1958, Hart 1973). In the Eastern Pacific the greatest densities appear to be found between 500N to 600N (Blagoderov 1994, Nakano and Nagasawa 1996, Neave and Havanan 1960). They apparently favor water temperatures ranging from 50C to 180C and occur from surface waters to at least 150m (~ 500ft) (Farquhar 1963, McKinnel & Waddell 1993, Nakano & Nagasawa 1996, Robinson & Jamieson 1984). The seasonal distribution of the salmon shark is difficult to assess but there appears to be some annual north-south movement in both the Eastern and Western Pacific (Ken Goldman 1998 pers. comm.). Female salmon sharks in the Eastern Pacific apparently migrate south to pup in the spring off Oregon and California (Goldman and Human in Press). While the recognized range of the salmon shark extends to the coastal waters of the Gulf of Alaska, the dramatic increase in salmon shark observations in PWS during the 1990’s are unprecedented in recent memory. Salmon Shark Encounters in Alaska Salmon sharks are particularly well known to Alaska’s commercial salmon fishermen who curse them for damaging gillnets and for ripping fish and gear from trollers. The encounters are usually costly to the fishermen and deadly for the shark. Salmon sharks in Alaska are most commonly observed during seasonal surface aggregations in southern inshore waters from July through September. Temporal abundance of salmon sharks in Prince William Sound is poorly documented. They have been observed associated with the sac roe herring fishery in spring (April-May) and during the fall herring bait fishery (September-October). Peak abundance appears to occur during July and August, corresponding with the return of adult salmon to Prince William Sound. Apparently, an occasional salmon shark is taken in trawl gear during the winter pollock fishery in Prince William Sound (Robert Bercelli pers. comm.). 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 tracked down 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 gear. When handling so many sharks at a time proved to be a dangerous and chaotic endeavor, 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. Capable of high-speed runs and aerobatic leaps when hooked, salmon sharks are becoming the hot "new" Alaskan big game fish. In recent years several hundred sharks averaging 7’ in length and 400+ pounds have been taken between two charter companies with which I am familiar (Luke Borer 1988, Bob Kandopolous 1998, pers. comm.). Many more have been caught and released. 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. Currently, no Federal Management plan exists for the GOA and the Aleutian’s, and closure of commercial shark fishing in Federal waters is also being considered. One of the primary objectives of this study is to investigate spatial and temporal patterns of shark abundance in the Gulf of Alaska and Prince William Sound based on survey data and commercial by-catch records. Although trawl, gillnet, and seine fisheries are probably responsible for a considerable number of salmon shark interactions, historical records of by-catch are poorly documented. As a fast swimming pelagic species, salmon sharks are not well represented in commercial catches. The low "catchability" factor of salmon sharks by gear types employed by Alaska’s commercial fishermen, and their patchy spatial and temporal distribution, confound efforts to extrapolate existing catch records to trends in relative abundance. Nonetheless, I am optimistic that through the cooperation of commercial fishermen and spotter pilots, an annual survey of salmon shark sightings and observations will provide a valuable contribution to the understanding of salmon shark ecology and document predator/prey interactions in the Prince William Sound and Gulf of Alaska ecosystem. [ Next Thread | Previous Thread | Next Message | Previous Message ] |
| Subject | Author | Date |
| Re: SALMON SHARKS | Randall Holman | 928366823PDT |
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