Something fantastic is happening with sharks in the waters of the Gulf of Alaska.
Shark abundance in the gulf has been apparent to fishermen throughout the 1990s. Spiny dogfish sharks (Squalus acanthias), the predominant shark species in near-shore Alaska waters; Pacific sleeper sharks (Somniousus pacificus); and salmon sharks (Lamna ditropis) have dramatically increased in abundance in the eastern gulf and Prince William Sound.
Salmon shark numbers also appear to be up in Southeast Alaska. Fishermen have been targeting them in areas where they congregate, sometimes off streams with returning salmon. Last summer there were reports of salmon sharks plying the waters off the mouth of Peterson Creek at the time of returning chum salmon.
Spiny dogfish are commonly taken as bycatch in commercial fishing gear in Alaska. They are particularly well represented in the pelagic trawl Pollock fishery and in longline fisheries for sablefish, halibut, Greenland turbot and Pacific cod.
International Pacific Halibut Commission longline survey data indicate an increasing trend in relative abundance of dogfish sharks along the eastern and central gulf coast of Alaska in the 1990s. Dogfish numbers appear to increase during the spring near the Copper River where they feed on eulachon. Fishermen in the Yakutat and Copper River areas have problems with dogfish swamping salmon gill-nets.
Dogfish bycatch have presented a formidable problem for Halibut Commission statistical analyses of halibut abundance in recent years. Halibut surveys often catch many more dogfish sharks than halibut.
Another shark species that has increased in abundance in recent years is the Pacific sleeper shark. Sleeper sharks are one of the few sharks found in polar waters year-round. They are related to the Greenland shark. They are a large bottom-dwelling species generally inhabiting deep water, although they occasionally come to the surface at high latitudes.
Researchers from National Marine Fisheries Service and the International Pacific Halibut Commission in Alaska have caught specimens in the 6 meter range (about 18 feet) although they average 1.8 to 2.4 meters (6 to 7 feet) in length in Prince William Sound sablefish surveys.
Sleeper sharks are opportunistic predators whose diet consists primarily of groundfish, squid and salmon. They have been found eating hooked halibut up to 40 pounds. They are also known to prey on marine mammals, including harbor seals and southern right whale dolphins.
Alaska Department of Fish and Game sablefish survey data also indicates an increasing trend in sleeper shark abundance since the survey began in 1996.
While finding empirical data for relative trends in sleeper shark and dogfish bycatch in Alaska is difficult, it is particularly hard to come by data for salmon sharks.
Salmon sharks are rarely caught in commercial hear and information on trends in abundance is largely anecdotal. However, salmon sharks appear to be the predominant large predatory pelagic fish in the coastal Gulf of Alaska. A member of the family Lamnidae, salmon shark has a similar related species in the Atlantic Ocean, the porbeagle shark. They are also closely related to white and mako sharks. Throughout the 1990s, salmon shark abundance in the northern gulf increased dramatically.
The vast majority of salmon sharks aggregating in surface waters of the gulf are adult females. They have been reported to reach 3 meters (9 feet) in length, although normal size range appears to be between 1.8 and 2.4 meters (6 to 7 feet).
Salmon sharks maintain an elevated body temperature and studies have shown that they may have the highest body temperature of any shark, as much as 80 degrees Fahrenheit above ambient water temperatures. Because of this, they likely possess a relatively high metabolic rate and daily ration. Their diet consists primarily of salmon, squid and groundfish.
What brought about the increase in abundance of sharks in coastal Gulf of Alaska?
Are sharks the new dominant apex predator in the gulf ecosystem, having shifted from marine mammals to sharks?
These are some of the many questions being asked about the fantastic increase in abundance of sharks in the gulf.
Bruce Wright is chief of the Office of Oil Spill Damage Assessment and Restoration for the National Marine Fisheries Service. More information on sharks is available at www.fakr.noaa.gov/oil/sharks.htm. Rod Flynn of the Alaska Department of Fish and Game will present a slide program on marten and their habitat on Chichagof Island at the Juneau Audubon Society March meeting at 7 p.m. Thursday at the Dzantik'l Heeni Middle School library.
© 2017. All Rights Reserved. | Contact Us