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Alaskan Waters Growing Hospitable To Sharks While Seals And Sea Lions Decline

Date:
February 20, 2002
Source:
University Of Washington
Summary:
Salmon shark fins cut the surface of the water. A spotter in an Alaska Fish and Game plane on an annual survey of sea otters estimates there are hundreds of fins in the small bay near Port Gravina, Prince William Sound. More sharks mass – unseen – below the surface. All are likely feasting on a run of salmon returning to spawn in nearby rivers and creeks.

Salmon shark fins cut the surface of the water. A spotter in an Alaska Fish and Game plane on an annual survey of sea otters estimates there are hundreds of fins in the small bay near Port Gravina, Prince William Sound.

More sharks mass – unseen – below the surface. All are likely feasting on a run of salmon returning to spawn in nearby rivers and creeks.

“That aerial count would be a high number of sharks in one spot for any place in the world,” said Vince Gallucci, University of Washington professor of fisheries and aquatic sciences. Gallucci, who has studied shark population dynamics for more than a decade, the last two years in Alaska, organized and is a speaker in the Saturday, Feb. 16, session “Not Enough Sea Lions, Too Many Sharks: Global Warming Signal?” during the American Association for the Advancement of Science meeting in Boston.

In the last two years of work in Prince William Sound, with cooperation from the Alaska Department of Fish and Game and NOAA's National Marine Fisheries Service, Gallucci says six salmon sharks an hour could be caught during certain times of day.

And sometimes, lines were in the water less than 2 minutes before a salmon shark bit.

Meanwhile the number of Pacific sleeper sharks encountered by halibut fishing vessels has increased every year since 1997, more than doubling, according to a database made available to Gallucci by the Pacific Halibut Commission.

All this in a place where fishermen haven't talked of sharks in such abundance in the past. Gallucci says a Fairbanks-based audio producer specializing in environmental stories has reported that fishermen, both commercial and tribal, don’t remember encountering many sharks while harvesting salmon and groundfish.

“Fishermen wouldn't forget 200-pound animals bending hooks and wrecking their nets,” Gallucci said displaying a No. 3 steel halibut circle-hook almost straightened by a salmon shark.

Salmon sharks, 6 to 8 feet long as adults and related to the great whites, Pacific sleeper sharks, which can grow to 24 feet in length, and dogfish are the most plentiful members of the shark family in Alaskan waters.

No one knows how many sharks there are because the necessary surveys haven't been done, something Gallucci's group is working on. Still, fieldwork by Gallucci and colleagues, along with surveys by others showing declining numbers of sea lions, seals and other pinnipeds (animals with finned feet), is enough for Gallucci to say top predators in Alaska's sub-arctic waters have shifted to a new “steady-state.” Instead of only orcas, pinnipeds and a few sharks, the line up is now orcas, increasing numbers of sharks and declining numbers of pinnipeds.

“Increases in salmon sharks and Pacific sleeper sharks, both sub-arctic northeast Pacific shark species, don’t represent ecological invasions and they aren’t range extensions since both sharks are endemic,” he says.

He believes population changes are tied to the decades-long swings in climate caused by the Pacific Decadal Oscillation, but exactly how sharks got such a finhold while pinnipeds struggle is not known. It also is probable that global warming is a factor, he says, however the Pacific Decadal Oscillation is the far more dominant effect.

The Pacific Decadal Oscillation is a regional climate shift that changes the temperature of surface waters in the central and north Pacific. It takes 20 to 30 years to shift between cold and warm phases, and the Pacific Decadal Oscillation has been in the latter since 1977. There is some evidence in just the last two years that the regime may be shifting again.

The environmental changes that follow such shifts, as well as human activities such as fishing, could be reasons Alaskan waters may have become more hospitable to sharks, Gallucci said. His research into shark population dynamics is funded through the North Pacific Universities Marine Mammal Research Consortium, which receives money from those in the fishing industry interested in the relationship of fishing and marine mammals and from NMFS.

There have been changes in the populations of fish these animals eat. Climate conditions in recent decades have favored salmon in Alaskan waters for a number of years and heavy hatchery production has added millions more fish, to the benefit of both pinnipeds and sharks. On the other hand, commercial fishing of pollock in competition with sea lions may have indirectly benefited sharks, Gallucci says.

"Sharks, being the more efficient eaters, just may be able to take greater advantage of changes in the food that's available," he says.

Then there are other factors such as the 1992 moratorium on drift-net squid fisheries that eliminated a source of incidental shark mortality. And, while there is no direct evidence of sharks preying on sea lions or seals in Alaskan waters, sharks do feed on pinnipeds in other parts of the world. Gallucci says more investigation is needed to know for sure.

The interactions that have been occurring, and those possible in the future, are impossible to determine without knowing the life history of the sharks, Gallucci says. Fieldwork last summer, for instance, revealed that most of the salmon sharks caught were females but researchers don't know why. Among other things, Gallucci's group is trying to find ways to analyze hormones from blood samples that might allow them to determine the age, sexual maturity and other information. Such life-history data can then be used to model shark dynamics.

Unraveling the basic biology of the animals also may give scientists insight on how these large predators might shape the ecosystem in the future.

Collaborators on Gallucci's AAAS presentation on shark abundance are Lee Hulbert, with NMFS in Juneau, and Bruce Wright, co-owner of Environment Alaska of Juneau.


Story Source:

The above story is based on materials provided by University Of Washington. Note: Materials may be edited for content and length.


Cite This Page:

University Of Washington. "Alaskan Waters Growing Hospitable To Sharks While Seals And Sea Lions Decline." ScienceDaily. ScienceDaily, 20 February 2002. <www.sciencedaily.com/releases/2002/02/020218094758.htm>.
University Of Washington. (2002, February 20). Alaskan Waters Growing Hospitable To Sharks While Seals And Sea Lions Decline. ScienceDaily. Retrieved August 20, 2014 from www.sciencedaily.com/releases/2002/02/020218094758.htm
University Of Washington. "Alaskan Waters Growing Hospitable To Sharks While Seals And Sea Lions Decline." ScienceDaily. www.sciencedaily.com/releases/2002/02/020218094758.htm (accessed August 20, 2014).

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