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, dont represent ecological invasions and they arent 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 Consorti
Contact: Sandra Hines
University of Washington