Tuesday, July 12, 2011

Hawai`i causes vast ocean eddies, and whales take advantage of them

As winds and currents sweep across the central Pacific, the Islands cause disruptions in their flow—creating vast eddies.

Those eddies can bring nutrient-rich waters up from the deep, and create an extensive food chain.

(Image: Three melon-headed whales. Credit: © Robin W. Baird.)

New research indicates that melon-headed whales take advantage of this food resource. Satellite tagging indicates they regularly feed around the fringes of some eddies, and near the centers of others.

Researchers placed satellite tags on 10 adult melon-headed whales that are part of a population that stays around the main Hawaiian Islands.

In analyzing the tracks of the whales, they found that several of them spent significant amounts of time around several of the major eddies around the Hawaiian Islands, and were presumably feeding there, since it is known that these eddies concentrate fishery resources.

The whales tended to use the edges of counter-clockwise (cyclonic) eddies that have cold centers, and to go to the hearts of clockwise-turning (anticyclonic) eddies that have warm centers.

What’s going on at these locations? The authors write of the fringes of one such eddy: “This edge region is characterized by convergence of nutrients and phytoplankton upwelled in the eddy’s divergent center. Several studies identify this convergence zone as a fruitful foraging ground for species such as sea turtles, marlin and tuna, seabirds, and cetaceans.”

The paper, “Eddies as offshore foraging grounds for melon-headed whales (Peponocephala electra),” was written by: Phoebe Woodworth of NOAA’s Pacific Islands Fisheries Science Center in Honolulu; Gregory Schorr, Robin Baird and Daniel Webster off the Cascadia Research Cooperative; Daniel McSweeney of Wild Whale Research Foundation; Bradley Hanson of NOIAA’s Northwest Fisheries Science Center in Seattle; Russel Andrews of the University of Alaska Fairbanks’ School of Fisheries and Ocean Sciences; and Jeffrey Polovina, of the Pacific Islands Fisheries Science Center in Honolulu. It was published in the journal Marine Mammal Science.

“Both warm and cold core eddies are continually recurring features in the lee of the Hawaiian Islands. Wind stress from easterly trade winds is intensified by the islands’ topography, leading to the formation of eddies,” the paper says.

These kinds of research often turn up other mysteries that need solving. In this case, some of the whales went to feed in an area west of Ni`ihau, where no eddies are known to occur. What’s happening there? That’s a new subject for inquiry.

“The melon-headed whale tracks … indicate a potentially favorable habitat southwest of the Hawaiian island of Ni‘ihau, although the factors contributing to this area’s desirability are unclear,” the authors wrote.

© Jan TenBruggencate 2011

1 comment:

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