Wednesday, February 6, 2008

Whales crowd seamounts off Hawaii at night

Bumps on the ocean floor, the biggest ones called seamounts, are known to attract many kinds of marine life—and new research indicates that this includes whales.

(Image: Cuvier's beaked whale. Credit: National Marine Fisheries Service Southwest Fisheries Science Center.)

Seamounts seem to create changes in current flows, concentrate some kinds of marine life, and attract predators of that marine life to those concentrations.

Fishermen have long known that they could improve their catch of certain species by seeking out places where the ocean floor changes elevation.

Researchers recently furthered our understanding of what goes on at seamounts by putting microphones on seamounts and listening for who showed up.

They used Cross Seamount, a feature about 100 miles south of O'ahu. The seamount rises from ocean two miles deep all around it to a depth of just 1,200 feet. It has a flattish summit about three by four miles across.

A sound recording system, referred to as a high-frequency acoustic recording package, was installed at the seamount for six months in 2005. It recorded for five minutes at a time, turning on every 25 minutes.

The results of the recording study was published this week in the journal Biology Letters, in a paper entitled “Temporal patterns in the acoustic signals of beaked whales at Cross Seamount.” The team conducting the research included Dave Johnston of the University of Hawai'i's Joint Institute for Marine and Atmospheric Research, Mark McDonald of Whale Acoustics, Jeff Polovina and Reka Domokos of NOAA's Pacific Island Fisheries Science Center., and Sean Wiggins and John Hildebrand of the Marine Physical Laboratory at Scripps Institution of Oceanography.

Scientists have long known that things happen around seamounts.

“Seamounts can have profound effects on the local physical and biological environment,” the authors write. “They can structure the velocity and vorticity of ocean currents and alter the vertical structure of water properties. These changes in the physical environment can alter local biological and ecological phenomena.”

But while a lot is known about what can happen, there's not a lot known about what actually does happen around the 4600 or so seamounts in the Pacific Basin alone. Previous studies shows that tuna fitted with electronic data recorders visit seamounts frequently, as do sharks. Above the surface, seabirds congregate there.

The fact that all those predators show up at seamounts suggests they provide a food source.

Not much was known about whales and seamounts, although there have been studies showing baleen whales are found in the seamounts of the Mediterranean and blue whales travel between deep ocean canyons and seamounts.

The Cross Seamount sensors recorded the echolocation signals and feeding signals of beaked whales, possibly Cuvier's beaked whales and Blainville's beaked whales, both of which have been spotted in Hawaiian waters.

Previous research by Robin Baird of Cascadia Research Collective, has found that both species appear to be semi-permanent residents of Hawaiian waters. The smaller Blainville's whale grows to 14 to 15 feet in length and can weight a ton. The Cuvier's whale reaches 18 to 19 feet and can weight three tons.

The Cross Seamount studies indicated the whales were primarily working the summit of the seamount at night, and that their sounds included both echolocation and what are called “feeding buzzes.”

The animals were present during the entire April to October period during which the microphones were in place.

The researchers' best guess is that the whales are attracted to the seamount for its tendency to have higher quantities of food resources than the surrounding ocean.

These are fish, crustaceans and other forms of life that scientists call nekton.

Nekton are ocean creatures capable of movement, like shrimps, fish and whales themselves. The term distinguishes them from plankton, which are smaller creatures that mostly move with the water and lack the ability to travel independent of currents.

“Concentrations of micronekton (those roughly 1 inch to four inches in length) were aggregated over the seamount in near-surface waters at night, and dense concentrations of nekton were detected across the surface of the summit,” the paper says.

The seamount may also be a convenient feeding spot in part because predators can “trap” prey against the surface of the seamount summit, making feeding easier, they say.

© 2007 Jan W. TenBruggencate