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updated 6/6/2012 6:20:48 PM ET 2012-06-06T22:20:48

Killer whales in the Pacific Northwest are jonesing for their favorite food, Chinook salmon, suggests new research finding the whales' stress levels increase when the fish food supply dwindles.

Having more prey around and lower stress levels are important to keeping the killer whale population numbers high, the researchers said.

"Recovering their Chinook salmon prey is critical to assure long-term killer whale recovery," study researcher Samuel Wasser, of the University of Washington, said in a statement. "Everything, including boats and toxins, matters more when prey is low."

Researchers had previously suggested three reasons for the whale's decline: Possible chemical contamination of the waters, a decline in prey species due to commercial fishing, or increased marine noise from whale-watching boats and large shipping vessels.

Whale poop

The researchers studied a population of killer whales off the Northwest coast of North America, confusingly called the Southern resident killer whales, for three summer seasons. These orcas are listed as endangered by the U.S. Fish and Wildlife Service. The clan includes approximately 89 individuals, a number that dropped about 20 percent between 1995 and 2001.

They are usually seen in the waters off the coasts of Oregon, Washington and British Columbia during the summers. During that time, Chinook salmon makes up 97 percent of their diet. The recent study used a poop-sniffing dog to collect fresh fecal samples from the whales during three summer seasons. (The dog can sniff out fecal matter from a mile away, with scientists steering their boat in the direction of the pup's nose.)

They tested the samples for stress hormones, including thyroid and glucocorticoid hormone levels. Animals stressed out by something in their environment, like noise or being hungry, have high glucocorticoid levels. When they run low on food, their thyroid hormone levels drop to slow their metabolism and preserve energy.

Stress levels

When whale-watching boats and other vessels were most numerous in the summer, glucocorticoids should have spiked if the whales were bothered. Instead, glucocorticoids declined, driven by a sated belly from an increase in the number of salmon present, the researchers said. They saw thyroid hormone levels increase during that time, too, indicating the whales were well-fed and less stressed.

"The data support Chinook salmon being a more important driver of physiology than vessel traffic for the Southern resident killer whale population," study researcher Katherine Ayres, a graduate student at the University of Washington, said in a statement. "However, vessel traffic may cause added physiological stress during times of low prey availability."

To save the whales, conservationists need to focus on keeping salmon populations at consistently higher levels, the researchers said. If the salmon levels are up, the loud noise from boats is much less of a problem, they said.

The study was published today in the journal PLoS ONE.

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