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Orca mom and calf
A southern resident killer whale calf accompanies its mother in 2004. (NOAA Photo)

What’s killing the killer whales? After following the whales and analyzing their poop for years, scientists say the Pacific Northwest’s population is dwindling primarily due to a chronic lack of Chinook salmon.

The killer whales, also known as orcas, aren’t dying of starvation. Rather, the scientists say the stress of not getting enough to eat is causing orca pregnancies to fail.

Other factors, such as marine pollutants and disruptive ship traffic, contribute to the whales’ woes as well. But in a paper being published in Thursday’s issue of the journal PLOS ONE, the researchers say the data point most directly to nutritional stress.

“I think our study is quite conclusive about the role that lack of prey is having on the killer whales,” lead author Samuel Wasser, a biologist at the University of Washington and director of UW’s Center for Conservation Biology, told GeekWire today.

Wasser and his colleagues painstakingly built up a chain of evidence by observing the orcas that spend their summers in the sheltered waters around northwest Washington state and southwest British Columbia. This body of water, known as the Salish Sea, includes Puget Sound and the Strait of Juan de Fuca, which are magnets for whale-watchers.

The orca population that hangs out in the Salish Sea from May to October, known as the southern resident killer whales, has declined from 98 whales in 1995 to 78 at last count, despite stepped-up efforts to protect the endangered species.

Biologists from UW, the National Oceanic and Atmospheric Administration’s Northwest Fisheries Science Center and the Center for Whale Research started gathering data in 2007 to look into the causes of the decline.

Every spring, orcas would arrive from their wintering grounds in the open Pacific Ocean off the West Coast. The research team noted how many orcas returned and how many calves were born, and kept tabs on all the orcas until they headed back out to the Pacific in the fall.

To take readings on the whales’ health, the researchers trained dogs to ride on boats trailing the killer whales and sniff out blobs of fresh orca poop. The scat was retrieved and analyzed to determine what the whales were eating, what hormones they were secreting, and what toxins they were absorbing.

Even the DNA in the poop was decoded, to determine which orcas were responsible for which leavings.

The team took a particularly close look at hormones linked to stress and to pregnancy. The hormonal readings showed that there were a total of 35 pregnancies over a period ranging from 2007 to 2014. But only 11 new calves were spotted during that time frame, suggesting that the 24 other pregnancies failed.

Scientists have long known that the southern resident killer whales have a lower birth rate than other orca populations – but there’s been a long-running debate over the reason why. Was it a lack of salmon, which is the dominant fare in the orcas’ diet? Was it the prevalence of pollutants such as polychlorinated biphenyls, or PCBs? Or was it the disturbances caused by passing ships?

To seek an answer, researchers compared the levels of two hormones that were extracted from the scat of the pregnant orcas – a thyroid hormone, as well as a glucocorticoid hormone (also known as cortisol) which is more strongly linked to nutritional stress.

The comparison showed that the stress indicator was dramatically elevated in the scat from orcas whose pregnancies ended up failing. What’s more, the pattern of the rise and fall in hormone levels pointed to poor nutrition rather than ship traffic as the dominant stress factor.

“The lack of fish is a much stronger signal,” Wasser said.

Wasser said pollutants could be a contributing factor, potentially related to the orcas’ nutritional state.

“Those toxins are stored in fat, and they accumulate over their entire lifetime,” he said. “Dumping the toxins out of fat exacerbates the abortion rate. … But if you keep the fish high, the toxins stay locked up in the fat.”

The study suggests that the fate of the southern resident killer whales is strongly linked to the fate of Chinook salmon runs, which have been hard-hit by environmental factors ranging from overfishing and dam construction to pollution and invasive species.

“It’s all about the fish, and we have to do something about how to increase fish runs,” Wasser said. “One of the things we need to look at seriously is, what is the impact of the Snake River dams on the Chinook?”

He said the whales’ woes may shed light on wider woes as well.

“Some of the Native American tribes … we know that a number of them have had reproductive problems that parallel what happens to the killer whales,” Wasser said. “So there’s a lot of reason to think that bringing the fish back would help a lot more than the killer whales.”

In addition to Wasser, the authors of the PLOS ONE study, “Population Growth Is Limited by Nutritional Impacts on Pregnancy Success in Endangered Southern Resident Killer Whales (Orcinus orca),” include Jessica Lundin, Katherine Ayres, Elizabeth Seely, Deborah Giles, Kenneth Balcomb, Jennifer Hempelmann, Kim Parsons and Rebecca Booth.

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