Groundbreaking research finds that imperiled Chinook salmon are crucial to the survival of Southern Resident orcas.
The survival of the world’s most endangered killer whales may hinge on whether another species threatened with extinction can also be saved.
A new study has definitively shown for the first time that Chinook salmon—which are endangered from northern California to Washington and inland to Idaho—comprise 70 percent of the summer diet of the Southern Resident killer whales, three pods of about 89 orcas that live mainly in the inland marine waters of the Pacific Northwest.
The Southern Residents prefer Chinook even at times when other salmon species, such as sockeye and coho, are abundant, according to research published in the journal PLOS ONE.
Scientists and other observers have long known that salmon are the favored prey of the Southern Residents, said Michael Ford, a biologist with the United States National Marine Fisheries Service, who led the study. But the study “reconfirms the importance of Chinook salmon to this particular group of killer whales.”
“To save the orcas, we need healthy salmon runs,” agreed Miyoko Sakashita, oceans director and senior counsel with the Center for Biological Diversity. “I think one of the key things is passage through dams and diversions. We need restoration of the native salmon runs. We also need to take into account water flow and make sure that salmon are getting enough water flow downstream.”
The center is one of many conservation groups advocating for the removal of dams on Washington state’s lower Snake River, arguing that fully unblocking the river is crucial to restoring salmon runs.
Chinook salmon began returning to their historic spawning grounds on Washington state’s Elwha River in 2014, the same year an effort to bring down a century-old dam on the river was completed.
To figure out the role fish play in the orcas’ diet, Ford and his team reviewed DNA analysis of 175 fecal samples—whale poop—collected in the wild between 2006 and 2011.
As past research had proven that these orcas were almost exclusively fish eaters, the team tested the samples for the DNA signatures of different regional fish species to learn which ones the killer whales ate most
“With fecal samples, we get around some of the uncertainty of whether remains on surface [of the water] are representational of the entire diet,” Ford said. “They integrate their diet over, say, a day, rather than a single feeding event.”
Failing Chinook runs over the past few decades have helped hinder the recovery of the Southern Residents, while recent abundant runs may have helped spur a baby boom in 2015. A seventh newborn orca was spotted among the Southern Residents in early December, and an eighth about a week later.
“U.S. and Canadian research have found a statistical significance between Chinook numbers and killer whale birth rates,” Ford said.
Over 1.2 million Chinook were counted at the Bonneville Dam, which straddles the Oregon-Washington border, as they swam downstream in 2015, nearing 2013’s record run of more than 1.3 million—the biggest run since 1938.
The federal recovery plan for the Southern Resident killer whales, which dates from 2008, states that the whales need around 200,000 salmon during the summer to thrive, including 143,000 Chinook.
“If they’re fat and happy and getting enough salmon, they’re not using their blubber stores” for energy to forage or to nurture young, said Lynne Barre, the Southern Resident killer whale recovery coordinator with the federal fisheries service.
That’s important because the Southern Resident killer whales have been exposed to toxic pollutants called organochlorines, manufactured chemicals that persist in the environment. These substances impair reproduction and overall health in mammals (including people). When nursing female whales don’t have enough salmon to feed on, they burn their blubber stores to generate fat-rich milk for their calves instead, thereby offloading the toxic chemicals to their offspring.
Excessive exposure to a class of organochlorines called polychlorinated biphenyls, or PCBs, has caused reproductive failure among killer whales in the United Kingdom, and scientists expect them to go extinct as a result.
Ford noted that the Northern Resident killer whales, another group of Pacific coast orcas that prefer Chinook salmon and have faced many years of lean salmon runs, seem to be reproducing more successfully than their cousins to the south.
“It doesn’t appear that lack of salmon has kept them from fairly robust population growth,” Ford said. This suggests to him that Chinook recovery is one of several keys to the recovery of the Southern Residents.
The whales are also dealing with habitat disturbances, including commercial shipping activity, coastal development, and U.S. Navy sonar training exercises in both Puget Sound and the Olympic Coast National Marine Sanctuary on Washington’s outer coast.
In February, the Obama administration announced a plan to include about 9,000 miles of Pacific coastline in the officially designated critical habitat for the Southern Residents. The move came in response to a petition from the Center for Biological Diversity, which used satellite-tracking data to show that the whales travel as far south as Point Reyes, California, in search of fish to eat.
Wildlife officials are now gathering and studying the information they need to finalize the habitat expansion, said Barre, including how much Chinook and other salmon the whales can forage when they leave the inland marine waters of Puget Sound during the fall and winter.
Officials expect to finalize the plan in 2017, Barre said.
Ford said his research team is currently analyzing poop samples collected from the Southern Residents during past winters, aiming to identify the mix of fish they are eating when swimming between California and southern Canada, and that the research will likely play a role in the expansion of the whales’ critical habitat.
“I’m sure the National Marine Fisheries Service is gathering and reviewing the science,” said Sakashita, “but we’d like to see them move a lot faster for the survival of the Southern Resident killer whales.”