Both killer whales and Chinook salmon are endangered in the Pacific Northwest. And one of the biggest problems facing both animals is that one eats the other.
According to a new study, a single small and vulnerable group of whales may eat close to a quarter of the salmon run in British Columbia’s Fraser River -- and that’s just in the summertime.
The findings emphasize the importance, when trying to save one creature, of looking out for everything that hunts and is hunted by it. In this case, whales can't rebound unless the fish bounces back, also. But saving both of them is not that simple.
"This is a case where one endangered species is eating another endangered species," said Rob Williams, a marine conservation biologist now at the University of St. Andrews in the United Kingdom. "We're not going to get anywhere if we have single-species management that doesn’t recognize that what is good for one species may be bad for another."
Southern resident killer whales, which live off the coast of Washington State and British Columbia, are one of the most critically endangered groups of marine mammals living in American waters. At last count, the population consisted of just 87 whales.
Part of what makes the massive hunters so vulnerable is that they are highly specialized, said Williams, who conducted the study while at the University of Washington, Seattle. Chinook make up more than 80 percent of their diet, according to previous work. And more than 90 percent of the Chinook they eat come from the Fraser River population.
In fact, southern resident killer whales will swim right by schools of herring and small pink salmon to seek out the biggest, oldest and fattiest Chinook they can find. Trouble is, many runs of Chinook salmon are also endangered, including the Fraser River population.
To get a sense of just how many fish the whales need to both sustain their population size and multiply, Williams and colleagues started with data on the nutritional requirement of captive whales. Then, they combined this information with the population structure of the wild whale population and the known make-up of their diet.
Results, published in the journal PLoS One, showed that southern-resident killer whales consume somewhere between 12 and 23 percent of Fraser River Chinook between the months of May and September.
Each day, the group of 87 whales eats an estimated 662 Chinook, and an average Chinook weighs about 19 pounds. That's close to 145 pounds of fish per whale per day. If the population recovers to include 155 whales by 2029, as some recovery plans are shooting for, the whales' demand for Chinook would increase by about 75 percent.
There are plenty of uncertainties in these estimates. Whale mothers, for example, need 42 percent more calories when they are lactating. And wild whales may be more active and need more food than captive whales do.
Still, by taking a stab at quantifying the impact of whales on salmon, studies like this one should help scientists refine their assessments of how sustainable various fisheries really are. Chinook are popular in the human diet, too.
"It gives us a metric," said marine biologist Lynne Barre, leader of the Southern Resident Killer Whale recovery program at NOAA Fisheries in Seattle. "If a fishery reduces its prey abundance by this much, here's how it will affect the growth of killer whales."
As conservation programs continue to focus on helping either endangered killer whales or endangered Chinook salmon, the best way to protect both species, Williams said, is to look at the entire ecosystem and preserve the habitats they share. And that’s a message that applies to plenty of threatened pairs of predators and prey around the world.
"This really speaks to the value of ecosystem-based management," he said. "I would love for killer whales and Chinook salmon to be an icon for that."