One Endangered Species Eats Another: Killer Whales and Salmon
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January 22, 2013
With clear skies above and a crystalline view of the Seattle skyline to the east, Brad Hanson motors along in a Zodiac inflatable, following a respectful distance behind a pod of killer whales. As the whales feed on Chinook salmon, Hanson and his crew skim what’s left of the whales’ meal off the water: fish scales, shreds of salmon, whale feces. “It’s very strange to be out with these huge predators right in the middle of an urban area,” Hanson says. It’s also very practical for data collection. With the samples he scoops from the water, Hanson will extract detailed information about the killer whales and their prey.
Hanson is a marine mammal biologist with NOAA’s Northwest Fisheries Science Center. The animals he’s studying are southern resident killer whales—the endangered population that spends much of the summer in and around the Puget Sound. There are only 81 of them, and their population is recovering very slowly. Hanson and his colleagues are trying to figure out why.
One of the main suspects is a lack of prey. Southern residents are highly selective in what they eat, preferring bigger, fattier Chinook salmon to the more abundant Pink and Sockeye. Many populations of Chinook salmon, like the whales that eat them, are also threatened. As a result, there may not be enough Chinook to support growth in the killer whale population.
Researching the Relationship Between the Species
Much recent research has been aimed at understanding the relationship between these two species. Brad Hanson’s research—a collaboration with Canadian colleagues and academic and non-governmental partners—is an example. Many different populations of Chinook salmon mingle in the Pacific before separating into spawning runs. If scientists knew which salmon populations the killer whales depended upon most, managers could adjust salmon recovery efforts or fishing limits in a way that would most benefit the whales. By analyzing the salmon DNA in the samples he collects off the water, Hanson is able to sort out which Chinook populations the killer whales are eating.
Hanson’s results indicate that during the summer months, Chinook make up roughly 80 percent of the whales’ diet. Of those, about 90% are Fraser River Chinook, named for the river basin in British Columbia where they spawn. The Canadian government has listed several Fraser River stocks as having medium to high conservation concern, meaning that their populations are at risk of decline.
Although Hanson’s approach reveals with great specificity what the southern resident killer whales are eating, it does not say how much. Dawn Noren, a physiologist also based out of the Northwest Fisheries Science Center, has tackled that question. Noren models the metabolic rates and prey requirements of killer whales. She does this by observing the activities and swimming speeds of wild killer whales. With collaborators at universities and other research institutions, she also extrapolates the food consumption rates of captive killer whales to their wild cousins. Both methods yield similar results, and together they allow scientists to estimate more precisely than before how much prey wild killer whales consume.
Noren and her co-authors estimate that, during the summer months, the southern residents consume between one-eighth and one-quarter of returning Fraser River Chinook. In other words, the whales are taking a big bite out of the Fraser River Chinook run. And they will require still more if their population grows.
The Big Unknown
For all the new detail that these research projects reveal, they have one big weakness. The results are limited to the summertime. That’s when the southern resident killer whales live among us, so that’s the side of them we know. But there are two sides to this story. When winter comes the whales head out to the open ocean, leaving the scientists and the rest of us behind.
“This is one of the most intensively studied populations of marine mammals in the world,” says Eric Ward, a population ecologist also at NOAA’s Northwest Fisheries Science Center. We have records of the birth year, death year, and line of matrilineal descent for every southern resident killer whale born since the late 1970’s. “But,” Ward says, “we don’t know much about what they’re doing for six months of the year.”
For all the data about the southern residents relying on Chinook in summertime, it’s not clear that that's the factor limiting their population growth. “The big unknown,” says Hanson, “is where the whales are and what they’re eating in winter.”
Hanson and his colleagues do briefly follow killer whales out to sea in the winter. But winter conditions on the Pacific require a large NOAA research vessel. This precludes a close approach to the whales, and in any case, vessel time is hard to come by. During the week or two of good weather they usually have, Hanson and his colleagues locate the whales using bigeye binoculars and hydrophones that listen underwater for whale calls. When they detect the whales, they turn the ship about and close in.
“Once you get crossways in the swell all hell starts to break loose,” Hanson says of the North Pacific in winter. “Even on a 220-foot vessel. Everything’s falling off the shelves. No one’s getting any sleep.” Based on limited evidence from tissue samples, scientists know that the whales are eating Chinook salmon and possibly other species in winter. But which populations and how much remain a mystery.
Hanson and his colleagues also recently started using satellite tags to track the movements of one or two whales in wintertime. But shooting a tag into the dorsal fin of a whale is an invasive procedure, so they limit the number of tags placed each year. They have also moored a handful of listening devices to the seafloor. These have detected passing killer whales several dozen times in the past five years, the most location data to date. But the whales’ known range is large and the number of recorders is small. In the wintertime, the Pacific Ocean jealously guards its secrets.
Ecosystem-based Management is the Only Solution
Marine biologist Lynne Barre heads up NOAA’s southern resident killer whale recovery program. She had hoped that some of the new science would bring to light management actions that would directly benefit killer whales. For instance, if scientific findings pointed to a specific Chinook population as the factor limiting killer whale population growth, then managers might have an effective lever in fishery regulations.
Simple answers, however, remain elusive. An independent science advisory panel funded by NOAA and Fisheries and Oceans Canada recently concluded that the jury is still out on whether prey availability is the limiting factor. The panel also found that reduced fishing for Fraser River Chinook might not help much anyway since other predators, such as seals and sea lions, would compete for the extra fish. Also, relatively few Fraser River Chinook are caught in the ocean to begin with, so fishing can’t go down much farther. “There remains a lot of uncertainty, even with some of the new work and analysis we’ve done,” says Barre. “Unfortunately, there’s no easy answer.”
There may not be an easy answer, but there is an obvious one. Killer whales and Chinook salmon are both high level predators sharing the same ecosystem, and both need that ecosystem to be healthy if they are to thrive.
“This situation perfectly demonstrates the need for ecosystem-based management,” says Dawn Noren, the NOAA physiologist. She’s referring to a management approach that considers all components of an ecosystem, including humans and the interactions between them. “We can’t fix this situation simply through fishing regulations. We also need management actions that increase salmon abundance, such as freshwater habitat restoration. That will help the salmon, and it will also help killer whales in the long run.”
In the meantime, Brad Hanson and others will continue trying to follow the southern resident killer whales out into the Pacific Ocean. Perhaps the whales will reveal more about what they eat in wintertime, allowing managers to prioritize the restoration of freshwater salmon habitat. For now, though, the winter is a mystery. That’s the thing about killer whales, that contradictory animal, half black and half white. They live with us in close, almost intimate proximity for half the year. But then they leave us, and we’re in the dark.