SEATTLE — In the decade since Puget Sound’s southern resident killer whales were protected under the Endangered Species Act, scientists have figured out where they go in winter, learned that they eat mostly chinook and have documented the many ways orcas shift their behavior in response to noise from boats.
Despite that vast increase in knowledge since the cetaceans were listed as endangered in 2005, the region’s orca population — already a fraction of what it was in the 1960s — still is not growing, according to a new synopsis of research on the troubled whales by the National Oceanic and Atmospheric Administration.
Scientist are “trying to understand … why the whales haven’t increased more than they have,” said Mike Ford, with the conservation biology program at NOAA’s Northwest Fisheries Science Center.
Back in the early 1970s, when Puget Sound’s killer whales were still being captured for sale to marine parks, there were 71 orcas left. Capture was outlawed a few years later, and orca populations climbed to 99 in the mid-1990s. As of last year, 82 remain.
Scientists have tried to understand the factors contributing to their decline and have confirmed that there are three major drivers: a decline in their food, the build up of pollutants in their system and disturbance by boats.
But understanding all the subtle ways these and other changes in the marine world affect Puget Sound whales continues to be complex.
For example, while contaminants tend to build up in the bodies of whales, making them some of the most polluted animals on Earth, not all pollution is the same.
“They pick up contaminants from where they’re going,” Ford said. “L and K pods tend to pick up California-type contaminants, compared to J pod, which doesn’t, and that’s consistent with their distribution.”
Meanwhile, as these orcas decline, other marine mammals in the Northwest that also rely on fish — seals, sea lions, even other killer whale populations — actually are on the rise. A population of northern resident killer whales, which have the same diet and share portions of the southern residents’ environment, has tripled to nearly 300 since the 1960s.
In fact, “it’s possible that some of those increases influence the rate at which southern resident populations grow,” Ford said.
But many mysteries remain. For starters, when killer whales die, they rarely wash up on the beach for researchers to find and study, said Lynne Barre, the head of protected resources for NOAA’s Seattle office.
Instead, they just disappear, and scientists have no way of learning what caused their deaths.
And Brad Hanson, a whale expert with the Northwest Fisheries Science Center, said their behavior is often quite unpredictable.
“Why do they sometimes decide to turn out toward the ocean and stay there for two weeks?” he asked.