Northern orcas getting first crack at the best chinooks: study

Killer whales native to the waters of northern B.C. and Alaska are selectively eating millions of large, nutritious chinook salmon long before the fish make their way to the feeding grounds frequented by our dwindling southern resident killer whales, according to new research.

Healthy northern orcas are devouring chinook salmon that sustain southern resident killer whales, new research finds.

Conservation efforts aimed at both killer whales and chinook salmon could be having “unexpected consequences,” said the study’s lead author, Jan Ohlberger, from the University of Washington’s School of Aquatic and Fishery Sciences.

The problem is that booming populations of northern resident killer whales and Alaska residents are getting first crack at the best chinooks, those more than 76 centimetres long.

Unlike the southern resident group, which is hovering at about 73 individuals, there are 300 northern residents and 2,300 Alaska residents, about three times as many as 30 years ago.

“Chinook from the Columbia River and B.C. river systems move north along the coast to the Gulf of Alaska where they put on most of their mass,” said coauthor Daniel Schindler, a UW professor of aquatic and fishery sciences.

“It’s pretty clear that Alaska killer whales and northern residents get first shot at [the chinook] before they head south through a gauntlet of predators, and that’s when the dregs show up in the Salish Sea,” he said.

In recent years, Canadian and American researchers have observed that the southern residents appear to be malnourished. Three members have perished this year, including one that showed obvious signs of body-fat depletion.

All three resident killer whale groups prefer large chinook and together eat about 2.5 million adult chinook salmon per year.

That feeding pressure has led to a decline in the body size of chinook and the number of large chinook over the past 40 years, according to the study published Monday in the Proceedings of the National Academy of Sciences.

Fishing pressure has eased over that same period, with little effect.

Chinook have declined 10 per cent in length and at least 25 per cent in weight, which is likely affecting their ability to reproduce. Smaller fish are less attractive to the resident killer whales, which typically do not eat chinook until they are at least 63 centimetres long.

“Killer whales don’t show a lot of interest in chinook until they reach a certain size, and then they focus intensely on those individuals,” said Ohlberger.

Both chinook salmon and resident killer whales have been the target of intensive conservation measures, but the success of the northern orcas appears to be coming at the expense of chinook and southern residents.

Eleven B.C. chinook stocks are considered endangered or threatened.

“We can’t expect every species to be abundant; there are limits and trade-offs,” he said. “These are ecosystems that are in flux and things are going up and down all the time, even without human influences.”

The recovery of seal and sea lion populations in the Salish Sea has also been implicated in the general decline of the chinook populations.

However, in the UW analysis, most arrows point to the northern residents.

“Most of the discussion about the southern residents in Vancouver and Seattle almost pretends that they exist in their own universe,” said Schindler. “In fact, they are competing [for food] with whales elsewhere that are doing really well, with incredibly high population growth rates.”

Taking a wider view of the northern Pacific, orcas are thriving, he said.

“There are more orcas out there now than there have been in decades,” he said. “The southern residents could be suffering from competition or changes to their environment.”

This article by Randy Shore Vancouver Sun
Published in the Times Colonist