A team from the University of British Columbia says the assumption that southern resident killer whales are in decline due to lack of chinook salmon in Canadian waters does not hold up under scrutiny.

Instead, their new study suggests declining chinook stocks are only part of the problem facing the critically endangered orcas, and that researchers need to look beyond the Salish Sea for answers.

Andrew Trites, co-author and director of UBC’s Marine Mammal Research Unit, called the findings published earlier this month in the Canadian Journal of Fisheries and Aquatic Sciences unexpected. 

“It doesn’t fit with what anybody believed was going on,” he said.

In the last decade, the number of southern resident killer whales has dwindled to just over 70. Observers believe many in the group are increasingly malnourished and skinny. 

Over the past five years, the population has been returning to its Salish Sea summer feeding ground later every year.

These factors, combined with declining chinook stocks, have led to the assumption that the orcas were staying away from the Salish Sea because they just weren’t finding enough to eat.

Wanting to test that conclusion, the UBC team set out to compare the southern residents to their thriving northern resident killer whale relatives, who also feed primarily on chinook salmon, and whose number has grown to over 300.

Members of the A5 pod of northern resident killer whales, as shown in this recent handout image provided by Jared Towers, a scientist with the Department of Fisheries and Oceans. (handout: Department of Fisheries and Oceans-Jared Towers/Canadian Press)

“[The northern resident killer whales] are fatter on average and the population is increasing,” said Trites.

“And so the presumption has been even though they too are feeding on the declining population of chinook, they’ve got more fish available to them and that’s why they’re doing better.”

By using ship-based multifrequency echosounders — what Trites calls the world’s most expensive fish finder — the researchers evaluated the size and availability of chinook to each killer whale population in 2018 and again in 2019. 

They focused on two geographical pinch points the fish use for migration: for southern residents they looked at Juan de Fuca Strait; for northern residents they studied Johnstone Strait.

Researchers evaluated how many chinook salmon were available to southern resident killer whales by looking at Juan de Fuca Strait. For the northern resident killer whale population they looked at Johnstone Strait. (UBC)

Contrary to expectations, both habitats showed similar distributions of schools of chinook.

Even more surprising was discovering the schools in the southern habitat contained four to six times the number of fish as the northern counterpart.

“It’s a really important finding because we’re the first team of researchers to go out to test the belief that there, in fact, is a shortage of chinook. And we didn’t find it,” said Trites.

“So it tells us then the reason why [the southern resident killer whales] are not coming back in is not because there’s an absence of salmon. It’s something else going on.” 

Southern resident killer whales are officially listed as a species at risk by the Canadian government.

Concern over their survival has prompted action on many fronts, including changes to marine traffic and numerous chinook salmon conservation and restoration efforts.

One of the more interesting campaigns saw U.S. biologists and a First Nation attempt to feed an ailing southern resident known as J-50 live chinook salmon off a boat.

Trites is quick to point out that the study does not indicate that chinook stocks are healthy.

“It doesn’t mean that salmon are in great condition because they’re not,” he said.

“But the fact is that there are more fish available to [southern residents] here than to the growing northern resident killer whale population. So that should get people to open their eyes and now ask questions.”

Source From CBC News

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