UBC researchers find killer whales have plenty of prey

Photo: Andrew Trites

The popular theory that the endangered southern resident killer whales are suffering from a lack of chinook salmon in the Salish Sea may not hold water.

Scientists at the University of British Columbia claim their research indicates there is plenty of prey — at least during the summer — in the orcas’ traditional habitat in the Juan de Fuca Strait.

A lack of chinook has been blamed for the southern residents’ declining population and low birth rates. The orcas rely almost exclusively on chinook as a food source. Only 73 of them remain, in three pods.

In a paper published in the Canadian Journal of Fisheries and Aquatic Sciences, UBC researchers say the numbers of chinook in the Salish Sea in summer are four to six times more abundant for southern resident killer whales than northern resident killer whales.

“People have been talking about a prey shortage as if it’s a fact, but this is the first study to quantify and compare the amount of their preferred prey, chinook salmon, available to southern and northern resident killer whales,” lead author Mei Sato said in a statement.

The increasing northern resident population numbers about 300. In recent years, southern residents have also been returning later than normal to the inside coastal waters of British Columbia and Washington.

Andrew Trites, professor and director of the Marine Mammal Research Unit at UBC, said measurements from drone footage have shown the southern residents are thinner on average than the northern residents, which supports the popular belief that the southern residents are experiencing a food shortage.

“Our findings suggest that this food shortage is probably not occurring during the summer when they have traditionally fed in the Salish Sea,” he said.

Researchers spent the summers of 2018 and 2019 surveying two areas known to be important foraging habitats for resident orcas — the Johnstone Strait for northern residents and the Juan de Fuca Strait for southern residents, where the orcas intercept migrating chinook returning to the Salish Sea.

Using fish finders, the UBC team compared the number and size of Chinook salmon in the two straits.

“Killer whales use a similar technique to locate the fish — only it’s much better than the electronic one we use because they can distinguish between different fish species in the water,” said Sato.

Researchers captured sample fish to validate the acoustic signals from their fish finder and distinguish between different fish species, as orcas prefer larger, older chinook.

They found that while the distribution and sizes of fish were similar in both straits, there were four to six times as many fish in patches of the Juan de Fuca Strait.