Chinook salmon study reveals shifting diets across B.C.’s Salish Sea

In this undated photo a chinook salmon fights while on a fishing line near Brookings, Ore. (Jamie Lusch/The Medford Mail Tribune via AP)

Deep-sea lanternfish turning up in Chinook salmon stomachs is one of the more unexpected findings in a new study of what the fish are eating along B.C.’s coast.

The species normally lives hundreds of metres below the surface, but researchers say it may occasionally be pushed upward by strong tidal currents, bringing it into range for feeding salmon.

The finding comes from a University of Victoria and Pacific Salmon Foundation study that examined more than 7,000 Chinook salmon stomachs collected across the Salish Sea since 2017.

Led by UVic PhD candidate Wesley Greentree, the research shows Chinook salmon diets are far from random. Instead, what they eat depends heavily on where they are and the time of year.

Herring stood out as the most important food source overall.

“We think of salmon as being not picky eaters,” Greentree said. “They like to eat what is available to them and what will provide a good amount of energy for them.”

But the study found clear regional differences. Anchovies were more commonly found in salmon near Howe Sound, while sand lance were especially important around Victoria and Sidney.

Researchers say those patterns line up with how water moves through the Salish Sea, including river outflows and tidal mixing.

“The boundaries that we found between different fish communities align closely with oceanographic features that divide the Salish Sea,” Greentree said. “We suspect anchovies are taking advantage of freshwater from the Fraser and Squamish rivers flowing into the Strait of Georgia.”

One of the biggest surprises was the appearance of lanternfish in salmon diets in areas like Campbell River and Victoria during winter.

“The thing that surprised me the most was the odd fish that we saw,” Greentree said. “We saw some fish that are called ‘lantern fish.’ They typically found very deep in the ocean.”

Scientists believe strong tides may be lifting these deep-water fish closer to the surface, where salmon can feed on them.

The study also shows Chinook diets change with the seasons. In winter, salmon eat a wider variety of prey compared to summer.

Sand lance, for example, were far less common in winter diets because they bury into the sand and become harder for salmon to reach.

Researchers say these shifts help show how closely salmon are tied to the movements and behaviour of their prey.

Looking ahead, scientists are watching for bigger changes in the ecosystem, including whether sardines — once common in salmon diets decades ago — could return to B.C. waters as the climate warms.

“One thing that we are on the eye out is to see if sardines make a return to British Columbia waters,” Greentree said. “They were common in salmon diets in the 1930s and ‘40s but have disappeared in B.C. waters.”

If that happens, it could signal a broader shift in the food web.

“If a fish species that is important to salmon disappears, we will catch that because it will no longer be present in the salmon diet and vice versa,” he said. “If a new species shows up in B.C., we’ll see salmon feeding on those, and so we think it gives a good canary in the coal mine to see how our ecosystem is changing.”

The findings were published in Fisheries Oceanography.