UBC researchers are tracking wild chinook salmon on their expected migratory route in the Salish Sea. They're trying to find out why salmon-eating southern resident killer whales appear to be malnourished.

The study is the first to track the movements of chinook salmon while killer whales are hunting prey. CBC joined researchers on the water as they collected data from the fish.

Salmon are being fitted with high-tech acoustic tags a short 20-minute boat ride from Port Renfrew, on the west coast of Vancouver Island.

Southern resident killer whales are officially listed as a "species at risk" by the Canadian government and "endangered" by the U.S. government, according to the Pacific Salmon Foundation.

They say the recent poor health of southern resident killer whales suggests they are not getting enough chinook salmon.

Chinook and chum salmon are the main food source of the whales during the summer and fall. Killer whales use underwater sound frequencies to hunt their prey.

The Pacific Salmon Foundation says it is unknown if their poor health is due to declines in chinook abundance or if it is due to vessel traffic and underwater noise cutting off their food source.

UBC's Stephen Johnston, left, and Brian Hendriks, right, spend their days catching chinook salmon, collecting data and fitting them with high-tech acoustic tags. (Sterling Eyford/CBC)

"When we look at the overall numbers of [salmon] returns mathematically, there appears to be enough fish to go round," said Andrew Trites, director of the Marine Mammal Research Unit at UBC's Institute for the Oceans and Fisheries.

"So, if some of these fish are doing well, how come the killer whales aren't finding them?"

How tracking works

Stephen Johnston, a UBC masters student, is working on the research team.

"We're just trying to understand a little bit more about the migratory behaviour of the chinook and how long it takes them to get from point A to point B. How long they reside in certain areas ... areas where we know there's high killer whale density," Johnston told CBC's Sterling Eyford.

The acoustic tag chinook salmon are fitted with. (Sterling Eyford/CBC)

The tag's signals are picked up by receiver stations the team has left throughout the fish's expected migratory path or spawning channels, all the way up into the Fraser River.

Right now, researchers know how chinook salmon move as a group or population but have little understanding of how they move in the marine environment as individuals.

"The killer whales are hunting these fish at an individual level. It's important to know how that relates to the whales and potentially how their behaviour changes in the presence of whales or in the presence of ourselves in both a fishing sense and a transportation sector sense."

The research hopes to find out what areas of the Salish Sea boats should avoid so they might be able to allow killer whales more access to chinook salmon, not creating too much noise and disrupting sound frequencies killer whales use to hunt.

Stephen Johnston collects data from a caught chinook salmon. (Sterling Eyford/CBC)

Researchers use sport fishing techniques like line fishing and trawling with nets to catch the fish. They bring the fish onboard and place a tag just behind the dorsal fin on the back of each fish. They take DNA and scale samples to determine where they're coming from, how old they are and how long the fish have been at sea.

Once all the data is collected, they make sure the fish is healthy and then release it back to the sea.

The Pacific Salmon Foundation is supporting the research with a $150,000 grant, under the auspices of the Government of Canada's new 'Whale Science for Tomorrow' initiative.

For more on the future of the southern resident killer whales check out a new CBC British Columbia podcast, out July 18. Killers: J pod on the brink is hosted by Gloria Macarenko.