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At a lab an hour’s drive from Vancouver, British Columbia, Erika Eliason is watching her team chase a young male sockeye salmon around a tank. He keeps dodging the scientists’ hands in a bid for freedom, but there’s nowhere to go but in circles.

This salmon, plucked from the Fraser River just a few days earlier, will spend the rest of his short life in this lab. His peers—the free salmon still in the Fraser—will spend theirs dodging anglers’ poles and hungry predators as they race upstream to their ancestral birthplaces. With any luck, these wild salmon will birth a new generation before they succumb to their exhaustion.

The salmons’ upriver journey is never easy, but this year has been particularly hard. It’s been a hot summer for everyone on the coast—the fish are making the journey in water that is four to five degrees celsius warmer than normal. The salmon are being squeezed, pressed up against the upper ceiling of their temperature tolerance. Yet this summer is just a glimpse of things to come. Over the next 60 to 80 years, the water in the Fraser is expected to warm by another two degrees.

“Two degrees doesn’t seem like much,” says Eliason. “But to a salmon, it’s everything.”

The salmon are already having trouble, and scientists are not sure whether they’ll adapt to a warmer future. That’s where the fish in this lab come in.

Eliason has been studying Fraser River sockeye for a decade, trying to understand how they respond to warming temperatures. So far, marine biologists, including Eliason, have learned a lot: hot water is deadly, but even water a few degrees warmer than normal stresses the fish. Sixteen degrees, she says, is the perfect temperature for most Fraser River salmon. At 19 °C, however, some salmon are likely to die of heart failure during their journey to reproduce. And at 22 °C, even the most robust salmon struggle to survive.

This past summer, temperatures in the Fraser River just outside Vancouver hit an unusual 21 °C in early July. But even as summer comes to an end, the temperature has hovered stubbornly between 18 and 21 °C.

Eliason and her colleagues aren’t just documenting the salmons’ plight. They’re hoping to learn how the fish deal with stress, and whether they will adapt to rivers warmed by climate change. So, with nets in hand, they have been plucking fish from the Fraser.

The scientists fit the fish with heart rate loggers, and split them into groups: some go into tanks with water at a comfortable 16 ˚C, and others into water at a potentially dangerous 19 ˚C. Then, in a separate pool, they chase each fish for three minutes, haul it out of the water for a minute, and then put it back in the pool to measure its automatic responses: do its eyes follow movement; if you flip it upside down, does it try to right itself? The point of this study, overseen by Dalhousie University student Tanya Prystay, is to simulate the challenges a sockeye faces during its migration—including how stressful is it for a salmon to be caught, hoisted up for a photo shoot, and tossed back in the river.

The salmon are put back in their respective tanks and, after a few days, the research team measures their recovery rates. Eliason anticipates that how each salmon reacts to a warmer than normal environment will vary. As her previous research has shown, even within the confines of the Fraser River, salmon are very different. Each spawning ground is home to a distinct population, and each has evolved its own thermal tolerance. Some salmon, she’s found, are better prepared for a warming world than others.

She suspects many of the salmon the team recently caught were headed to Chilko Lake, nearly 150 kilometers inland (something they will later confirm through DNA analysis). Because those fish are used to both the warmer water of the late-summer Fraser and the colder water of the glacier-fed lake, they have the highest and broadest thermal tolerance of all the Fraser sockeye Eliason has studied. But other salmon—the fall migrators that only encounter cool waters—have a lower thermal tolerance and are likely less resilient to climate change.

The salmon born into the sweltering Fraser water this year will struggle to survive. Some will live to pass on their genes and begin rebuilding the population with a higher thermal tolerance. But by the end of the century, the Fraser River is going to be a very different place.

The genetic material from the fish in this lab will not be a part of that future. But Eliason and her team hope that what they learn from these fish will make it an easier journey for the rest of their species.