No coach potato. Sockeye salmon have evolved the heart and physiological traits needed for its particular upstream journeys.

Matt Casselman

Some Salmon Can Take the Heat

The Chilko sockeye is the Michael Phelps of salmon. Once a year, it swims 650 kilometers up British Columbia’s Fraser River, fighting rapids and strong currents, to reach a spot where it can lay and fertilize its eggs. New research reveals that the fish is well-adapted to this journey—it has a bigger, better heart and uses oxygen more efficiently than do other local salmon. And thanks to these attributes, the Chilko sockeye may be more likely than these other fish to survive a warming world.

Not all of the Fraser River’s salmon swim as far as the Chilko sockeye. Some stay relatively close to the coast; others swim a bit farther upstream to spawn. There are so many different migration distances that the fish have split into 100 distinct populations—one of which is the Chilko—with different swimming behaviors and body types.

Hot summers take a toll on these migrations. In 2004, for example, 80% of some salmon populations died of heat stress before reaching their spawning destinations. Water temperatures in the Fraser River have risen 2°C in the past 60 years, and, with global warming, researchers expect even bigger die-offs to come. Erika Eliason, a graduate student at the University of British Columbia, Vancouver, in Canada, wanted to know whether different populations of Fraser River salmon were better than others at handling the heat.

Over three summers, she caught 97 salmon heading upstream and gave them stress tests in a portable “fish treadmill” mounted on a boat trailer. With the fish in this enclosed tank, she could increase the current speed of the water and raise the water temperature from 8°C to 26°C to determine how well the fish swam at different temperatures. At the same time, Eliason monitored the amount of oxygen in the water to learn how well their bodies were utilizing oxygen, a measure of athletic ability. She then dissected some fish to look at their hearts. All together, she studied individuals from eight populations.

The Chilko sockeye was the most versatile. It swam best at 17°C, a moderate river temperature, but it could cope with the hottest water tested, 26°C. The Weaver sockeye, which spawns downstream of the river’s big series of rapids, collapsed in water above 21°C. Its heart was smaller and had a poorer blood supply than did the hearts of Chilko and other salmon populations that had to fight the rapids, Eliason and her colleagues report online today in Science. Moreover, Chilko hearts appear to be more sensitive to adrenaline, making it easier for them to keeping going when overheating. “I was surprised at how much variation there was between populations,” Eliason says.

“The message is pretty clear that these sockeye salmon are highly adapted to the energetic demands of their upstream migration,” says Brian Riddell, a fisheries scientist who heads the Pacific Salmon Foundation in Vancouver. “I am continually amazed at how well adapted these animals are to their environment.”

Evolutionary biologist Michael Kinnison of the University of Maine, Orono, says it’s useful to know which populations are most vulnerable to climate change. The Chilko sockeye may do okay, but the Weaver sockeye will likely have a much harder time surviving if the river continues to warm. Still, these vulnerable fish may have other traits—such as disease resistance—that might be valuable under other conditions. So Riddell is calling for protection of all of the Fraser River’s salmon populations. That may mean restricting fishing during unusually hot summers to reduce the stress on these fish, Eliason says.

But ecologist Thomas Quinn of the University of Washington (UW), Seattle, questions whether climate change is threatening salmon as much as some experts think and therefore says corrective measures might not be necessary. Aquatic biologist Daniel Schindler of UW agrees, saying that salmon may change the timing of when they go upriver to spawn to avoid extreme river temperatures.