This article is part of a special National Geographic News series on global water issues.
British Columbia’s Fraser River hosts more than 100 different sockeye salmon populations, each with its own unique and heroic migration story. But thanks to climate change, its water is becoming warmer, and that could spell doom for some of the fish.
A new study says the salmon that navigate the most arduous routes for their once-in-a-lifetime migration—up to 680 miles (1,100 kilometers) upstream with an elevation gain of 3,000 feet (1,000 meters) against raging whitewater--may be the best suited to cope with warming waters.
The Fraser River has heated up by nearly 2 degrees Celsius (about 3.5 degrees Fahrenheit) since the 1950s, and salmon migration mortalities have approached 95 percent in some populations during the warmest years.
“There are several studies showing a correlation between temperature and mortality,” said Erika Eliason, a doctoral candidate at University of British Columbia in Vancouver and lead author of the new study. “We’re trying to look at the mechanism of that mortality.”
The paper, which appears today in Science, reports that the fish that do survive the temperature spikes have larger and stronger hearts compared to those that perish.
(Read more about the Fraser River in the NatGeo News Watch Blog: “To Save the Heart of a Mighty River.”)
Millions of sockeye salmon spend the majority of their three- to four-year lifespans in the ocean before swimming up the tumultuous Fraser River to reproduce and die.
Eliason and her colleagues plucked members of eight salmon populations out of the Fraser River just a couple of days after they’d begun their journeys, but before they had reached any of the most arduous parts of the river, like the highly constricted channel called Hell’s Gate that no sane rafter would try to run.
The researchers placed their study subjects in massive swim tunnels mounted behind a boat trailer—basically exercise treadmills for fish. The researchers were able to regulate the water temperature and measure the fishes’ swimming performance and vital signs.
Members of a population called Chilko, named for the Chilcotin watershed where their challenging migration occurs, emerged as “what I call my superfish,” Eliason said.
“They migrate in the middle of the summer, when the water temperatures . . . can get up to 20 or 21 degrees Celsius (68 to about 70 degrees Fahrenheit),” she said. “Then they break left and head up this massive, massive mountain stream to ascend over 1,000 meters (3,280 feet) in elevation. They spawn in a glacial lake in one of the remote parts of British Columbia, grizzly country.”
The coldest reaches of the Chilko waters dip to 57 degrees Fahrenheit (14 degrees Celsius). In the swim trials, the Chilko withstood the greatest range of temperatures before they began to experience cardiovascular decline. All of the fish were dissected and analyzed following the experiments, and the Chilko were revealed to have the largest hearts and the most robust cardiorespiratory systems of all the fish populations examined.
Coastal ‘Couch Potatoes’
In contrast to the Chilko superfish are the Weaver sockeye salmon, which come in to spawn in coastal tributaries–a journey that’s wimpy in comparison.
Eliason and her team found that these fish, which they dubbed the “coastal couch potatoes,” swim optimally at about 57 degrees Fahrenheit (14 degrees Celsius) and can only cope with temperatures up to about 62 degrees Fahrenheit (17 degrees Celsius).
When temperatures reach that maximum, “at some point, they’re not going to be able to swim well, and they’re not going to make it to the spawning ground,” Eliason said. “We think that cardiovascular collapse occurs, and the fish can’t swim.”
Race to Evolve
The big remaining question is whether the kind of adaptation that has allowed the Chilko salmon to spawn so far upstream and therefore navigate such a wide range of temperatures during their migrations could also occur in other salmon as a response to climate change.
“We’d like to know the dates of evolution of these different traits,” she said, “so we can have an idea of how quickly they can evolve and whether they will be able to cope with current warming trends.”