Sockeye salmon in the Fraser River are facing such critically warm water in the summer that populations will either have to adapt or die as climate change pushes temperatures even higher, according to new research at the University of British Columbia.
With oceans, lakes and rivers warming worldwide, the study holds a warning that fish stocks are facing increasingly dire environmental challenges.
To test the thermal tolerance limits of salmon, researcher Erika Eliason placed them on "fish treadmills" in the lab, where temperatures and simulated currents could be adjusted.
What Ms. Eliason found was that sockeye, which migrate up to 1,200 kilometers in the Fraser, are already "near their upper limit" for warm water and any further increases could lead to the disappearance of some populations.
The PhD candidate in UBC's Department of Zoology looked at eight distinct populations of sockeye salmon, and found each group had evolved to deal with local conditions.
There are more than 100 sockeye populations in the Fraser and over time they have evolved in response to the specific environmental conditions they face during migration.
Ms. Eliason said one group - the Chilko stock, named for the lake and river system they spawn in - was able to handle such a wide range of conditions that she began calling them "super salmon."
The Chilko fish run up the Fraser far into the northern Interior, and then hook back to the West, in the Chilcotin River, before entering the Chilko River, where they spawn near a glacial lake, high in the Coast Range.
"So they are swimming in the Lower Fraser when it's the warmest [water] temperatures of the entire year, in the middle of August, going through Hell's Gate, in 20 degrees plus," she said. "Then they . . . head up the Chilcotin, which can be as cold as 12 degrees Celsius, so it's a huge range in temperatures. I think it's this massive range of temperatures that they have adapted to, that makes them super."
She said the Chilko stock have "very large hearts," compared to other populations in the Fraser and they are more streamlined and athletic. By contrast the Weaver Creek sockeye, which run a short distance and spawn in the Lower Fraser, are less fit and appear especially susceptible to warming waters.
Ms. Eliason said it isn't known how long it takes a population to change physiologically to adapt to environmental conditions - and it might not be possible for any of the populations to change fast enough to survive in the long run.
"The current challenge is determining whether the rates and extents of physiological adaptation for Fraser River sockeye salmon will allow them to adapt quickly enough to cope with the current warming trend," concludes the paper.
The Fraser has warmed by about two degrees over the past two decades and the trend is expected to continue. The river is usually over 19 degrees in the summer, and often hits highs of around 21.5 degrees.
"They are all near their upper limit. . . 21.5 degrees C is already higher than the optimal temperature for every single population in the Fraser," she said. "There is not much room there, for anybody."
The study, published Friday as the cover article in Science, found the optimal temperature for each fish population matched the historical river temperatures they had encountered.
Above their optimal temperature the fish had increased difficulty swimming and could suffer cardiovascular collapse, said Ms. Eliason, who worked on the study with co-authors Tony Farrell, a professor in the Department of Zoology and Scott Hinch, a professor in the Faculty of Forestry, both at UBC.