Rising water temperatures in southcentral Alaska’s rivers are causing noticeable changes in the behavior of the invasive northern pike. As water temperatures rise, these predators are feeding more, raising concerns about the future of native fish such as salmon.
Researchers led by the University of Alaska Fairbanks examined the stomach contents of northern pike collected by the U.S. Fish and Wildlife Service from the Deshka River in the summers of 2021 and 2022. They compared their findings with samples taken from pike in the same river about 10 years ago.
Their analysis showed that as temperatures rose, pike of all ages increased their fish consumption. This change was especially noticeable among young fish, with yearling pike consuming 63 percent more fish than before.
The research results were published in a magazine biological invasion.
“Significant warming is predicted to occur in the future, and with it the amount of fish consumed by pike will increase,” said study leader Benjamin Rich, who is pursuing a graduate degree at UAF’s College of Fisheries and Marine Sciences.
Increase in air and water temperature
A steady warming trend is already observed in the study area. Average summer temperatures have increased about 3 degrees Fahrenheit since 1919, including an increase of 0.8 degrees Fahrenheit over the past decade. Water temperatures in the Deshka River, which flows into the Susitna River, have also been above historical averages in recent years, Rich said.
Looking ahead, scientists expect this warming to continue throughout the 21st century. Models suggest that northern pike could increase their food intake by an additional 6% to 12% by 2100.
Warm water increases the appetite of predators
The increased appetite of pike in the Deshka River mirrors patterns seen in other freshwater systems. As water temperature increases, the metabolism of predators accelerates, increasing their energy needs and causing them to feed more aggressively.
This change is particularly problematic in southcentral Alaska. Northern pike were illegally introduced there and now share habitat with Chinook and coho salmon, which are already in decline.
Interestingly, the number of Chinook and Coho salmon found in pike stomachs has decreased over the past decade. The researchers suggest this likely reflects a decline in the river’s salmon population, rather than a decrease in predation.
Salmon face multiple pressures
Peter Westley, a fisheries professor at UAF, said salmon are already under strain from the effects of global warming. More aggressive predation adds further pressure to an already difficult environment.
“We know that invasive species and climate are each associated with freshwater fish extinctions,” said Westley, a co-author of the study. “Those influences are likely to work together into the future.”
Changes in complex ecosystems
Eric Schoen, a researcher at the UAF International Arctic Research Center, emphasized the importance of understanding these interconnected influences. Salmon are an important species, but they are just one part of a broader ecosystem that will be affected by rising temperatures.
“A lot of research has been done on how changes in temperature directly affect salmon. This is very important, but salmon are not the only species in these rivers,” said Shane, who contributed to the paper. “It is also important to understand how these changes are affecting salmon indirectly through predators, prey, and pathogens.”
Other contributors to the study include Adam Sepulveda and Jeffrey Falke of the U.S. Geological Survey and Daniel Rinella of the U.S. Fish and Wildlife Service.
