Alaska's warming rivers are turning invasive pike into hungrier predators

University of Alaska Fairbanks College of Fisheries and Ocean Sciences

The Deshka River winds through Southcentral Alaska as a tributary of the Susitna, its waters home to Chinook and coho salmon that have sustained ecosystems and communities for generations. But the Deshka also harbors an unwelcome resident: the northern pike, illegally introduced decades ago and now firmly established. As Alaska warms, these predators are getting hungrier - and the salmon they prey on are running out of room to cope.

Sixty-three percent more fish in young pike stomachs

A team led by Benjamin Rich at the University of Alaska Fairbanks College of Fisheries and Ocean Sciences compared the stomach contents of northern pike caught by the U.S. Fish and Wildlife Service in 2021 and 2022 with samples collected a decade earlier from the same river. The pattern was consistent across every age class: pike were eating more fish. The most dramatic shift appeared in year-old pike, which consumed 63% more fish than their counterparts had ten years prior.

The mechanism is straightforward biology. Warmer water accelerates a fish's metabolism, which means it needs more energy, which means it eats more. Mean summer air temperatures in the Deshka region have risen roughly 3 degrees Fahrenheit since 1919, with 0.8 degrees of that increase occurring in just the last decade. Water temperatures have tracked upward in tandem.

A double bind for salmon already under stress

Here is where the story gets uncomfortable. Chinook and coho salmon in the Deshka are already declining. When researchers looked at what pike were eating, they actually found fewer salmon in pike stomachs compared to a decade ago - but that likely reflects the fact that there are simply fewer salmon available, not that pike have lost their taste for them.

Peter Westley, a UAF professor of fisheries and co-author of the study, framed the problem bluntly: salmon are being hit from multiple directions simultaneously. Warming water stresses salmon directly, reducing the quality of their habitat, altering migration timing, and lowering survival rates. Now add increasingly aggressive predators sharing those same waters, and the compounding effect becomes a serious concern.

Modeling carried out as part of the study projects a 6% to 12% increase in pike consumption rates by 2100, assuming warming trends continue along predicted trajectories. Those numbers may sound modest, but they layer onto an ecosystem already under considerable strain.

Invasive species and climate change - a dangerous partnership

The Deshka River findings echo observations from other freshwater systems worldwide. Warmer water tends to benefit invasive cold-water predators disproportionately, because many of them - pike included - are metabolically primed to capitalize on temperature increases that stress native species.

Erik Schoen, a researcher at UAF's International Arctic Research Center and a co-author on the paper, emphasized that the scientific community has spent considerable effort understanding how temperature affects salmon directly. That work matters. But salmon do not exist in isolation. They share rivers with predators, compete with other species for food, and face pathogens whose prevalence also shifts with temperature. Understanding these indirect effects is essential to building an accurate picture of what warming actually does to freshwater ecosystems.

The convergence of invasive species pressure and climate change is particularly worrying because the two threats reinforce each other. Invasive species and climate change are each independently associated with freshwater fish extinctions. When they operate simultaneously in the same system, the combined impact may exceed what either would produce alone.

What this study cannot tell us

Several important caveats apply. The study examines a single river system in Southcentral Alaska, so its findings cannot be automatically extended to other watersheds, even nearby ones with different thermal profiles and species compositions. Stomach-content analysis captures a snapshot of predation at the moment of collection; it does not track total predation pressure over a full season or year. The consumption projections for 2100 rely on climate models that carry their own uncertainties, particularly at the regional scale relevant to individual river systems.

The study also does not establish a direct causal link between increased pike predation and salmon population declines. The correlation is suggestive and biologically plausible, but disentangling predation effects from the many other factors driving salmon declines - habitat loss, ocean conditions, fishing pressure - requires different study designs.

Managing a river with two crises at once

For fisheries managers in Alaska, the practical challenge is daunting. Northern pike removal programs already operate in some Southcentral Alaska waterways, but pike are prolific breeders and extremely difficult to eradicate once established. If warming waters make surviving pike more effective predators, removal efforts may need to intensify just to maintain the same level of protection for native fish.

The study, published in Biological Invasions, was co-authored by Adam Sepulveda and Jeffrey Falke of the U.S. Geological Survey and Daniel Rinella of the U.S. Fish and Wildlife Service, in addition to the UAF researchers. It adds quantitative weight to what many biologists in the region have suspected for years: that climate change is not just a background condition in Alaska's freshwater ecosystems but an active force reshaping the relationships between species in ways that are difficult to predict and harder to reverse.