New data points to rising freshwater temperatures as a cause of chinook salmon decline

For the last decade, chinook salmon, commonly known in Alaska as “king salmon,” has been in decline, a trend that has stumped researchers and biologists across the state as to what is causing the salmon’s low returns. Although it is commonly thought that ocean conditions are the culprit for the chinook’s demise, a recent study led by University of Alaska researchers from UAA and UAF in collaboration with Cook Inletkeeper, a community-based nonprofit organization combining advocacy, education and science to protect the Cook Inlet watershed, provides the first evidence that freshwater habitats may also be a contributing factor.

The effects of chinook salmon's decline have been felt throughout the state from economic fallout in the tourism and commercial fishing industries to sport and subsistence use. The impacts have been significant as the Alaska Department of Fish and Game (ADF&G) has been closing commercial and public fisheries statewide since the late 2000s to preserve the valuable salmon runs.

But with the research team of seven’s findings, recently published in the journal Global Change Biology, “Watershed‐scale climate influences productivity of chinook salmon populations across southcentral Alaska,” the project, which examined Cook Inlet chinook salmon in 15 watersheds in Southcentral, may provide valuable insights on cumulative effects driving the decline of chinook salmon.

According to the team’s findings, high spawning abundances, ocean conditions and freshwater habitats all played an important role in chinook productivity or decline. The team investigated data collected from 1980-2009 for 15 different chinook populations across Southcentral and examined how freshwater conditions, like water temperature and streamflow, impacted the salmon’s productivity. Productivity was defined in each population by how many offspring survived by each spawning adult and were able to determine which years were associated with larger or smaller salmon runs. 

The team found consistencies throughout the region and discovered that years where heavy rains occurred in late summer and fall, a time when chinook salmon are spawning and eggs are incubating, led to decreased productivity, possibly due to reduced egg-to-fry survival from streambed scour or being crushed by heavy sediment loads. Conversely, the team found that chinook salmon were more productive when juveniles experienced above-average summer rainfalls. The researchers hypothesized that juvenile salmon were able to thrive because higher streamflows create more habitat, allowing them to access more side channels and floodplain habitats that are rich in food resources.

Additionally, stream temperatures had variable effects on chinook productivity in hotter-than-average years in Cook Inlet streams. In hotter summers, lowland rivers had decreased productivity, while in the colder, mountain streams, they were more productive. However, the study found when stream temperatures rose above 64 degrees Fahrenheit for a week or longer during spawning, chinook salmon populations in the warmest streams suffered a substantial decline. 

Rebecca Shaftel, research aquatic ecologist with the Alaska Center for Conservation Science (ACCS) at UAA, and Leslie Jones, geospatial information officer with the State of Alaska and lead aquatic ecologist with ACCS at UAA from 2017-2020 and current affiliate professor at UAA, were part of the seven-person research team and co-authors on the recently published paper. Both said they hope their research can not only shed light on the importance of freshwater conditions but also that their research is a catalyst for other related salmon research projects in the state.

According to Shaftel, the project idea came together because of collaborative efforts between UAA and Sue Mauger, director of Cook Inletkeeper, who has been monitoring stream temperatures in Cook Inlet for the past two decades. The team expanded to include fisheries biologists from UAF and ADF&G. With Mauger’s data and long-term data collected by ADF&G, the team had the baseline data needed to execute the study.

 “Sue was interested in looking at the effects of stream temperature and fall precipitation on the chinook salmon decline in Cook Inlet,” said Shaftel. “There was a period of unusually warm summer temperatures from 2003-2007 and three significant regional flooding events that occurred in 2002, 2006, 2012. All of which set the stage for decreased salmon returns in the future.” The researchers and paper co-authors included Erik Schoen with the Institute of Arctic Biology at UAF, Curry J. Cunningham with APU, Daniel J. Rinella with U.S. Fish and Wildlife Service and Adam St. Saviour at ADF&G, proposed evaluating the effects of freshwater conditions on chinook salmon in response to a call put out by the Pacific States Marine Fisheries Commission (PSMFC), who funded their research. Besides PSMFC funding, the group received funds and grants from the State of Alaska, Alaska EPSCoR National Science Foundation, Pollock Conservation Cooperative Research Center at UAF, Pollock Conservation Cooperative at Alaska Pacific University (APU), Groundfish Forum at APU, ACCS at UAA, Institute of Arctic Biology at UAF and Cook Inletkeeper.

In 2017, Jones was hired by ACCS to lead the Aquatic Ecology program and lend her expertise on freshwater habitats for salmonids. Her extensive career has focused on computational ecology to understand relationships between climate, biophysical processes and the landscape. 

“There’s a hypothesis that most of the declines are being caused by conditions in the ocean during that life stage,” Jones said. “We wanted to see if there were any indications that conditions in freshwater habitats during spawning or rearing may have impacted productivity, to see whether or not streamflow or stream temperature might have contributed to chinook declines.”

With Alaska experiencing climate change effects at two times the rate as the global average, the team’s research points to specific events, like warmer temperatures and increased precipitation during certain times of the year that significantly impact chinook salmon habitats in Southcentral watersheds and may have contributed to the salmon’s decline in local rivers and streams.

“We know now that there are complex relationships to salmon productivity decline and it is not just a marine thing,” said Shaftel. “We found correlations — that are not causative — that freshwater conditions can impact chinook salmon.” 

Shaftel and Jones said that Alaska’s salmon populations are unique and that their team’s research is not only important for scientists, wildlife management officials and biologists but for all Alaskans who are impacted by salmon. 

“There are actions we can take on the landscape to provide resiliency for streams against climate change,” said Shaftel. “Like maintaining natural vegetation and shade along stream banks and protecting wetlands, which provide critical streamflow in the summer and buffer streams from flooding in the fall.”