Climate change is impacting the Skagit River ecosystem and the salmon and eagles that congregate there each winter, according to a recent study.
The study, published online Oct. 16 by the Journal of Applied Ecology, states the majority of chum salmon and the bald eagles that eat them are being seen in the Skagit River about two weeks earlier than they were in the 1980s.
That shift in the timing of peak salmon and eagle migration to the Skagit River is likely in response to climate change, according to the study.
The study, written by wildlife biologists with the North Cascades National Park Service Complex and the U.S. Geological Survey's National Climate Adaptation Science Center, used about 30 years of data about salmon and eagle populations in the mid-to-upper reaches of the Skagit River watershed in Skagit and Whatcom counties.
"Our results show that the timing of important seasonal events is getting earlier for both salmon and bald eagles," study co-author Madeleine Rubenstein of the USGS said. "The eagles are likely coming earlier because they are following their food source — the salmon migration. Salmon, in turn, are most likely migrating earlier because of climate change: They are responding to higher stream and ocean temperatures."
Rubenstein, along with Jason Ransom and Roger Christophersen of the North Cascades National Park Service Complex, analyzed winter bald eagle counts, chum salmon counts and flood data gathered from Sedro-Woolley to Newhalem.
That section of the Skagit River is known as important habitat for the nation's previously endangered bald eagles. A large concentration of the Pacific Northwest's bald eagles flock to the river each winter to feast on chum salmon that spawn and die along its banks.
"Our analysis demonstrates the strong links between the local bald eagle abundance, the timing and abundance of salmon runs, and the number and timing of flood events in the Skagit," the study states.
The study authors noted a direct relationship between number of floods and number of eagles.
Years with more flood events tended to have fewer eagle observations, Rubenstein said. This could be because flooding that occurs after chum salmon have spawned can wash the carcasses downstream, out of the upper reaches of the Skagit River where the eagles are accustomed to finding them.
According to the 30 years of data used in the study, flooding appears to be more frequently occurring after spawning.
"It’s possible that as salmon peak earlier in relation to flood timing, the floods are increasingly removing an important food source for eagles," Rubenstein said.
During that same 30-year period, the peak of eagle sightings in the area has come earlier, suggesting the birds are responding to changes in the presence of the salmon they rely on for food.
Rubenstein said other scientists have over several decades documented wildlife and plants changing the timing of their seasonal events in response to increasing temperatures due to climate change.
"As our seasons are getting warmer faster, many plants and animals are responding by shifting important events earlier ... because temperature is an important cue — like a clock — for determining when species start to breed, bloom or migrate," Rubenstein said. "Climate change is like the clock getting set a little bit earlier every year."
The clock seems to be shifting for the eagles and chum that frequent the Skagit River.
"Our results are consistent with a large body of research on climate change," Rubenstein said.
Ransom said armed with this new information, Seattle City Light that operates three hydroelectric dams on the upper Skagit River could in the future manage flows for the benefit of bald eagles, similar to the way it currently manages flows for fish.
"Our results may help inform future hydroelectric management towards accommodating resource needs for both eagles and salmon," he said.