Heavy rainfall and above-average stream temperatures contributed to the recent decline of Cook Inlet king salmon, a new study published Monday by the University of Alaska Fairbanks suggests.
The peer-reviewed study, which was published in the scientific journal Global Change Biology, modeled the relationship of nine different indicators, including stream temperatures and precipitation levels, to the populations of 15 of the 23 chinook, or king, salmon populations monitored by Alaska’s Department of Fish and Game within the Cook Inlet.
The study was the first comprehensive look of its kind at the impact of freshwater conditions on chinook salmon in the area, Sue Mauger, one of the study co-authors, said.
“There’s not been any research that has been able to look at the freshwater conditions across many different watersheds that would represent different chinook populations,” Mauger, who is the science and executive director of local environmental advocacy group Cook Inletkeeper, said Wednesday. “So that’s really one of the strengths of this paper: looking at the overall regional story of the decline, and then looking for those watershed-scale drivers that are actually part of the story of why we have that decline.”
Chinook salmon populations across the Cook Inlet have experienced significant fluctuations but overall have been in decline over the past 30 years. The total harvest of king salmon in Cook Inlet peaked at 134,489 fish in 1993, but by 2012 the harvest was only 10,838 fish. A federal fishery disaster was declared that year, and for the past two years king salmon fishing has been heavily restricted on the Kenai Peninsula due to low returns.
The most notable finding from the study showed that maximum rainfall amounts during late summer months, when spawning and incubation take place, had a “strong negative association” with productivity rates for spawning salmon populations.
In other words, heavy rainfall was consistently associated with poor brood years across all 15 chinook populations.
Monthly maximum precipitation was above-average for the majority of the 15 streams during the 2004, 2005 and 2006 brood years, and the timeframe between 2003-2007 saw an average decline in productivity of 57% in these waters.
The heavy rains likely cause increased flooding in waterways, according to the study, which can then damage or displace salmon embryos. Increased deposits of fine sediments in the water can lead to diminished oxygen levels.
While heavy rains appear to have a negative impact on incubating salmon eggs, a slight increase in precipitation was shown to benefit, rather than harm, the juvenile salmon that had already hatched and were spending a year in the freshwater before traveling to the ocean.
“In the summertime we also found that a little more rain can really help the juveniles find some of those side-channel habitats and places where they can maybe feed a little bit out of the main flow of…