Salmon returning to Alaskan rivers have become significantly smaller over the past 60 years, according to a new study published in the journal Nature Communications. The research found that due to climate change and competition from hatchery fish, wild salmon are spending less time at sea and are returning to spawning grounds at younger ages — a trend that could have important economic and ecological impacts.
The study, led by scientists at the University of Alaska at Fairbanks and the University of California Santa Cruz, examined measurements from 12.5 million fish collected by the Alaska Department of Fish and Game from 1957 to 2018. The data showed that four of Alaska’s five wild salmon species — Chinook, chum, coho, and sockeye — have shrunk in size.
For example, Chinook, Alaska’s official state fish, historically stayed out at sea for seven years to mature, but many are now returning to rivers to spawn at just four years old. As a result, the fish are on average 8 percent smaller now than they were before 1990.
Scientists say a range of factors are driving this shift, impacting species in different ways in different regions. “There’s not a single smoking gun,” lead author Krista Oke, a biologist at the University of Alaska at Fairbanks, said in a statement. “Small contributions from a lot of factors are adding up to drive these changes.”
But two factors seem to have universal impact — how climate change is altering ocean productivity and competition with farmed pink salmon in the ocean.
The shift in salmon size has significant consequences for people, the economy, and ecosystems in Alaska. Wild salmon is a staple food for many Alaskan residents, particularly Indigenous groups. In addition, the state produces nearly all of the nation’s wild salmon. Commercial fishermen harvested more than 206 million salmon in 2019 and sold them for $657 million, Reuters reported. The fish are also an important food source for bears and other wildlife, and their spawning migration plays a critical role in nutrient transportation in Alaska’s riverine ecosystems.