New research suggests Alaska's salmon are spending fewer years in the ocean and returning early to spawn.

As a result, Alaska's salmon are smaller than they were 30 years ago, according to an analysis published Wednesday in the journal Nature Communications.

In addition to feeding local communities and putting money in the pockets of commercial fishers, salmon help bring ocean nutrients inland and fertilize terrestrial ecosystems. Smaller salmon leave ecosystems starved for nutrients and local communities with less to eat.

To find out why Alaska's salmon are shrinking, researchers analyzed the measurements of 12.5 million fish collected between 1957 and 2018 by the Alaska Department of Fish and Game.

The analysis showed salmon aren't spending as much time cruising the open waters of the Pacific.

"There are two ways they could be getting smaller -- they could be growing less and be the same age but smaller, or they could be younger," corresponding author Eric Palkovacs said in a news release.

"We saw a strong and consistent pattern that the salmon are returning to the rivers younger than they did historically," said Palkovacs, a professor of ecology and evolutionary biology at the University of California, Santa Cruz.

The research revealed a range of possible factors responsible for driving the shift in salmon size.

"There's not a single smoking gun," said first author Krista Oke, a postdoctoral scientist initially at University of Alaska Fairbanks. "Small contributions from a lot of factors are adding up to drive these changes."

The data suggests climate change and increased competition among wild and hatchery salmon in the ocean are two of the main contributing factors.

Surprisingly, the influence of commercial fishing on salmon size has been varied and mostly minimal, according to the new analysis. Likewise, rebounding salmon predator populations have had a negligible effect.

"We know that climate drives changes in ocean productivity, and we see a consistent signal of climate factors associated with decreasing salmon size," Palkovacs said. "Another consistent association is with the abundance of salmon in the ocean, especially pink salmon. Their abundance in the North Pacific is at historic highs due in part to hatchery production in Alaska and Asia, and they compete with other salmon for food."

Palkovacs and his research partners suggest the ocean is becoming an increasingly risky place for bigger, older salmon. Scientists hope followup studies well help them understand exactly why that is.

"That's the next hard step I hope we can get to soon," Oke said. "It could be that they're having to spend more time feeding, which is putting them in risky places. Lots of things could be happening to increase the overall risk of mortality in the ocean, but we weren't able to pin that down."

Smaller salmon are obviously bad for commercial fishers. Smaller salmon fetch lower prices, as they often can't be converted into high-value products. But humans aren't the only organisms dependent on salmon for their wellbeing.

Bears, insects, birds, trees and juvenile salmon themselves all count on the nutrients provided by salmon returning to spawn.

"Salmon go up into these small streams, and whether they are caught by predators or die after spawning, their nutrients are transferred into the forests and freshwater ecosystems," Palkovacs said. "It's a classic salmon ecosystem service, and the amount of nutrients they deliver depends on their body size."

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