A new study found that salmon returning to rivers in Alaska has grown drastically smaller in the past 60 years - and their time at sea appears to be the reason.
Researchers from the University of California, Santa Cruz (UCSC), and the University of Alaska Fairbanks (UAF) collaborated for the study, tracking the size and its effect on the local communities and ecosystems. The details and results of their work are published in Nature Communications on Wednesday, August 19.
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The Value of Salmon in Alaska Communities
Researchers noted the importance of salmon in both the human communities and local ecosystems. Natural salmon supply is essential for both commercial and traditional subsistence fisheries in the area. As a part of the ecosystem, they transport nutrients from the sea back to the inland when they migrate through connecting tributary rivers.
However, less of these salmon means less food for the people and fewer nutrients for the inland ecosystems. While locals have long observed their wild salmon getting smaller, they were not able to pinpoint what caused this decline.
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The UCSC and UAF team poured through collected data for sixty-one years, from 1957 to 2018, covering 12.5 million fishes from data provided by the Alaska Department of Fish and Game. This previously untapped and large dataset offered data for four species of salmon - Chinook (Oncorhynchus tshawytscha), chum (Oncorhynchus keta), coho (Oncorhynchus kisutch), and sockeye (Oncorhynchus nerka).
From the data, researchers observed that the salmon growing smaller is because of the fishes returning to their spawning grounds at much younger ages compared to previous years. These fishes spend less time at sea. Alaska salmon usually stay at sea for up to seven years. However, the study notes that the average stay of these fishes at sea varies with species.
While at sea, salmon feed and mature, growing until adulthood and migrating up to the North Pacific Ocean. These fishes, upon reaching adulthood, soon return to inland rivers to spawn the next batch of fingerlings.
Natural Selection at Work
"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-and we saw a strong and consistent pattern that the salmon are returning to the rivers younger than they did historically," said Eric Palkovacs, the corresponding author in the study. Palkovacs also teaches ecology and evolutionary biology. He also serves as the associate director of the Fisheries Collaborative Program under the UCSC Institute of Marine Sciences.
The researchers have identified a number of factors that contribute to the Alaska salmon getting smaller over the years. However, first author Krista Oke, formerly from UCSC and currently at UAF, noted that there is no main cause for the shift.
"Small contributions from a lot of factors are adding up to drive these changes," Oke said.
However, Palkovacs cited two factors - climate change and competition with increasing population - as clear contributors to salmon growing smaller in all species and across all regions. Meanwhile, commercial fishing appears to only have a significant impact on select salmon populations.
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Palkovacs added that the salmon's shorter stays in the ocean might suggest that the ocean is becoming riskier for the species. "Natural selection has always pushed in both directions, but the balance between the two is changing, pushing harder against the older, larger salmon," the corresponding author added.
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