The results showed that the decreases in body size are primarily due to salmon returning to their spawning grounds at younger ages than they have in the past. Alaskan salmon can spend up to seven years at sea, although this varies by species. During this time they feed and grow to maturity, migrating great distances in the North Pacific Ocean before returning to fresh water to spawn.
“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 corresponding author Eric Palkovacs, professor of ecology and evolutionary biology and associate director of the Fisheries Collaborative Program in the Institute of Marine Sciences at UC Santa Cruz.
The researchers identified a range of factors that appear to be driving this shift, some acting across all regions and others affecting only certain species or populations.
“There’s not a single smoking gun,” said first author Krista Oke, a postdoctoral scientist initially at UC Santa Cruz and now at University of Alaska Fairbanks. “Small contributions from a lot of factors are adding up to drive these changes.”