Alaska salmon returning smaller amid climate change and competition with hatchery fish, study finds

Alaska salmon have gotten smaller in recent decades, a downsizing that appears to be largely driven by climate change and increased competition for food as hatcheries release some 5 billion young fish into the North Pacific each year, according to a study published this month by U.S. and Canadian researchers in the science journal Nature Communications.

Alaska provides the vast majority of the United States’ wild salmon, and their smaller size is reducing the number of eggs that these fish produce and their value to commercial and other fishermen.

That decline encompasses salmon runs all over the state but varies by species and region. Chinook returning across a broad expanse of western and northern Alaska were some 10% smaller than the average size before 1990. Meanwhile in southeast Alaska, sockeye salmon declined — on average — by only about 2%.

Many of these salmon appear to be returning from the ocean earlier to freshwater spawning grounds, and that’s why they are smaller as they reach coastal-area harvest zones.

“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, study co-author and professor of ecology and evolutionary biology at the University of California, Santa Cruz.

The study used Alaska Department of Fish and Game sampling information gathered between 1957 and 2018 from 1,014 locations across the state. The study looked at 12.5 million size and age measurements from chinook, coho, chum and sockeye, four of the five species of salmon that return to spawn in freshwater.

Chinook are the largest salmon. The study estimates that the body-size declines observed in chinook have decreased egg production by 16%, knocked 21% off their value and reduced by 26% the meals that these fish can provide to rural Alaskans.

“Reductions for other species were less dramatic but still substantial,” the study stated.

Others researches also have looked at the decline in chinook size. A study by University of Washington and federal fishery researchers published last year in the proceedings of the National Academy of Sciences looked at chinook that returned to spawn in West Coast river systems but migrated to feed in ocean waters much farther north.

That study pointed to orcas, which prey on the chinook, as a big cause for the decline in the size of these fish.

The southern resident orcas, which depend on chinook for their survival, are struggling and are listed as an endangered species. Other causes for the decline of the southern residents, which frequent Puget Sound, include noise masking their ability to hunt as well as pollution.

But off British Columbia and Alaska, orcas are doing well. Fish-eating resident populations have nearly tripled their numbers over a 50-year period, according to Jan Ohlberger, a UW professor of aquatic and fishery science and a co-author of the study.

“Something has to be affecting the survival rates of the oldest fish,” said co-author Daniel Schindler, a UW professor of aquatic and fishery science, in a statement released last year. “It’s clear there are lots of unanswered questions, but if you take a weight-of-evidence approach, most arrows are pointing to marine mammals — and killer whales, in particular.”

Palkovacs notes that the study published in Nature Communications has a different focus — looking at fish caught off Alaska and not those harvested in Northwest waters — and used a different methodology. He said that it’s possible orcas could have an impact in some select populations of salmon returning to spawn in Alaska’s freshwater. But he said there is not good enough data on the diet of Alaska orcas to determine if they are having any kind of broad-scale affect on size of Alaska salmon.

Also, off Alaska, there are lots of reports of orcas feeding on fish other than chinook. Alaska fishermen, for example, report orcas frequently strip black cod from the baited longlines set by Alaska fishermen.

“The limited diet data available for Alaska resident killer whales suggests that they show lower selectivity on Chinook salmon than do killer whales from Washington and British Columbia,” stated the study Palkovacs co-authored.

Much of this study discusses the dual impacts of climate change and competition as salmon migrate from freshwater, where they hatch from young eggs into ocean feeding grounds. This cycle is completed when some fish survive to return to freshwater to spawn.

The researchers say the data they analyzed indicates that many salmon opted to forgo additional time at sea to return at younger ages to spawn. Thus, they had less time to fatten and grow.

The researchers say that may reflect, in part, climate change, which has included periodic warming of ocean temperatures that has reduced the availability of prime food resources. Meanwhile, there has been increased competition among some species of salmon for the food that is available.

Hatcheries in both Asia and North America also have been expanding their release of young salmon to augment the harvests.

The co-authors wrote that there are difficult decisions to be made about the scale of hatchery releases into the ocean, They found that new “tools” are urgently needed to quantify “the apparent trade-offs between the releases of one species and the impacts of size and productivity … on other species.”