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article imageScientists find out why Alaska's wild salmon are getting smaller

By Karen Graham     Aug 21, 2020 in Science
Fairbanks - Alaska’s highly prized salmon - a favorite of seafood lovers the world over - are getting smaller, and climate change is suspected, a new study reported, documenting a trend that may pose a risk to a valuable fishery, indigenous people and wildlife.
Salmon are critically important to both people and the ecosystem, not only in Alaska but in British Columbia and other areas of western North America. Salmon support commercial and subsistence fisheries and transport nutrients from the ocean to inland areas, fertilizing the ecosystems in and around the rivers where they spawn.
In a study published in Nature Communications on Wednesday, a team of researchers led by University of Alaska at Fairbanks (UAF) scientists, found that four of Alaska’s five wild salmon species have shrunk in average fish size over the past six decades, with stunted growth becoming more pronounced since 2010.
The hardest hit was Alaska's official state fish, the chinook, or king salmon. Chinook are on average, 8.0 percent smaller than they were before 1990. Alaska's sockeye, coho and chum salmon are also shrinking, according to the report.
A school of Chinook salmon.
A school of Chinook salmon.
Zureks (CC BY-SA 3.0)
The researchers analyzed data collected over six decades (1957 to 2018) from 12.5 million fish by the Alaska Department of Fish and Game. With this huge database, the team was able to see patterns of body size changes for the four species of salmon--Chinook, chum, coho, and sockeye--across all regions of Alaska.
Their results are interesting and intriguing at the same time. Alaskan salmon can spend up to seven years in the ocean before returning to their freshwater homes to spawn. during those seven years, the salmon feed and grow to maturity, migrating great distances in the North Pacific Ocean.
However, the scientists found that the decrease in body size was primarily because the salmon were returning to their spawning grounds at an earlier age than they had in past years.
Al Brewer displays the 35-pound chinook salmon his wifr caught in Puget Sound in this 1973 photo tak...
Al Brewer displays the 35-pound chinook salmon his wifr caught in Puget Sound in this 1973 photo taken by Doug Wilson with the EPA.
U.S. National Archives and Records Administration
"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, according to Eurekalert.org.
A range of factors at play
The research team identified a number for factors to explain the smaller size of salmon driving the change. Some of the factors cover almost all areas while others may be specific to certain species of salmon.
"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."
Juvenile Pacific salmon. Environmental quality is crucial to their survival.
Juvenile Pacific salmon. Environmental quality is crucial to their survival.
The two major factors behind this change in sizes is climate change and competition from hatchery-raised salmon in the ocean. Palkovacs says these two factors have definitely contributed to the decline in sizes.
Other factors, like the effect of commercial fishing appears to be important only for some salmon populations. Similarly, the results were mixed for another proposed driver of size declines, the recovering populations of marine mammals that prey on salmon.
"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."
The bottom line is that the ocean has become a riskier place for salmon. If salmon were to stay in the ocean longer, they would grow larger, thereby giving them greater success in spawning and laying more eggs. However, each additional year increases the risk of not returning to reproduce at all.
Black bears
Black bears
Bryan Wilkins (CC BY-SA 2.0)
Consequences to people and the environment
Alaska produces almost all the wild salmon consumed in the United States. Last year, commercial fishermen harvested over 206 million salmon and sold them for $657.6 million, reports Reuters, citing Alaska state officials.
Salmon are also a dietary staple for Indigenous peoples, and the red-fleshed fish are also eaten by bears and other wildlife. However, smaller fish means fewer meals per fish for subsistence fishers, lower profits for commercial fishers, and fewer eggs to sustain salmon populations.
UAF’s Krista Oke, the study’s lead author, says “It is impacting things that eat eggs, but it also impacts the salmon population itself. Smaller fish is a real problem for people who depend on salmon for their food and well being," Oke said. "For commercial fishers, smaller fish tend to fetch lower prices, and below a certain size they can't be made into high-value products and might have to be canned."
On the ecosystem's side, there are also consequences. Marine nitrogen from salmon runs provides critical nutrients for bears, insects, birds, trees, and juvenile salmon themselves.
"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."
As the study warns, "The downsizing of salmon and other organisms is a global concern, and current trends may pose substantial risks for nature and people."
More about Salmon, Climate change, Competition, hatcheryraised salmon, smaller size
 
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