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Smaller, younger salmon due to climate change and competition, new study says

Salmon are returning to their spawning grounds smaller in size and younger, and researchers say it's because they are spending fewer years in the ocean.

'Chinook salmon in particular have shown a really rapid decline in size'

Peter Westley, one of the co-authors of a new study looking at chinook salmon, is photographed at the Yukon River with a chinook salmon, caught near Stevens Village. (Submitted by Peter Westley)

Pacific salmon are returning to their spawning grounds in Alaska, B.C., and Yukon, smaller in size and younger, and researchers say it's because they are spending fewer years in the ocean.

Researchers from the University of Alaska and the University of California Santa Cruz say it's due to climate change and competition.

"The only two variables that had strong evidence were variables related to climate and competition with pink salmon in the ocean; those were the two drivers that really seemed to be highly associated with the changes in size and age," said Peter Westley, one of the co-authors of the study from University of Alaska Fairbanks.

The new study was published on Aug. 19 in the journal Nature Communications. 

The study took three years to complete, and looked at nearly 60 years of data from 1957 to 2018. 

Over that span of time, the Alaska Department of Fish and Game, fishery biologists and technicians measured and weighed nearly 12.5 million salmon in Alaska. With the wealth of information, researchers could see patterns in body-size changes in coho, chinook, chum, and sockeye salmon. 

Fish biologists are also trying to figure out why the salmon are around 20 per cent smaller than they should be, especially over the last 20 years.

"Chinook salmon in particular have shown a really rapid decline in size," said Westley.

Side-by-side images of same person and same spot with a large salmon caught about 10 years apart, according to Peter Westley. (Submitted by Peter Westley)

Researchers say one of the main reasons salmon are smaller in size returning to their spawning grounds is because the chinook salmon are spending less time in the ocean.

"The changes in size are really driven by the fish being younger than they were in the past," said Westley. "The largest fish that used to come back after seven years after being spawned in the gravel and spending many years in the ocean, those fish just are not in the population anymore," said Westley.

He said chinook salmon returning to spawn are only about three to five years old.

Chinook normally would start to enter fresh water around six to seven years of their lifecycle.

He said more research needs to be done to find out why they are maturing so early.

Westley said with the smaller salmon, there are fewer eggs to start the cycle again. 

"There is photo documentation of fish that are close to 100 pounds in Dawson City," said Westley.

He said the new normal for chinook entering the Yukon river system to spawn is around 10 or 11 pounds.

Elder remembers big chinook

Champagne and Aishihik Elder Chuck Hume remembers just how big chinook salmon were when he was a child. He would use a gaffing hook attached to a long pole to catch the salmon in the Tatshenshini River near Dalton Post in Yukon.

He said his parents knew how to pull them in from the river by gaffing them in the gills, but for him as an older child and being inexperienced, it was a different story.

"You feel them, then your delay time, next thing you know you gaffed them by the tail and away you go," said Hume, laughing. 

Hume said chinook and sockeye salmon were big in size and weight. He estimated the average size for a chinook salmon was around 40 to 50 pounds but sometimes in the 70-pound category.

Hume said the chinook and sockeye spawning runs are small these days, just like the size of the fish. It worries him as well, because his grandchildren may only hear stories of the salmon from the Tatshenshini River system and its tributaries. 

Study co-author Westley said researchers want to find out why salmon are returning to the spawning grounds early, and what can be done to help with the overall management of rebuilding salmon species.

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