Chinook salmon. (Photo: NOAA)

Chinook salmon from west coast have reduced their size in four decades

UNITED STATES
Wednesday, February 28, 2018, 22:50 (GMT + 9)

A new study by the University of Washington reveals that the largest and oldest chinook salmon have mostly disappeared along the West Coast.

Researchers analyzed around 40 years of data from hatchery and wild chinook populations from California to Alaska, looking at patterns that emerged over the course of four decades and across thousands of miles of coastline.

In general, they found that chinook salmon populations from Alaska showed the biggest reductions in age and size, while salmon from Washingtin were in second place.

“Chinook are known for being the largest Pacific salmon and they are highly valued because they are so large,” said lead author Jan Ohlberger, a research scientist in the UW’s School of Aquatic and Fishery Sciences. “The largest fish are disappearing, and that affects subsistence and recreational fisheries that target these individuals,” he added.

Chinook salmon are born in freshwater rivers and streams, then migrate to the ocean where they spend most of their lives feeding and growing to their spectacular body size. Each population’s lifestyle in the ocean varies, mainly depending on where they can find food.

California Chinook salmon tend to stay in the marine waters off the coast, while Oregon and Washington fish often migrate thousands of miles northward along the west coast to the Gulf of Alaska where they feed. Western Alaska populations tend to travel to the Bering Sea.

After one to five years in the ocean, the fish return to their home streams, where they spawn and then die.

According to the study, most populations analyzed showed a reduction in the average size of the returning fish over the last four decades,  up to 10 percent shorter in length, in the most extreme cases.

Researchers said these similarities, despite the different life histories of the different populations, point to a cause that transcends regional fishing practices, ecosystems, or animal behaviors.

“This suggests that there is something about the larger ocean environment that is driving these patterns,” Ohlberger said. “I think fishing is part of the story, but it’s definitely not sufficient to explain all of the patterns we see. Many populations are exploited at lower rates than they were 20 to 30 years ago.”

The reductions in size could have a long-term impact on the abundance of Chinook salmon, because smaller females carry fewer eggs, so over time the number of fish that hatch and survive to adulthood may decrease.

Researchers also revealed that resident killer whales and other marine mammals that feed on salmon are probably contributing to these changes.

Scientists are still trying to understand the impacts of orcas and other marine mammals on Chinook salmon, and the ways in which their relationships may have ebbed and flowed in the past. It may not be possible, for example, for marine mammals and Chinook salmon populations to be robust at the same time, given their predator-prey relationship.

This new study was funded by the Pacific State Marine Fisheries Commission, and published in Fish and Fisheries magazine.


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 National Oceanic and Atmospheric Administration NOAA/NMFS

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