Sockeye salmon spawning in B.C.'s Adams River in 2010.

Ocean warming may be factor in sockeye decline

Study ties salmon productivity drop to 'shared mechanisms'

Sockeye salmon along the west coast are producing far fewer returning offspring than in the past and the SFU fishery scientists who have documented the trend say it suggests climate change may be a factor.

Randall Peterman said the study he co-authored found 24 of 37 sockeye stocks from Washington State to Alaska lost productivity since 1985, with the hardest hit runs no longer even replacing themselves.

He said the fact the decline has been widespread across both pristine and heavily disturbed watersheds points to non-local “shared mechanisms” as the more probable cause, rather than river-specific logging or pollution.

“It’s much more likely that what’s causing these changes is occurring over a large area,” said Peterman, a professor in SFU’s School of Resource and Environmental Management.

Warming oceans could be reducing the salmon food supply in the north Pacific, sending more predators towards the sockeye or increasing their vulnerability to pathogens, he said.

The culprit could also be affecting sockeye in freshwater, Peterman added.

A pathogen – either naturally occurring or spread by fish farms – could be amplified by climate changes and infecting sockeye in rivers that later die at sea.

Preliminary findings were presented in 2011 to the Cohen Inquiry, which reports in the fall with recommendations on halting the decline in Fraser River sockeye.

But Peterman said the newest analysis shows the pattern of declining productivity has spread northward to more watersheds over time.

“That trend of spreading northward is indicative of possibly climate-driven processes that become more extreme in the south first and work their way north,” he said. “The evidence is much stronger than it was.”

The theory of a fish food shortage on the high seas is backed in part because sockeye have tended to return significantly underweight in recent years. The food supply is expected to decline as the ocean warms.

But Peterman noted increased competition for the same food supply is another possibility.

The number of pink salmon feeding in the same area of the north Pacific has more than doubled, largely the result of “ranching” of pinks by Russians and Alaskans.

“Because they feed on similar food to sockeye salmon there may be increased competition,” Peterman said.

The study was published in the Canadian Journal of Fisheries and Aquatic Sciences.