What happens at the equator doesn’t stay at the equator. El Niño-associated changes in the ocean may be putting the biodiversity of two Northern Pacific salmon species at risk.
Researchers tracked the survival of Chinook and coho salmon from hatcheries in North America between 1980 and 2006.
Before the 1990s, ocean survival rates of Chinook and coho salmon varied from each other—but the new findings show that survival rates of the two species have become increasingly similar.
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Coho Spawning on the Salmon River. Credit: Bureau of Land Management Oregon and Washington
Says lead author Patrick Kilduff, a postdoctoral scholar working with Louis Botsford in the wildlife, fish and conservation biology department at University of California, Davis, at the time of the study:
“Two species that historically have had different responses and seem to be very different in their coastal-wide patterns now appear to be more synchronized. When salmon populations are synchronized, it’s either good for everyone or bad for everyone—similar to the stock market.”
From an economic perspective, it means that when catch of one species is low, catch of the other also will tend to be low. This synchronous response to ocean conditions represents a loss in biological diversity that can’t be addressed directly by freshwater management actions.
It’s not yet well understood what is causing the increasing similarity, but it could reflect a change in coastal ocean food-web linkages or perhaps a change in the salmon species themselves.
Historically, many Pacific salmon species were thought to be influenced by the Pacific Decadal Oscillation (PDO), an El Niño-associated eastern Pacific warming pattern. As the nature of El Niños has changed, another ocean indicator, the North Pacific Gyre Oscillation (NPGO), has grown increasingly important, but its impact on salmon has not been well understood.
The new study, published in the Proceedings of the National Academy of Sciences, shows that coho and Chinook salmon survival rates along the West Coast are more strongly connected to the NPGO than to the PDO.
“Changes in equatorial conditions lead to more of the large-scale Pacific Ocean variability being explained by North Pacific Gyre Oscillation, and it’s influencing the survival of salmon from Vancouver Island south to California,” Kilduff says.