Sockeye salmon repeatedly surprised the Northwest in recent years by returning to the Columbia River in tremendous numbers that far surpassed all predictions. Now scientists at NOAA’s Northwest Fisheries Science Center have teased out an explanation why.
Young sockeye arriving in the ocean feed on creatures lower on the marine food chain than many other salmon, gobbling up tiny zooplankton, while Chinook and coho salmon go after small fish. The scientists suspect this helps sockeye benefit more quickly from improved ocean conditions that nourish the food chain from the bottom up. The result is that far more sockeye may survive their difficult years in the ocean than traditional indicators used for other salmon might suggest.
Sockeye may also benefit more immediately from smaller areas of improved ocean conditions, for much the same reason.
"Sockeye eat further down in the food chain, so they seem to respond to these changes more directly and more quickly than we see with coho and Chinook," said John Williams, lead author of the research published in the May edition of the journal Fisheries Oceanography. "We may get good returns for sockeye even when other fish aren’t showing the same response."
Nearly 400,000 sockeye returned to the Columbia River in 2010, more than in any other year since counting began in the late 1930s when Bonneville Dam was built. Sockeye surprised the region again in 2012 when more than 500,000 fish flooded back to the Columbia. The boom generated wide speculation about the factors at play.
To unravel the likely causes, NOAA researchers looked for relationships between sockeye returns over more than 25 years and 19 different factors that affect conditions in the river and ocean, including river flow, temperatures, and the spill of water past dams. In the ocean they examined factors including the upwelling of cold, nutrient-rich seawater and the large-scale Pacific Decadal Oscillation, a marine climate pattern known to drive ocean conditions and affect salmon.
They found the factors with by far the greatest influence on sockeye returns were the April upwelling and the Pacific Northwest Index, a regional climate indicator developed about 20 years ago to assess factors affecting oyster growth. Although the Index is based on terrestrial measurements such as snowpack, it reflects ocean temperatures and wind patterns circulation, Williams said.
"These things producing terrestrial conditions are the things occurring right off the coast, and the things happening right off the coast are the same ones affecting sockeye," he said.
It was still not clear exactly what conditions at sea affected sockeye most. But the resulting strong returns benefited the majority of Columbia River sockeye that return to Lake Wenatchee and Lake Osyoos on the upper Columbia and the far fewer and endangered Snake River sockeye that return to mountain lakes in Central Idaho.
"Something to do with the ocean was definitely driving these returns, and we saw its influence in both in the Snake and the Columbia," said Rich Zabel, director of the Fish Ecology Division at the Science Center and a coauthor of the study.
The ocean influences were so strong when juvenile sockeye entered the ocean in 2008 that 23.5 percent of the fish eventually returned as adults, a very high return rate. Conditions in the river when the young fish migrated toward the ocean showed very little influence by comparison, the study found.
"We got a huge return from not that many juveniles going out, which really tells you that the ocean is very important," Williams said.
The study did find a positive trend of increasing survival of juvenile sockeye in the Columbia River above McNary Dam, suggesting that efforts in the United States and British Columbia to improve habitat and dam passage are paying off by helping more young sockeye migrate safely downriver. And the more juvenile sockeye that reach the ocean, the more that benefit when conditions there are good.
"Some freshwater factors may be relevant, but the dominant factor clearly seemed to be the ocean," Zabel said. "That doesn’t mean that freshwater isn’t important, but it does suggest that the results are really dependent on what’s happening in the ocean."