A new study suggests that anomalous ocean conditions two years ago, which led to the lack of food for juvenile Chinook salmon, may have resulted in significant mortality that will show in their return this year to the Columbia River.
Pacific Decadal Oscillation values, which reflect warm and cold sea surface temperatures, suggest 2015 was one of the warmest nearshore oceans encountered by migrating Chinook salmon dating back to at least 1900. About 80 percent of a typical spring Chinook run on the Columbia River come from fish that went out to sea as yearlings two years earlier.
"When juvenile salmon first enter the ocean, it is a critical time for them," said Elizabeth Daly, a senior faculty research assistant with the Cooperative Institute for Marine Resource Studies, jointly operated by Oregon State University (OSU) and the U.S. National Oceanic and Atmospheric Administration (NOAA) out of the Hatfield Marine Science Center in Newport, Oregon, in the U.S. Pacific Northwest.
"They are adjusting to a salt-water environment, they have to eat to survive, and they have to avoid becoming prey themselves. When we sampled juvenile salmon in May and June of 2015, the fish were much smaller and thinner than usual, and many of them had empty stomach. There just wasn't anything for them to eat," Daly said.
Two key statistics stand out from 2015, noted the researchers, who reported their findings in the journal Marine Ecology Progress Series. The California Current system off the U.S. West Coast was more than 2.5 degrees Celsius, or 4.5 degrees Fahrenheit, warmer than normal, and the juvenile Chinook were smaller and skinnier than during a cold-water year, weighing an average of 17.6 percent less.
When the oceanic waters off Pacific Northwest states of Oregon and Washington are cold, young salmon primarily feed on readily available fish prey such as Pacific sand lance and smelts, which triggers their growth spurt. When waters are warmer, there is less food available, and they primarily eat juvenile anchovies and rockfish, which are less-desirable prey than cold-water species. However, by the time the juvenile Chinook salmon migrated to the ocean later in spring of 2015, the larval anchovies and rockfish had all but disappeared, making even backup food sources for the salmon scarce.
The researchers theorize that these larval fish died off because they themselves had little to eat.
"During warm years, there is typically less upwelling that brings cold, nutrient-rich water to the surface," Richard Brodeur, a biologist with the NOAA Northwest Fisheries Science Center and co-author on the study, was quoted as saying in a news release from OSU. "Salmon populations may be able to handle one year of warm temperatures and sparse food. But two or three years in a row could be disastrous," Brodeur said.
