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NEWPORT, Ore. - Juvenile salmon survival increases dramatically when cold-water zooplankton species are dominant in the northeast Pacific Ocean, and scientists at Oregon State University's Hatfield Marine Science Center believe the copepods' high lipid levels may boost the oceanic food chain and allow salmon to grow fast enough to survive their first year at sea.

The scientists will present their research later this month at the American Geophysical Union's Ocean Sciences Meeting Feb. 20-24 in Honolulu.

William Peterson, an oceanographer with the NOAA Fisheries Program at the center, says that cold-water copepods hibernate during the winter - much like bears - in a period of inactivity known as diapause. To survive the winter they store high amounts of lipids, or fats.

These copepods, in turn, are eaten by juvenile anchovies, herring, smelt and euphausiids (krill), boosting the fat content of those species and making them highly nutritious delicacies for young coho and Chinook salmon, as well as other predators.

"A fat salmon," Peterson said, "is a happy salmon."

For years, scientists, the fishing industry and even the public have been generally aware that "good ocean conditions" are critical to salmon prosperity and that good years are associated with strong upwelling that brings nutrient-rich deeper waters to the surface. But studies by Peterson and his colleagues have taken that several steps further, shedding new light on just what make those conditions favorable for juvenile salmon.

And one of those keys to survival is rapid growth for salmon once they enter the ocean, says Peterson, who is a courtesy professor in OSU's College of Oceanic and Atmospheric Sciences.

"The salmon are roughly six or seven inches long when they enter the ocean and are about the same size as adult herring and anchovies that make them ideal prey for birds and larger fish," Peterson said. "But salmon have the ability to grow tremendously fast and they have to, just to limit predation. When they feast on lipid-rich anchovies and smelt - that fed on cold-water copepods - they grow rapidly.

"Warm-water copepods do not enter diapause during winter; rather they spend their entire lives in the upper layers of the ocean and thus don't store lipids in the same way as cold-water species," he added. "This nutritional differential is a new hypothesis and one we'd like to test over the next 3-4 years."

Research has shown that the first year is critical for salmon survival. The salmon must not only grow fast enough to limit predation during their first summer at sea, they must then store enough fat to prevent starvation during their first winter.

Cold-water copepods are most abundant during "the negative phase" of the Pacific Decadal Oscillation, a multi-factor index that monitors many ocean conditions, including sea-surface temperatures. When the PDO is in its positive phase, the waters off Oregon are warm, as in the strong El Nino year of 1997-98 and during much of the 1990s, when salmon were struggling to survive. Upwelling of deep-water nutrients is limited and copepod biomass is down. The numbers of salmon and steelhead returning to coastal rivers usually plummets.

Conditions can change rapidly, however, and in 1998 for example, the northern Pacific experienced a rapid transition to a negative PDO that brought strong upwelling and cold-water copepods to the region. Not coincidentally, Peterson said, the number of adult Chinook salmon returning to the Columbia River system in the first years of this century were the highest since the 1950s. Many other river systems experienced outstanding fish runs.

In the fall of 2002, conditions reversed again and warmer waters have been dominant since. The copepod biomass has shrunk, and salmon and steelhead runs have decreased significantly. There may be good news on the horizon, however.

"There are signs that it is getting cold again, which is encouraging," Peterson said. "But it's still too early to tell. We won't know until April or May whether the colder water temperatures we're seeing now will continue or disappear, as they did in 2005.

"Last year was a strange year in many ways," Peterson added. "The ocean was cold until February, then it warmed quickly, and the upwelling came very late. The whole system was out of whack. It was almost like the El Niño year of 1998 in terms of ocean production."

Robert Emmett, a NOAA colleague of Peterson, says another factor related to temperature may affect juvenile salmon survival. When ocean conditions are warm, Emmett says, Pacific hake tend to come onto the continental shelf at night and prey on young salmon, which may be small because of the lack of food. When the waters are cold, the hake stay in deeper water and feed on euphasiids.

"It's kind of a double whammy," Peterson said, "but it's all related to temperature. In one way, it's simple - cold water is good, and the earlier the upwelling, the better."

The research is part of a NOAA program to study salmon growth and survival in the ocean. Funded by the Bonneville Power Administration, the program also studies the effects of the Columbia River plume on juvenile salmon survival and growth.