CORVALLIS, Ore. - The record return of coho and chinook salmon to the Oregon coast has been credited to superb ocean conditions as returning salmon encounter a virtual smorgasbord of herring, anchovies, zooplankton and even sardines, which had virtually disappeared from West Coast waters.

Now an interdisciplinary, inter-agency group of scientists believe they may have an answer for why the ocean conditions are so bountiful. They call it a "climate regime shift."

From 1977 to 1998, the low pressure system that sits off Alaska's Kodiak Island every winter - known as the Aleutian Low - was larger and more intense than it had been since the mid-1940s, according to William T. Peterson, an oceanographer with the National Oceanic and Atmospheric Administration at Oregon State University's Hatfield Marine Science Center in Newport.

This 1,000-mile wide low pressure system was characterized by strong, circling winds that pushed nutrient-rich waters north into Alaska and delayed the upwelling off Oregon and Washington which helps feed the nutrient cycle in spring and summer. The effect created good ocean conditions for salmon in Alaskan waters, while less-than-ideal conditions off Oregon and Washington.

The Aleutian Low became even larger and more intense in the fall and winter of 1997-98, during a strong El Nino episode.

Then in the winter of 1999, the pressure system suddenly shifted west to Kamchatka, a Russian peninsula. And the ocean conditions - and biology - changed almost overnight. Different zooplankton appeared off the Pacific Northwest coast, and in much greater numbers, the scientists say.

"During much of this intense Aleutian low period, the waters off the Oregon coast were dominated by 'southern' copepods that are more common off central California," said Peterson, who also is a professor in the OSU College of Oceanic and Atmospheric Sciences. "These species are typical of weak currents, weak upwelling warm water and low productivity. Then, in 1999, bang. Overnight the southern copepods disappeared and were replaced by boreal, or northern copepods.

"The actual biomass of the copepods has doubled in the last couple of years," Peterson added. "And suddenly, the anchovies begin to spawn again, herring are everywhere, and sardines have flourished."

This intersection between ocean conditions and biology is of particular interests to scientists involved with the Global Ocean Ecosystem Dynamics, or GLOBEC program. Funded by the National Science Foundation and NOAA, GLOBEC is a national program that has West Coast components in Alaska and the Pacific Northwest.

Richard Brodeur, a NOAA fisheries biologist who also has a courtesy faculty appointment at OSU, studies salmon survival and feeding habits in the ocean. During the 1980s and most of the 1990s, the poor ocean conditions led to a low survival rate, he said. During the El Nino year of 1998, things hit rock bottom.

"When we looked inside the stomachs of juvenile salmon that had entered the ocean in 1998, they were pretty empty," Brodeur said. "They had some small prey - a few juvenile rockfish - but mostly small copepods and jellyfish. It wasn't their usual diet."

What salmon usually eat, Brodeur said, are juvenile rockfish, smelt, anchovies, sardines, crab larvae and krill. Starting in 1999, those prey reappeared in the stomachs of fish the scientists examined.

"Having an abundance of baitfish actually does two things," Brodeur said. "They obviously are an important food source for the juvenile salmon. Salmon need to grow fast early on to avoid becoming prey of other fish and birds.

"Baitfish have another useful purpose," Brodeur added. "When they are abundant, they become an alternate prey for birds and groundfish like rockfish and hake that may eat them instead of juvenile salmon."

The biological chain of events boosting salmon runs seems fairly clear. The abundant reappearance of northern copepods off the Northwest coast has led to huge numbers of "baitfish," including herring, anchovies and sardines. The presence of these baitfish appears to significantly boost salmon survival in the ocean.

The climatic and oceanic mechanisms behind this phenomenon are not as clear, the scientists say.

"These 'regime shifts' are part of a cycle, but we don't have enough data to know much about them historically," Peterson said. "We know there were Aleutian low pressure cycles from approximately 1923-47, and from 1977-98, but we don't know their history prior to the 1920s. And we think they typically last about 20 to 25 years, but what triggers these shifts - both in and out of the cycles - is still a mystery."

One way physical oceanographers track changes in the ocean is through an index called the Pacific Decadal Oscillation, which monitors several conditions, including sea surface temperatures. From the period of 1977-1998, every year was warmer than normal except one, Peterson pointed out. For the past three years - including 2001 - the waters off Oregon have been colder than normal.

"When things shifted in 1999, the California Current became stronger," Peterson said. "There was more upwelling, more nutrients and greater productivity and - equally important - an infusion of northern copepods from the Gulf of Alaska.

"Not coincidentally, ocean conditions for salmon in Alaska began to decline at the same time."

Ted Strub, a professor of oceanic and atmospheric sciences at OSU, has monitored changes in the Pacific in another way - the height of the ocean - measured by the TOPEX/POSEIDON altimeter. It is the same satellite that first detected the El Nino signal in the western equatorial Pacific in early 1997.

"There is no question that ocean conditions are different from what they were for most of 1977-98," Strub said. "Our satellite data shows that since 1998, the coastal ocean off the U.S. west coast is a few centimeters lower than it was from 1993-98, the beginning of the TOPEX data. That's because water becomes denser as it gets colder, occupying less space."

Strub says studying ocean conditions is like listening to music. There are different frequencies and layers that - examined separately - don't fully represent the big picture.

"If you look at the climatic effect, for example, you have to look at 20- to 25-year cycles, then overlay the influence of El Nino and La Nina events, which have a three- to seven-year time scale, and then look at seasonal variations, which are enormous," Strub said. "Satellite data only go back 10-20 years, so we're just now beginning to get the baseline data that we need.

"A hundred years from now, we may understand how all this works."

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Bill Peterson, 541-867-0201