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FAIRBANKS -- One of the ways scientists make predictions about future changes is by studying what happened in the past when natural cycles caused waters to warm.

In the winter of 1976/1977, a dramatic shift in the Pacific climate brought warm air up from the south and raised water temperatures in oceans off Alaska. Years later, a fish biologist and a climatologist at the University of Washington identified the shift as part of a natural cycle, which the biologist named the Pacific Decadal Oscillation.

The shift in the late ‘70s and others earlier in the century showed up just as well in records of fish stocks as they did in temperature records. During “warm” phases of the PDO, when Alaska waters were warm, salmon did well in Alaska and less well in California, Oregon and Washington. In cool phases, it was the opposite.

Since the late ‘70s, cold-water species like crab and shrimp have generally declined in numbers in Alaska, while groundfish and warmer-water species like pollock and cod have done well, according to Gordon Kruse, a professor at the University of Alaska Fairbanks’ School of Fisheries and Ocean Sciences. During the El Nino in 1997 and 1998, which also raised ocean temperatures, ocean sunfish showed up in the Gulf of Alaska and fishermen caught albacore tuna off Kodiak Island.

Many scientists believe the Pacific Decadal Oscillation has already switched back to its cool phase. But ocean temperatures in Alaska have not noticeably cooled, and Kruse says some waters are now warming beyond the ranges seen in the past.

National Marine Fisheries Service surveys show king crab and snow crab have both shifted north in the Bering Sea over the past few decades, Kruse said. Other studies by Kruse and a graduate student of his suggest Pacific herring are changing their migration patterns as sea ice melts earlier in the year.

Warming waters and melting sea ice drove Alaska’s two U.S. senators, Ted Stevens and Lisa Murkowski, to push for the creation of an international management entity to oversee arctic fisheries that could exist in the future.