Climate cycles point to change

Posted: Thursday, July 19, 2001

Every fisher on Cook Inlet knows that tides influence the ways salmon move.

Recent evidence suggests that salmon also move to the rhythm of larger natural cycles. Oceanogra-phers are forcing biologists to rethink the importance of the years salmon spend at sea in building salmon runs and to wonder if Alaska fisheries are on the brink of a major change.

"It used to be most of the fishery research was done in fresh water," said Bruce Finney, an associate professor of marine science at the Institute of Marine Science at the University of Alaska Fairbanks.

"I think the whole mindset has really changed."

More powerful computers, satellites and years of carefully collected measurements are giving scientists new tools for understanding the rest of the salmon story. Scrutini-zing oceans, climate and biological cycles, they are discovering new patterns with major ramifications for fisheries management.

The tropical climate cycles known as El Nino and La Nina have gotten attention for their effects on the weather. But different cycles, on much longer time scales, may have more effect on northern waters and Alaska fish.

In the 1990s, a team from the University of Washington's Joint Institute for the Study of the Atmosphere and Oceans began to look at North Pacific fish.

Steven Hare, then a graduate student, compared salmon production to climate. He coined the term Pacific Decadal Oscillation (PDO) in 1996 to describe the links and cycles he found.

Hare and his colleagues concluded that populations of salmon and other marine life such as pollock, crab, sea lions and some seabirds are linked with large weather patterns that last from 20 to 30 years.

In the warm PDO mode, Alaska salmon thrive, while those farther south dwindle.

The "regime shift" that stunned Gulf of Alaska fisheries in the late 1970s, heralding a decline in shellfish, a boom in bottom fish and record Cook Inlet salmon hauls, correlates exactly with the Washington oceanographers' conclusion that the winter of 1976-77 marked a climate turning point and the beginning of what they call a "positive" PDO.

"For Alaskan salmon, the typical positive PDO year brings enhanced stream flows and near-shore ocean mixed-layer conditions favorable for high biological productivity. Generally speaking, the converse appears to be true for Pacific Northwest salmon," they wrote in a scientific paper published in 1997.

The positive PDO phase, which began in the late 1970s, is associated with warmer surface waters in the Gulf of Alaska, more winter storms, more coastal precipitation in Southcentral Alaska and more plankton in the gulf, where juvenile salmon from Alaska's rivers feed.

What is good for Alaska salmon is not necessarily good for other marine life. The positive PDO phase seems to mean fewer nutrients close to shore, making hard times for animals that forage in the shallows, such as harbor seals and kittiwakes.

The ocean climate researchers traced the previous fishery and climate records back about 100 years.

Cool PDO conditions prevailed from 1890 to 1924 and from 1947 to 1976. A warm PDO period ran from 1925 to 1946 and another began in 1976, they concluded.

Now oceanographers are questioning whether the 25-year-old warm PDO phase is ending.

This year's North Pacific ocean temperature measurements are average, Alaska salmon returns of the past couple years have faltered and the outlook is confused. The climate tide may be turning against Alaska's salmon fishers.

But UAF's Finney said it is too soon to tell, because so little is known about ocean systems that regime shifts are only clear in hindsight.

"I don't think anyone, to my knowledge, has gone out on a limb and said we have switched," he said.

"But there are certainly indications things are changing. We may be in a transitional mode."

The PDO is only part of the story, he cautioned.

Finney has been looking at residue on the bottom of Alaska lakes. By studying the thickness of layers of dead salmon parts in sediments, he can estimate relative run sizes. To compare the fish populations with climate, he measures tree rings. These sources allow him to look centuries into the past.

He is finding even longer cycles, about 50 to 100 years long, he said.

"There is another type of weather pattern that is not explained by this Pacific Decadal Oscillation," he said.

"Over that time period, I also see that salmon up here generally do well when things are warm."

The complex climate cycles and their fisheries implications are a hot frontier of scientific endeavor these days. In the North Pacific, an international science project called GLOBEC (GLOBal ocean ECosystems dynamics) already is collecting more information.

The Exxon Valdez Oil Spill Trustee Council has named such monitoring as a priority. As its original mandate for oil spill research and restoration expires this year, it is setting up an endowment for ongoing research. Its ambitious, interdisciplinary Gulf Ecosystem Monitoring program, scheduled to begin in the fall of 2002, will study what organizers call "the vital signs" of the sea and its coast.

Finney said research so far is bringing up more questions than answers. But he expects the next few years to dramatically expand understanding.

"Having a good, long-term set of data is really important to understand these things, because they are really complex," he said.

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