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Stanford researchers take new look at old puzzle of Bering Sea ecosystem

By Alexei Koseff | 1 May 2013

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Researchers catch walleye pollock during an acoustic trawl survey in the Bering Sea. (Photo courtesy of David Csepp, NOAA/NMFS/AKFSC/Auke Bay Lab)

Researchers catch walleye pollock during an acoustic trawl survey in the Bering Sea. (Photo courtesy of David Csepp, NOAA/NMFS/AKFSC/Auke Bay Lab)

The name may not ring familiar, but if you’ve had fish sticks, imitation crab meat or a fast food fish sandwich at some point in the past three decades, then you’ve already encountered the walleye pollock.

This two-foot, speckled fish found in the Bering Sea, between Alaska and Russia, is one of the most valuable catches in the world. As other marine fish populations have collapsed in recent years, the walleye pollock has grown into a billion-dollar annual fishery in the United States—and a menu mainstay wherever white, flaky fish is served.

The relatively recent economic importance of the Alaska pollock is largely the product of a population explosion in the 1970s—a time when other major fisheries, such as the Atlantic cod, were crashing. That enigmatic boom is still something of a scientific mystery, according to Zachary Brown, a Ph.D. student in environmental Earth system science at Stanford University. Data he presented at Stanford’s School of Earth Sciences annual Research Review earlier this month suggests that the prevailing theory is due for an overhaul.

Last year, Brown tested a decade-old hypothesis that a sudden warming of local temperatures in the mid-1970s caused the increase in walleye pollock population. But he found that the Bering Sea climate had already been in flux for several decades before that. Another explanation is needed for the pollock swell, he said.

Brown’s project stemmed from his own longstanding interest in the Bering Sea as a native of Alaska. Along with adviser Kevin Arrigo, a professor of biological oceanography at Stanford, he investigated records of sea ice loss over the last half-century. Shifting patterns in the seasonal melting of sea ice have been heavily linked to major ecosystem changes in the Bering Sea, including the collapse of crab stock and a decrease in marine mammals such as sea lions.

Each spring, as sea ice melts from the Bering Sea, a bloom of microscopic plants called phytoplankton forms the crucial base of the ecosystem’s food web. In warmer years, the sea ice melts earlier in the year, leading to phytoplankton blooms in open, warmer water. These warm-water blooms are more productive than the cold, ice-edge blooms of late-season sea ice retreat.

Zachary Brown presents his project at the 10th annual Stanford School of Earth Sciences Research Review on April 12. (Photo courtesy of Stanford School of Earth Sciences.)

Zachary Brown presents his project at the 10th annual Stanford School of Earth Sciences Research Review on April 12. (Photo courtesy of Stanford School of Earth Sciences)

The phytoplankton blooms support vast numbers of small animals called zooplankton. In warm water, the phytoplankton remain closer to the surface. This favors the type of zooplankton that feed walleye pollock, rather than those consumed by many sea mammals and larger fish found deeper in the ocean. Scientists have speculated that the warming period begun in the mid-1970s shifted the availability of food so as to ultimately benefit the pollock population.

“Phytoplankton fuel the growth of all the other species,” Brown said. The structure of the ecosystem “depends on where they end up.”

But conclusions about the effect of regional warming were based on satellite records of sea ice retreat that only extended back as far as 1979. Using surface air temperature measurements as a guide, Brown was able to test the theory by extrapolating annual conditions back to 1948.

Brown’s data showed a slow oscillation between colder and warmer eras in the Bering Sea, rather than a steady rise in temperatures. These shifts in the prevailing temperature were likely driven by the atmospheric fronts and ocean currents that impact the region, he said. Though annual variations in sea ice retreat had significant effects on the spring blooms, there was no indication of any long-term shifts in the timing of when sea ice melted each year.

Defying expectations—and warming in other regions, such as the Arctic Ocean further north—the Bering Sea has also entered another cold period over the past few years, Brown said. 2012 had the iciest conditions on record.

The value of this research, Arrigo said, is that it creates a longer timeline for conditions in the Bering Sea, providing broader context for what has been happening to the regional climate.

“You can almost get any trend you want if you don’t go back far enough,” he said.

Because the Bering Sea had experienced prior warm periods, Brown concluded that other factors probably caused the walleye pollock swell in the 1970s. He theorizes that the pollock population may instead have surged as result of devastating whaling in the region, which took off after World War II and only ended in the early 1970s. The pollock competes for food with some species of whale, and is preyed upon by several others; their disappearance from the Bering Sea could have allowed the fish to thrive.

Walleye pollock has become one of the world's most valuable fisheries, worth a billion dollars per year. (Photo courtesy of NOAA)

Walleye pollock has become one of the world’s most valuable fisheries, worth a billion dollars per year. (Photo courtesy of NOAA)

“We do a disservice when we ignore the direct impacts that humans can have on the environment,” Brown said.

Still, not everyone is convinced that Brown has ruled out a climate role in the pollock boom. Nicholas A. Bond, who researches air-sea interactions in the Pacific at the University of Washington, cautions against underplaying the impact of even temporary climate variations.

“The rules change in different climate regimes,” Bond said.

“Warmer conditions in the late ‘70s led to a whole shift in the ecosystem structure of the Bering Sea community,” he added. “The walleye pollock took advantage of it.”

While whaling may have contributed to the pollock population increase, Bond said, it’s difficult to measure how much any one element affects the system as a whole. That makes predicting and preparing for the sustainability of the walleye pollock fishery a challenge, especially if future warming once again drastically shifts the conditions that determine the Bering Sea food web.

For one of the world’s most important fish, it seems uncertainty is the only sure thing.

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