for National Geographic News
Sardines and anchovies swim to the beat of a 50-year climate cycle in the Pacific Ocean. The seesawing changes of these two tiny fishthe one abundant when the other is scarcehave highlighted a natural climatic pattern that may color our understanding of global warming.
While scientists have known that sardine populations along the California coast took big swings based on changing ocean temperatures, the study that appears January 10 in the journal Science spots an ocean-wide pattern behind these fishy fluctuations.
"Different people were talking about these cycles in their part of the ocean," said Francisco Chavez, a biological oceanographer at Monterey Bay Aquarium Research Institute (MBARI) and lead author of the paper. When Chavez's team pulled together 100 years of research from around the Pacific Rim, the sardine's boom-and-bust cycle kept reappearing. And around the ocean, anchovy populations stepped up as sardine levels sank.
"They were all marching to the same drummer," Chavez said.
Fishing for Climate Changes
Chavez and John Ryan of MBARI, along with researchers from Mexico and Peru, piled up a range of evidence to get to the bottom of this piscine pattern. The team checked fish catches, temperature measurements, ocean current records, and carbon dioxide concentration in the atmosphere. They looked at other species, such as anchovy-swilling seabirds, that might provide clues to changing fish numbers.
By using everything from high-flying satellites to seaworthy vessels, the researchers spotted a likely culprit that could sway fish populations: a naturally-occurring climate pattern that works its way across the Pacific.
This 50-year cycle resembles the El Niño events that take place every three to seven years. Along the eastern Pacific, sardine success starts when coastal waters surrounding California and Peru warm up, a pattern similar to El Niño's warming ways. And while scientists had spotted this cycle, known as the Pacific Decadal Oscillation, no one had tailed its influence across an entire ocean.
When things heat up, warm surface water traps nutrient-rich cold water in the deep spots. The nutrients feed phytoplankton, which becomes prime pickings for larger sea species. Decreasing levels of this smallest marine meal can tinker with a host of marine species, including the anchovy.
Schools of sardine and anchovy seem to switch off every 25 years, following rounds of warming and cooling ocean temperatures. During the mid-1970s, the warming Pacific sparked a "sardine regime." In the middle to late 1990s, the cycle swung the other way. Surface water cooled and anchovies started to return.
Chavez said natural warming and cooling like this may be missed when tallying climate change. "I worry a little bit that these fluctuations are confused with global warming," he said. The warming trend in the mid-1970s that ushered in a new batch of sardines came about the same time concern rose about global warming.
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