Predictions of climate impacts on sardines can be misleading. (Credit: Image courtesy of University of California – San Diego) Enlarge

Mar. 25, 2013 — In the early 1940s, California fishermen hauled in a historic bounty of sardine at a time that set the backdrop for John Steinbeck's "Cannery Row" novel. But by the end of the decade the nets came up empty and the fishery collapsed. Where did they all go? According to a new study led by scientists at Scripps Institution of Oceanography at UC San Diego, the forces behind the sardine mystery are a dynamic and interconnected moving target.

Publishing in the Proceedings of the National Academy of Sciences, Scripps graduate student Ethan Deyle, professor George Sugihara, and their colleagues argue that problems lie in seeking answers one factor at a time, as scientists have done for decades. What is the impact of climate on sardines? What is the effect of overfishing on sardines? Focusing on single variables in isolation can lead to misguided conclusions, the researchers say.

"Studying ecosystems in this piecemeal way makes it hard to find quantitative relationships, the kind that are useful for management and stand the test of time," said Deyle.

Instead, using novel mathematical methods developed last year at Scripps, the researchers argue that climate, human actions, and ecosystem fluctuations combine to influence sardine and other species populations, and therefore such factors should not be evaluated independently.

The technique developed by Sugihara and his colleagues, called "convergent cross mapping," takes multiple variables into account and avoids the centuries-old "correlation does not imply causation" issue that has plagued single-factor studies. For example, based on data from the Scripps Institution of Oceanography Pier, studies in the 1990s showed that higher temperatures are beneficial for sardine production. By 2010 new studies proved that the temperature correlation was instead a misleading, or "mirage," determination.

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