Fish Ear Bones Hold Clues to Migration

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Scientists have known about the daily rings since 1971. Recently the rings have provided clues to ancient climates. The otoliths of 6,000-year-old Peruvian sea catfish from shallow waters off the coast of Peru revealed that sea surface temperatures were three degrees warmer than now.

Thorrold analyzes the daily rings' geochemical signatures to track a fish's movements. If the rings' oxygen and carbon signatures match those of a particular tributary, river or coastal region, Thorrold can sometimes pinpoint to within six miles where a fish has been swimming.

Boundaries Important to Fish

If the carbon and oxygen signatures do not match a unique location, Thorrold examines trace metals like magnesium, manganese, strontium and barium to fine-tune the fish's whereabouts. The task is tougher in the open ocean.

Thorrold uses a laser to trace several rings—several days—at a time. The laser vaporizes part of the ring and sweeps the material into a mass spectrometer that can identify and quantify the metals.

The research provides information essential to commercial and recreational fishing, a $50 billion industry in the United States that's threatened by overfishing.

Of the 120 major fish stocks targeted in the U.S., 81 are considered overfished, according to a 2001 report by the National Oceanographic and Atmospheric Administration.

Understanding where fish spawn, where they migrate and whether they return to their natal waters helps create the strategies to rebuild fish stocks.

Today, national boundaries separate U.S. and Canadian fish stocks, which are managed separately. "But we need to identify boundaries that are important to the fish," Thorrold says.

For example, analyzing otoliths could reveal whether adult cod in the U.S. Georges Bank–Gulf of Maine fisheries originated in Canadian waters like the Scotian Shelf. Thorrold's research could identify spawning grounds and migration routes that warrant special protection.

"If we (the U.S. and Canada) share the same population," Campana points out, "then we should have similar management strategies."

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