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Seafaring Fish Breed at Birthplace

12 January 2001 7:00 pm

Lend me your ears. Weakfish otoliths, or ear bones, provide geochemical clues to the fish's birthplace.

Salmon and a few other fish have a remarkable ability to navigate back to their birth rivers after years at sea. Now, for the first time, scientists have documented similar "natal homing" in fish that spend their entire lives in salt water. The finding could prompt fisheries managers to rethink how they regulate catches.

It's not easy to track juvenile fish that are exclusively marine. Hatchlings are too small, too numerous, or too dispersed for easy tagging or recapture, and they generally don't display telltale genetic differences. To get around these difficulties, fisheries biologist Simon Thorrold of the Woods Hole Oceanographic Institution in Massachusetts and his team looked to chemical clues contained in otoliths, tiny concretions that form in the ears of many fish. As the otolith grows, each new layer of calcium carbonate captures the chemical signature of the surrounding water.

The researchers analyzed otoliths from hundreds of juvenile weakfish caught in five major estuaries along the east coast of the United States in 1996. Then, in 1998, the researchers returned to the same areas and captured 2-year-old fish fresh from their wintering grounds. The adults' otoliths revealed that up to 81% of the spawners had swum in the same waters as juveniles, the researchers report in the 12 January issue of Science.

Geographically distinct spawning populations might spell trouble for weakfish managers, who currently treat the population as a single coastal stock. "You can't assume that vagrants from one estuary can quickly replenish another estuary's stock that has been overfished," Thorrold says. Similarly, designers of protected reserves--often touted as nurseries that will supply fish to areas where fishing is allowed--may have to reshape plans. "Putting a reserve in one estuary may not do a lot of good" for another area's stocks, Thorrold says.

Although impacts on policy may not be felt for years, researchers say the new findings are another sign of the otolith's growing value to scientists. European researchers, for instance, have recently launched a multimillion dollar effort to use otolith signatures to track cod and other economically important fish. Steven Campana, a biologist at the Bedford Institute of Oceanography in Dartmouth, Nova Scotia, says otoliths "give you some very precise information not available from other kinds of studies."

Related sites

Simon R. Thorrold

Bedford Institute of Oceanography Otolith Research Laboratory

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