Fish Ear Bones Hold Clues to Migration

Like a tree's concentric growth rings, a small bone within a fish's ear also contains a record of its growth.

The bone, called an otolith, acquires a growth ring every day for at least the first six months of the fish's life.

Each ring contains a fingerprint of the water chemistry wherever the fish swam on a given day. Now scientists hope that these rings may yield a fish's diary, from spawning through early development.

"The daily rings in the otoliths of juvenile bluefish, for example, contain a geochemical signal that could be used to map their migration pathways," says Simon Thorrold, a fish ecologist at the Woods Hole Oceanographic Institution, in Massachusetts.

"Some fish may be making migrations as amazing as the monarch butterfly, but currently we have no way of knowing," says Thorrold, who pioneered the use of geochemical analysis of otoliths to determine marine fishes' spawning grounds.

"Thorrold's work is groundbreaking," says Robert Warner, a marine ecologist at University of California, at Santa Barbara. "This is one of the first times that the otolith has been used as a flight recorder."

The messages within a fish's ear bone diary have implications beyond the laboratory—in settling sometimes hotly contested international fisheries disputes and in establishing marine protected areas.

Natural Tags

A challenge for fisheries science is how to track fishes' migration routes and determine fish stocks. Tag-and-recapture methods are too labor intensive for large-scale surveys.

The otolith, however, is a natural tag—a protein skeleton that accumulates calcium carbonate and trace metals from the surrounding water and therefore reflects the fish's environment.

Otoliths range in size from one-tenth of an inch to one inch long and are found in the heads of all fishes except sharks, lampreys and rays.

Like the inner ear in humans, the otolith plays a role in hearing and balance. "It helps the fish know which way is up," says Steven Campana, a fisheries scientist and head of the otolith research lab at the Bedford Institute of Oceanography in Dartmouth, Nova Scotia.
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.

National Geographic Today(December 9, 2002)-Bijal P. Trivedi