National Geographic Today
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 laboratoryin settling sometimes hotly contested international fisheries disputes and in establishing marine protected areas.
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 taga 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.
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