Road map. The otolith's chemical composition reflects a fish's journey. (Blue-purple represents time spent in fresh water, green reflects brackish water, and orange-to-red colors mean the fish was in seawater.)

Listening to Fish Ears

SNOWBIRD, UTAH--Ask most biologists and you'll hear that salmon, trout, and many fish species make one round trip between fresh water and the ocean over their lifetime. But by looking in, of all places, their ear bones, scientists have learned that the European brown trout doesn't stick to such a simple itinerary: The fish seem to do whatever they please.

European brown trout start their life ranging through the rivers of Europe and Asia. Ecologists believed the fish first spent a year in fresh water, then moved to the ocean to mature and finally came home to reproduce. To test that scenario's accuracy, fisheries ecologist Karin Limburg of the State University of New York, Syracuse, studied the animals' otolith, a little bone under the brain that is part of its hearing and balance system. Because the otolith keeps growing throughout the animal's life, its chemical composition mirrors a fish's journey: Parts grown at sea have a higher strontium to calcium ratio than parts grown while the fish was in fresh water.

When they examined otoliths from adult brown trout caught near the Swedish island of Gotland in the Baltic Sea, Limburg and her colleagues found that most trout don't lead textbook lives. Some spend all their lives in fresh water, while others shoot out to sea immediately, only to come back later; yet others seem never to go back at all, Limburg reported on 9 August at the Ecological Society of America's annual meeting. In other work, Limburg found that eels and blueback herring in the Hudson River, too, have all sorts of travel patterns. "The otoliths are telling us that they're doing these things--but we don't know why yet," Limburg says.

Knowing precisely where fish travel over their lifetime is important for conservation, she says. It's certainly not a simple picture anymore. "The diversity of movements is more complex than anybody thought," comments John Musick, a vertebrate marine biologist from the Virginia Institute of Marine Science in Gloucester Point.

Related sites
Karin Limburg's home page. Has a link to her fisheries work and a link to a discussion of "otolithology."
Fishbase, a global information system on fishes