CORVALLIS, Ore. – An isotope in the ears of fish will allow researchers to track how steelhead choose mates and where they prefer to spawn, researchers say.

Fish have a bony structure within their ears called an otolith, where calcium carbonate accumulates. By examining isotopes of the molecule, biologists can amass critical data about the age of fish, where they have lived and what they have eaten, said David L.G. Noakes, an Oregon State University professor of fisheries.

“These otoliths are like flight recorders,” Noakes said. “They’re swimming recorders in fish.”

The researchers will study steelhead that are spawning in the new artificial stream channels at the Oregon Hatchery Research Center in the Alsea River basin.

Noakes said that a high ratio of one kind of isotope indicates that a fish has been living in the ocean, while a lower ratio suggests a recent freshwater history.

The isotope signatures may eventually be refined enough to pinpoint how long fish have been in freshwater, at what depth in the ocean they swam, and what types of prey they ate while at sea all critical to gaining a better understanding for how to protect different species, he said.

“It’s even more precise on smaller fish,” Noakes said. “We should literally be able to examine a fish and say that it is 112 days old, it left the hatchery on day 37, it entered the Alsea River on this certain day, and stayed in freshwater for this period of time.”

He said the isotope research can be extended to predators or to scavengers that eat fish that die after spawning.