Trout's ear bone reveals its life travels

Like a tree’s concentric growth rings, a small bone within a fish’s ear reveals a history of its growth. And according to a new study of westslope cutthroat trout in the Flathead River system, the bone also contains a record of its migration pathways – a kind of geochemical diary of its life. The bone, called an otolith, acquires a new ring every day of the fish’s life. All fish have them, and for decades scientists have counted the bands of the bone to determine the age of a fish, as well as estimate population growth. But a study published last week by a slate of Montana researchers in the Canadian Journal of Fisheries and Aquatic Sciences brings to light new information that the ear bones also contain a unique fingerprint of the water chemistry where the fish swims on a given day, which can be used to map the entire life history of a fish within a river network. When a fish leaves its natal stream and begins its life cycle, it drifts between geographically separate spawning and rearing beds, and embarks on the occasional foray to distant waters within a river system. During these sojourns, the fish absorbs the unique combination of isotopes and chemical elements contained within the water, and the otolith records them like a passport stamp. “It worked so well. The values in the water matched those in the otoliths, which grow like rings in a tree,” said Clint Muhlfeld, an aquatic ecologist at the U.S. Geological Survey's Glacier Park field office and lead author of the study. “As fish grow and move into new environments, the otoliths record that information and we matched that with stream statistics to reconstruct the entire life cycle of a fish.”...more