Scientists know relatively little about the thousands of fish species — many of which are rare, endangered, or threatened — living in the Amazon, the world’s largest river system. Assistant Professor Leandro Castello and colleagues report on the use of chemical analysis of ear-stones or “otoliths” as a way to tease out a fish’s life story, potentially revealing the migratory routes and environments the fish encountered in its travels.

Made of calcium carbonate, otoliths grow as the fish grows, forming rings each year that can be read much the same way as a tree’s rings. Their growth incorporates traces of other elements that reflect the inherent chemistry of the water in which the fish lived. Through X-ray fluorescence and mass spectrometry analysis, scientists can extract the story of the fish’s growth and movements among different environments.

The research focused on the migrations of Amazonian fish species, some of which travel as many as 2,000 miles of the Amazon River and cross international boundaries during the course of their lives. The vastness of the Amazon system means most locations are remote enough to make tagging fish impractical and expensive for many species. The best management decisions require knowledge of a fish’s life history from hatching through its larval, juvenile, and adult stages in addition to its spawning behavior.

The study of the chemistry of fish ear-stones is part of an emerging body of knowledge that lays critical groundwork for the conservation and management of threatened species. The goal is to provide fisheries managers and conservationists with better information about how to protect fish from threats such as deforestation, mining, oil drilling, construction of hydroelectric dams, and overfishing.

Read the full press release.