Unlocking the Secret Lives of Fish Using Nitrogen Isotopic Analysis of Otolith-Bound Organic Matter

Stomach content analysis and stable isotope analysis in fish tissues have served as the primary tools in trophic ecology for decades but provide only a limited temporal window into the dietary history of fish. In contrast, otoliths accrue over the entire life history of the fish and provide a time capsule of trophic information that integrates over space and time. Until recently, analytical constraints have limited isotopic analysis of otolith-bound organic matter due to the low organic content of these aragonite structures. We adapt a previously established method for measuring the nitrogen isotopic composition (δ¹⁵N) of organic matter in fossilized foraminifera and corals to a novel application in fish otoliths. Here, using paired otolith and tissue samples from both hatchery-reared and wild fish, we provide validation of the otolith method. We find that otolith-bound δ¹⁵N records the dietary origin of otolith δ¹⁵N albeit with a species-specific trophic fractionation factor, and that it generally correlates with known trophic level information from a variety of species. The application of this new otolith-bound proxy for trophic level holds promise for tracking life history variability, natal origins in migratory fishes, and reconstructing changes to ecosystem structure on contemporary and prehistoric timescales.