A framework for using tissue chemistry and biochronologies to reconstruct fish habitat use and growth

Understanding fish habitat use, migration patterns and growth are fundamental for effective fisheries management and conservation, yet reconstructing individual life histories in variable environments remains challenging. Recent advances in tissue chemical tracers have provided novel tools to retrospectively ascertain habitat use; however, these approaches are often deemed inaccessible to individuals without chemistry backgrounds or applied inconsistently between studies, often without clear rationale. Here, we present a framework for using chemical tracers from archival tissues to reconstruct the lifetime habitat use and growth histories of fish. We demonstrate the utility of the framework by applying it to brown trout (Salmo trutta) in a Norwegian system of interconnected lake, river and marine habitats. We describe the extent to which adult trout used two freshwater lakes as juveniles and explore variation in outmigration timing and growth rates between anadromous and resident forms. We illustrate that chemical tracers can provide novel and valuable information when applied correctly. Practical implication. This framework offers a clear, transferable tool for using chemical tracers to reconstruct habitat use and growth across diverse fish species. This is of use to both researchers and managers to deliver key insights into lifetime habitat use and growth in both freshwater and marine environments.