Revealing early life history of brown trout in an impaired watershed through otolith geochemistry

Abstract

Early life history data are critical for conservation and management of trout species in freshwater river networks and are imperative for planning habitat restoration in degraded watersheds. In this study, we use otolith geochemical signatures to assign natal provenance and determine age of tributary outmigration for fluvial brown trout ( Salmo trutta) in the Upper Clark Fork River watershed, a large impaired river network. Distinct spatial variations in chemical markers allowed for accurate assignment of fish to their stream of origin and often to specific natal sites. All 10 natal tributaries investigated in this study contributed brown trout to the mainstem and most fish migrated from tributaries to the mainstem during age 1. Mainstem natal habitats, previously thought to be limited by pollution, contributed 29% of the total natal production. Our results indicate that trout in the Upper Clark Fork River use spawning and rearing habitats that are spread throughout the basin. Thus, restoration strategies should focus on maintaining and enhancing a broad variety of habitats and life histories to be successful.