PNNL

Abstract

The negative consequences of hatcheries for native fish populations have been demonstrated across species and habitats. Despite this evidence, hatcheries are ubiquitous across North America and release billions of fish into the wild annually to augment harvest opportunities in sport, commercial, and tribal fisheries and to recover depressed populations. The Flathead River is considered a stronghold for Westslope Cutthroat Trout Oncorhynchus clarkii lewisi that have decreased by 90% across their historic range. The primary threat to Westslope Cutthroat Trout in the Flathead River is hybridization with invasive Rainbow Trout O. mykiss through subsequent genomic extinction. We collected Rainbow Trout in a section of Mill Creek, a tributary of the Flathead River, downstream from a hatchery that produces the species, and used otolith 87 Sr/ 86 Sr to estimate the relative abundance and entry timing of hatchery fish into the creek. We found that 78% of 106 Rainbow Trout we collected were of hatchery origin and most had spent limited time in Mill Creek. The heterogeneity in 87 Sr/ 86 Sr among the Mill Creek water, Mill Creek Scuplin Cottus bondi otoliths, and marine-derived fish food fed to hatchery Rainbow Trout provided clear discriminating power to classify unknown Mill Creek Rainbow Trout to hatchery or natural origin. Our results illustrate the utility of otolith 87 Sr/ 86 Sr to identify hatchery fish in natural habitats, and point to a source of invasive Rainbow Trout that has the potential to jeopardize Westslope Cutthroat Trout in a drainage where efforts are ongoing for conservation and restoration of native fish species.