Bioenergetic responses of a stream food web to habitat restoration: interactions between Brown Trout and invertebrate prey resources

Stream habitat restoration has the potential to rehabilitate degraded stream communities, but the bioenergetic impact of restoration on prey resources and utilization by salmonids has been understudied. We measured the responses of Brown trout (Salmo trutta) populations, aquatic and terrestrial prey resources, and prey resource utilization by trout, before (2008-2014) and after (2015-2020) large-scale habitat restoration in the Upper Arkansas River, a previously listed US EPA superfund site located in the western United States. We observed significant increases in Brown trout populations following restoration, as well as complex alterations to invertebrate prey resources that generally resulted in increased benthic biomass but reductions in adult aquatic insects and terrestrial invertebrates. Population-level metabolic demand by trout increased 25% after restoration, and prey utilization shifted with reduced consumption of adult aquatic insects and greater consumption of terrestrial invertebrates, relative to their availability. We observed reduced biomass of prey resources consumed by trout with increasing trout population size, and while the total number of invertebrates consumed increased after restoration, actual invertebrate biomass consumed decreased. Our results suggest that restoration was effective in increasing targeted Brown trout populations, but prey resources were altered by physical changes in habitat and increased resource utilization by trout. Habitat restoration projects that are primarily designed to increase trout populations may benefit by focusing on improving prey resources and utilization to support the bioenergetic needs of the restored fishery.