Wait until dark: influence of turbidity on the foraging ecology of trout in the Colorado River, Grand Canyon

Authors: Michael D. Yard, Lewis G. Coggins, Colden V. Baxter

Year: 2009 (xli)

Abstract

Nonnative fish interactions (competition and predation) are considered one of the major factors responsible for the decline of native fishes of the Colorado River. Glen Canyon Adaptive Management Program implemented a large multi-year experiment in Grand Canyon, designed to suppress nonnative fishes, particularly rainbow trout (Onchorhynchus mykiss) and brown trout (Salmo trutta) near the confluence of the Little Colorado River (LCR). The LCR confluence is an area of strong overlap between nonnative trout and native fishes, especially the humpback chub (Gila cypha, endangered species). Thus, the overlap among fish communities at this location and the application of a large-scale removal effort provided the context for determining trout foraging ecology, including piscivory on native fishes. We also evaluated how turbidity affects prey availability, detection, and utilization, including the degree of piscivory of these nonnative salmonids, by examining incidence of predation upstream and downstream of the LCR, a major source of turbidity. Over 20,000 nonnative fishes, of which 98% were salmonids, were removed from a 14 km segment of the Colorado River surrounding the confluence of the Little Colorado River (LCR). Rainbow trout diets were comprised of three general prey types: aquatic invertebrates (85.6%), terrestrial invertebrates (12.6%), and vertebrates (1.8%, aquatic and terrestrial origin). Although brown trout diet also consisted of the same prey types, the proportional use of aquatic invertebrates (73.0%), terrestrial invertebrates (8.7%), and vertebrates (18.3%) was significantly different from rainbow trout. Incidence of piscivory for brown trout was higher (8-46%) than for rainbow trout (0.5-3%); nonetheless, rainbow trout exert a larger cumulative predation effect than other less abundant but more piscivorous species. In modeling daily piscivory estimates, rainbow and brown trout cumulatively consumed nearly 30,000 fish during the first two years of the suppression study (699 days) (rainbow trout 16,157; and brown trout 13,205). On average trout consumed 85% more native than nonnative fish even though native fish were 30% less abundant. Turbidity may mediate piscivory directly by reducing prey detection, yet its influence is uncertain as our results suggest since rainbow and brown trout showed greater piscivory in reaches with higher turbidity. This may suggest a shift in foraging strategy under turbid conditions from drift-feeding to active foraging, and also in response to increases in prey availability due to dispersal of young fish from the LCR.