Trophic Structure of Fish Communities in Grand Canyon National Park Tributary Streams

Nonnative rainbow trout (Oncorhynchus mykiss) and brown trout (Salmo trutta) are often implicated in the reduction of native fishes through resource competition and predation.  However, relatively little is known concerning food web dynamics between native and nonnative species, particularly in systems with relatively undisturbed natural flow regimes.  Colorado River tributary streams, protected within Grand Canyon National Park (GCNP) in the Southwestern US, provide unique opportunities to study trophic interactions between native and nonnative fishes in hydrologically unaltered systems.  Our objectives were to 1) determine trophic structure of fish communities, 2) determine food resource overlap among the fish species, and 3) characterize feeding strategies of nonnative fishes in GCNP tributary streams.  We sampled all fishes in September 2010 in Shinumo Creek using stable isotopes ¹⁵N and ¹³C to infer trophic structure and food resource overlap between native and nonnative fishes.  Also, we analyzed rainbow trout gut contents to obtain short term trends in diet, and determine feeding strategy.  Mean δ¹⁵N and δ¹³C values were similar for native bluehead sucker (Catostomus discobolus; δ¹⁵N = 6.200‰ and δ¹³C = -21.082‰) and speckled dace (Rhinichthys osculus; δ¹⁵N = 6.697‰ and δ¹³C = -21.164‰).  Translocated humpback chub (Gila cypha) and introduced rainbow trout held similar trophic positions (δ¹⁵N = 9.127‰ and 8.941‰ respectively), but differed in carbon source (δ¹³C = -21.569‰ and -23.051‰ respectively).  The dominant prey items for small (< 100 mm TL) rainbow trout were Ephemeroptera, Lepidoptera, Trichoptera, and Diptera larvae, while Trichoptera and Megaloptera dominated the stomach contents of the largest 25% (> 178 mm TL) of individuals in the rainbow trout population.  Piscivory by rainbow trout was < 4% and only occurred in fish > 178 mm TL.  Our results suggest the predation rate by nonnative rainbow trout on native fishes is low, but competition for food resources may exist between native and nonnative fish in GCNP tributaries such as Shinumo Creek.