Determining resource intake of a nonnative fish highlights potential predatory and competitive interactions

Nonnative species are often perceived to cause the decline or impede management and recovery of native species, yet the ability to quantify the ecological impacts of nonnative species is often difficult. Disentangling the consequences of other stressors (e.g., habitat loss, climate change) and nonnative invasions on native fish communities might be accomplished by a thorough understanding of spatial and temporal variation in nonnative fish behavior and physiological rates of prey consumption mediated by environmental conditions. We examined the potential predatory and competitive threat of nonnative channel catfish ( Ictalurus punctatus ) on a native fish community in the San Juan River, New Mexico, Colorado, and Utah by examining how diet composition and prey intake varied across diel, seasonal, and annual temporal scales over a broad spatial extent (152 km). Prey intake rate and diet composition did not differ across diel temporal scales, but shifted across seasonal and annual scales as well as longitudinally. Estimated consumption of prey by the entire channel catfish population varied between 9.3 dry kg/ha/yr in 2019 and 16.5 dry kg/ha/yr in 2018 with consumption dominated by nonnative terrestrial plants (57.9%; mainly nonnative Russian olive seeds), aquatic invertebrates (13.4%), and aquatic plants (9.0%). The probability of prey in the diet of channel catfish was largely predicted by factors influencing their availability in the system, highlighting the opportunistic feeding strategy of channel catfish. Native fish were consistent in the diet spatially but increased with higher water turbidity and in larger catfish, and despite their relatively low proportion of total prey consumed (2.6%) the impact of this number depends on population sizes of the nonnative predator and native prey. Opportunistic feeding across a wide range of prey items likely allows catfish to sustain high abundances in this system, increasing the potential for negative interaction with native fishes. However, this foraging strategy is dominated by nonnative Russian olive seeds, which may also reduce predatory and competitive interactions with native fishes.