Complementary prey phenologies enhance invasional meltdown: a case study in the Columbia River Basin

Recent research shows that predators benefit from phenological diversity in their prey resources, which prolongs foraging opportunity. Here we apply this concept to invasive species, by exploring whether introduced prey with unique resource phenologies benefit an introduced predator. In the Columbia River Basin, expanding populations of invasive walleye (Sander vitreus) pose a growing threat to imperiled native fishes, including ESA-listed salmonids (Oncorhynchus Spp.) and Pacific lamprey (Entosphenus tridentatus). In addition to invasive piscivores such as walleye, invasive prey species such as American shad (Alosa sapidissima) and Siberian prawn (Palaemon modestus) have increased dramatically in recent decades. Indirect effects between invasive prey and native salmonids have not been robustly evaluated. Using year-round diet and stable isotope analysis, we quantified walleye trophic interactions with native and invasive prey in the lower Snake River, the largest tributary of the Columbia River. Walleye diets were dominated by Siberian prawn, American shad, salmonids, and lamprey with seasonal pulses in consumption driven by prey phenology. Stable isotopes revealed a cyclical shift in walleye tissue signatures, reflecting transitions among dominant prey over time. Both methods indicated that invasive prey species extended foraging opportunities and buffered seasonal gaps between native prey pulses, providing a novel example of the invasional meltdown hypothesis. These findings suggest that phenological complementarity among invasive and native prey may amplify predation pressure on native species and facilitate predator population growth. Our results underscore the importance of temporal dynamics and multi-species interactions in managing invasive predators and conserving native fish assemblages.