Unraveling multipredator impacts in salmon-bearing rivers using quantitative DNA metabarcoding.

Ecological impacts of invasive species are mounting as their numbers and geographic extent continue to increase. Across extensive parts of their range, Pacific salmon (Oncorhynchus spp.) smolts face an expanding gauntlet of nonnative predators during their seaward migration. Adopting multispecies, spatiotemporal perspectives is essential for understanding direct and indirect predation risks and prioritizing management actions seeking to reduce impacts. Using quantitative DNA metabarcoding, we investigated trophic interactions of commonly co-occurring nonnative and native fish predators of Pacific Northwest, USA, salmon-bearing rivers, addressing challenges for salmon recovery and questions related to single-species management. Chinook salmon (Oncorhynchus tshawytscha) were frequently consumed by nonnative smallmouth bass (Micropterus dolomieu), largemouth bass (Micropterus salmoides), rock bass (Ambloplites rupestris), and native northern pikeminnow (Ptychocheilus oregonensis). Among the focal predators, Chinook salmon were the largest contributors to smallmouth bass diets, ranking as their second most important prey. Chinook salmon consumption peaked during a year of relatively high smolt abundance, low discharge, and warm stream temperatures. The following year, under opposite conditions, Chinook salmon consumption declined, though predation remained disproportionately high in certain mainstem and tributary regions. Native species of conservation concern were frequently consumed by nonnative predators, including imperiled native lamprey (family Petromyzontidae). Across space and time, native prickly sculpin (Cottus asper) and largescale sucker (Catostomus macrocheilus) were generally the highest contributing prey for nonnative predators. Intraguild predation was prevalent, most notably with smallmouth bass as the top prey for northern pikeminnow. Intraguild predation highlights potential risks of compensatory effects when predators are managed in isolation. Our study provides crucial insights into restoring food webs for native species while minimizing the likelihood of compensatory effects and demonstrates the value of quantitative DNA metabarcoding for understanding novel predator assemblages. As ecosystems worldwide face increasing pressures from co-occurring invasive species, integrating multispecies approaches into management strategies is essential for mitigating impacts and conserving biodiversity.