Direct and indirect effects of predatory young-of-year fishes in a dryland river food web

Young-of-year (YOY) fishes are sometimes numerically dominant vertebrate consumers in many large river systems, but their effects as predators are not as well understood as those of adult fishes. We predicted that YOY fishes influence community composition and abundance of invertebrate prey. Predation effects could be especially important in recruitment-driven dryland river systems, where YOY fishes seasonally comprise a large portion of overall fish biomass. We conducted a mesocosm experiment to quantify effects of YOY fishes on trophic dynamics and interactions with environmental factors in a dryland river food web. We manipulated presence of YOY fishes (an assemblage of cypriniform species) and supplemental allochthonous carbon (LEAF treatments) in 24 mesocosms, and measured invertebrate abundance and diversity over 6weeks. Experimental conditions mimicked a seasonal river drying regime that occurs during YOY fish growth. Seasonal drying in the Rio Grande frequently results in isolated pools with altered habitat complexity and riparian connectivity. Fishes exerted direct top-down control of invertebrate assemblage composition through differential prey selection, and indirect control mediated by non-lethal responses of invertebrates to predatory fishes. We observed enhanced water clarity in LEAF treatments associated with greater prey selectivity in fishes, presumably through enhanced visual feeding. Stable isotope analysis (C and N) was used to measure indirect impacts of YOY fishes on invertebrate prey items. YOY fishes caused a significant decrease in isotopic niche breadth among invertebrate taxa, suggesting that fish predators limit foraging opportunities for these consumers. However, allochthonous carbon sources were predominant in invertebrate diets in all LEAF treatments, suggesting consumer access to more biologically available autochthonous production is limited by factors other than fish predation. Although the roles of abiotic drivers and adult fishes in determining food web structure have received more attention, our results indicate that YOY fishes exert biologically relevant top-down control of invertebrate assemblage composition and trophic complexity in dryland river ecosystems against a backdrop of substantial changes in carbon availability and abiotic conditions (e.g. water quality and habitat volume) over the growing season.