1. Larval fishes have a rich history in ichthyological research. However, relatively little attention has been given to the role of young-of-year (YOY) fishes in ecosystem processes. Despite difficulties of sampling and identifying these small fishes, YOY may serve an important role in the dynamics of some aquatic communities. The specific role of YOY fishes in aquatic, desert food webs remains largely uncharacterized. 2. We hypothesize that YOY fishes may control abundance and community composition of lower trophic levels (i.e., aquatic invertebrates) because they are numerically dominant predators during the growing season. The goal of this study is to determine the role of YOY fishes in aquatic food webs and the impact they have on trophic dynamics of dryland river food webs using the Rio Grande as an example. 3. A mesocosm experiment was designed to assess the effect of YOY fish predators on meio- and micro- faunal invertebrate prey density and diversity. We manipulated presence and absence of YOY fishes and leaf litter, and compared invertebrate diversity and abundance across an array of 24 stock tanks. 4. Data showed that YOY fishes significantly influence the composition of invertebrate communities, as does allochthonous carbon. Community composition varied among treatments both because fishes preferred some taxa over others and because of instances of avoidance by invertebrates. Stable isotope analyses (carbon and nitrogen) suggest important indirect effects on the behavior of invertebrates in fish-treated mesocosms. 5. Results suggest that YOY fishes play an important role in aquatic food web dynamics and that community composition of aquatic invertebrates is, in part, subject to direct and indirect changes induced by the presence of YOY fishes
