Competitive interactions and community influences among invasive and native fishes at different spatial scales

Bighead carp (Hypophthalmichthys nobilis) are invasive, filter-feeding planktivores that established in the Mississippi River Basin following their unintentional introduction in the early 1970s. Their subsequent expansion has generated much concern about their potential to compete with native fishes due to their ability to efficiently remove zooplankton from the water column. Despite the reliance of fishes on zooplankton at various life stages, few studies have tested for potential influences of bighead carp on native filter-feeding planktivores, and no studies have addressed interactions between bighead carp and facultative planktivores. The goal of my thesis was to test for competitive interactions and community influences between bighead carp and facultative planktivores at different spatial scales. I conducted three competition experiments involving bighead carp, bluegill (Lepomis macrochirus), and common carp (Cyprinus carpio). In the first experiment, I used a response surface design to independently vary the densities of bighead carp and bluegill in mesocosms. This design allowed for the investigation of inter- and intra-specific competitive interactions for both species, as well as the influences of the fishes on zooplankton, macroinvertebrates, and environmental variables. My results suggested that bluegill growth was density dependent, and bighead carp benefited from the presence of bluegill, which was likely due to shifts in nutrient cycling. To test whether the same influences occurred at a larger spatial scale, inter- and intra-specific competition was examined for bluegill with bighead carp in 0.4-hectare experimental ponds. With increased environmental complexity and niche opportunities, my results from the pond experiment differed from the mesocosm experiment, as bluegill were found to benefit from bighead carp presence. Plausible explanations for my results include increases in macroinvertebrate density or biomass via sediment enrichment from bighead carp excretions, changes in macroinvertebrate composition, and/or differences in bluegill foraging behavior. My results suggest that differences in species foraging behavior can modify communities in unexpected ways through changes in energetic pathways. Competition and facilitation between species is possible due to these modifications regardless of species origin. I also investigated competitive interactions between two invasive species, common carp and bighead carp, using a response surface design in mesocosms. My experiment suggested that intra-specific competition had a greater influence on both species, suggesting coexistence is likely. In all experiments, bighead carp had strong negative influence on zooplankton densities, which supports concerns that this invasive species has the potential to reduce an important food resource and modify aquatic communities