Effects of omnivorous crayfish on fish populations and the structure of littoral communities

Crayfish are the largest invertebrate predators in many freshwater systems, they are broadly omnivorous, and are capable of dominating secondary production and biomass. Crayfish are widely distributed in waterbodies throughout the globe and have been introduced to many ecosystems, otherwise beyond their dispersal capacities, for a variety of reasons. In this study, I explored the effects of crayfish on another large freshwater predator (sunfish) and on community structure of shallow water habitats.; The effects of crayfish on fish populations were explored with a literature review and two replicated experiments in ponds. Crayfish can have a number of relationships with fish, and interactions are not limited to the traditional studies of fish as predators and crayfish as prey. Other interactions have been less well studied, and one potentially important interaction is the effect of crayfish as predators of fish eggs.; To look for evidence of the effects of crayfish on reproductive success and nesting behavior I performed two experiments (1999 and 2001) with two species of common sportfish, the pumpkinseed sunfish (Lepomis gibbosus ) and bluegill sunfish (L. macrochirus). Both species of sunfish nest in shallow littoral habitats of lakes and ponds and nests are guarded by the adult males. In both of the experiments, crayfish infiltrated nests, ate the eggs and had negative impacts on reproductive success of the sunfish. In the experiment with bluegill sunfish, crayfish stopped reproduction until predator-free nesting habitats (exclosures) were added to the ponds later in the summer. After the exclosures were added, adult bluegill in each pond found the exclosures and were able to reproduce.; Crayfish had additional strong effects on overall community development (succession) in six ponds studied in 2001–2002. Through direct and indirect effects, crayfish had significant effects on the biomass of zooplankton and phytoplankton assemblages and peak levels of dissolved oxygen. Crayfish also had strong negative effects on macrophyte establishment, metaphyton abundance and composition, gastropod biomass, and the density of bullfrog ( Rana catesbiana) tadpoles. Based on these results and those of other studies of crayfish in lakes and ponds, systems with abundant crayfish are expected to be structurally simple systems with few macrophytes and gastropods where filamentous green algae and some species of amphibians and fish will perform poorly, but other organisms may benefit.; In my last experiment I explored the relative impacts of native and exotic Orconectes crayfish grazing on a common plant type. Using feeding trials in cages I determined that adults of the exotic crayfish, Orconectes rusticus and the native O. virilis had similar grazing rates on Chara macroalgae in two ponds with and without bass, as long as no direct interactions with the bass were possible. However, when direct interactions were possible, O. rusticus had higher feeding rates than O. virilis. This result is consistent with differences in overall predation vulnerability of these species, and suggests that stronger per biomass effects of exotic crayfish might be obviated in the presence of predators.