Bluegill sunfish (Lepomis macrochirus) are common freshwater fishes in North America, but in some locations they encounter the non-native western mosquitofish (Gambusia affinis). Mosquitofish are highly aggressive and negatively impact native fishes. We explored interactions between these fish to better understand how mosquitofish affect communities in small temperate ponds. One mechanism by which mosquitofish may influence native fish is through competition, thus we considered potential interactions between bluegill and mosquitofish and their impacts on native zooplankton. Because both species exploit similar food resources and are known to have significant effects on zooplankton communities, we predicted a shift in zooplankton communities in the presence of each fish, characterized by declining large zooplankton abundance and increasing small zooplankton abundance. We conducted a mesocosm experiment in which we manipulated the presence or absence of three variables: bluegill, mosquitofish, and vegetation. Our study used juvenile bluegill, a life stage that most closely matches mosquitofish in size range and habitat overlap. The vegetation treatment provided an alternative habitat for the fish, possibly serving as a refuge for juvenile bluegill from mosquitofish agression. We predicted that juvenile bluegill might experience reduced growth in the presence of mosquitofish, but that vegetation would mitigate this effect.
We found no significant effect of mosquitofish on the growth or survivorship of juvenile bluegill. Unexpectedly, we observed that the presence of bluegill significantly decreased mosquitofish growth and resulted in a reduction in survivorship. The presence or absence of vegetation in the mesocosm did not influence the growth or survival of either fish. The zooplankton community in our mesocosms included large-bodied taxa such as Scaphaloberis, Daphnia, Ceriodaphnia, and Simocephalus, and small-bodied taxa such as Bosmina and Chydorous, and we observed that fish presence influenced the composition of the community. We found that both bluegill and mosquitofish significantly affected the abundance of large-bodied zooplankton over time, causing them to decline, while the presence of vegetation did not affect the abundance of large-bodied zooplankton. Additionally, the abundance of small-bodied zooplankton declined in treatments with mosquitofish, but small-bodied zooplankton density was not affected by bluegill or vegetation. These results suggest that non-native mosquitofish are negatively impacted by the native bluegill and that this effect is mediated through the zooplankton. Thus, it is plausible that competition between the two fish may be more detrimental for mosquitofish than for bluegill, thereby permitting the native bluegill to maintain populations despite introductions of non-native mosquitofish.