Competition between invasive mosquitofish and native bluegill sunfish

Background/Question/Methods

Invasive species are one of the biggest challenges confronting native biodiversity, and in many regions of the world the introduction of mosqutiofish (Gambusia spp.) is one of the greatest threats to native aquatic biodiversity.  In particular, throughout their introduced range, mosquitofish have been suspected or shown to have caused the decline or extirpation of native fishes, either through competition or predation.  Here we examine the potential interactions between western mosquitofish (G. affinis) and native bluegill sunfish (Lepomis macrochirus) using a mesocosm experiment.  We created mesocosms containing juvenile bluegill only, mosquitofish (males and females) only, and juvenile bluegill and mosquitofish (interspecific) at two total fish densities.  Thus, we could examine the effects of species composition and total fish density on each species.  At the end of the experiment, we assessed survivorship/recruitment, body condition index (mass/total length), and relative total mass change (final total fish mass/initial total fish mass) for each species.  Because both species consume zooplankton, we expected each species to have a negative effect on the other species, thus showing competition.

Results/Conclusions

Survivorship of the juvenile bluegill was not affected by the presence of mosquitofish, or by total fish density.  Mean bluegill body condition index and relative total mass change were highest in the low density, interspecific treatment; there were not many differences among the other three treatments.  For mosquitofish, survivorship tended to be high in all treatments; however, recruitment of young was only successful in the low density, mosquitofish only mesocosms.  The body condition index of male mosqutiofish was lowest in the low density, interspecific mesocosms; whereas the body condition index of female mosquitofish was not affected by any of the treatments.  Relative total mass change of the mosquitofish was greatest in the low density mesocosms and was not affected by the species composition of the mesocosm.  It thus appears that mosquitofish had relatively little, if any, negative effects on juvenile bluegill; rather, juvenile bluegill actually tended to do better in the presence of mosquitofish.  In contrast, juvenile bluegill appear to be able to limit mosquitofish recruitment, as does higher densities of mosquitofish, probably through consumption of mosquitofish offspring.