Common bully (Gobiomorphus cotidianus) and mosquitofish (Gambusia affinis) shift littoral zooplankton community composition to smaller bodied taxa

We investigated the effects of native common bully (Gobiomorphus cotidianus) and non-native western mosquitofish (Gambusia affinis) on zooplankton assemblages in an outdoor mesocosm experiment. Twenty-one 100 L mesocosms containing zooplankton were assigned either four G. cotidianus, four G. affinis, or were left as a control group (i.e. with no fish) and monitored for 14 days. Zooplankton community composition in the fish treatments was significantly different from the control tanks by day 14, but did not differ from one another. Both G. cotidianus and G. affinis reduced crustacean abundances, including Bosmina meridionalis, Daphnia pulex and Mesocyclops australiensis, relative to the controls. This reduced the effects of predation, competition, and physical interference by the crustaceans on rotifers, allowing rotifers to reach numerical dominance in the fish treatments. In contrast, zooplankton assemblages in the control mesocosms became dominated by cladocerans, indicating that these had a competitive advantage in the absence of the zooplanktivorous fish species. Our results suggest that G. cotidianus and G. affinis have similar preferences for zooplankton taxa and that G. affinis can alter littoral zooplankton assemblages in a comparable manner to G. cotidianus.