Zooplankton are a valuable food source for both native and exotic freshwater fish species and play a key role in the survival of species during larval and juvenile phases. Predatory fish can alter the abundance of different taxonomic groups and change community composition. This study describes the impact of Murray cod (Maccullochella peelii peelii) larvae and Gambusia (Gambusia holbrooki) juveniles on the composition of emerging zooplankton communities. Using replicated mesocosms we quantified the impact of Murray cod and Gambusia larvae at three different densities, high (100 zooplankton per fish), medium (250 zooplankton per fish) and low (400 zooplankton per fish).
Cladocerans were consumed by both Murray Cod and Gambusia. Gambusia addition significantly reduced the abundance of all microinvertebrate taxa. Murray cod significantly reduced densities of cladocerans but not other taxa. In the Murray cod treatments, rotifers began to increase in abundance by the end of the experiment, but this effect was limited to the low density treatment. All densities of Gambusia caused decreases in zooplankton abundance and this effect continued for 21 days whereas the higher densities of Murray cod larvae led to an initial decrease in zooplankton abundance but this was not sustained over time.
Our findings show that high densities of Gambusia affect both the density and composition of zooplankton communities, potentially disrupting food webs and reducing food availability for native fish.