Gambusia holbrooki, the “Mosquito Fish” introduced to Australian waterways from North America to control mosquito larvae, is widely distributed and abundant in both disturbed and undisturbed aquatic environments. Because it has become established in areas where Litoria aurea was once abundant, there is some suspicion that G. holbrooki's non-specific predatory behaviour may be instrumental in the decline of L. aurea.
Laboratory and field experiments were conducted to assess the impact of G. holbrooki predation on L. aurea tadpole survivorship. Laboratory trials involved placing 30 L. aurea tadpoles per 46-litre aquarium in each of the following four treatments: control treatments (two aquaria with just tadpoles and two aquaria with tadpoles and the pondweed, Egeria densa) and two predator treatments (two aquaria with fish and tadpoles and two aquaria with tadpoles, fish and pondweed). A daily census of the tadpoles was taken over four days. Based on ANOVA of six aquaria for each treatment, G. holbrooki significantly reduced L. aurea survivorship, but pondweed had no significant effect until the final census. The same experimental procedures were used for L. dentata. Again, tadpole number was significantly lower in aquaria housing G. holbrooki than in those without these fish, and pondweed influenced L. dentata's survival only on the final 72-hour census.
Field experiments were conducted by placing 30 L. aurea tadpoles in 1.5 x 0.6 x 0.6 m screened cages which permitted water-flow but excluded predatory fish. A pair of cages (one with six G. holbrooki and one without) were placed in three separate ponds on two occasions. A daily tadpole census taken over four days revealed that tadpoles exposed to G.holbrooki had significantly higher daily mortality than tadpoles which were caged alone.
These results show conclusively that G. holbrooki is a predator of L. aurea and L. dentata tadpoles. Future management decisions regarding L. aurea populations should include a consideration of G. holbrooki's impact as well as methods for their removal from breeding sites.