Gambusia holbrooki is a small, aggressive fish introduced into Australia in 1925 to control mosquitoes. It has been suggested that this species may also prey on tadpoles and be a contributing factor in the decline of some Australian frog populations. We examined the influence of tadpole body size (10 mm. 15 mm and 20 mm). predator prey ratio (1 : 1, 1 :2 and 1 :4) and nutritional status of the predator on the level of predation by G. holbrooki on tadpoles of Limnodynasfas peronii. Unfed fish attacked all three size classes of tadpoles without any significant preference but did so more vigorously when the predator/prey ratio was densest. Predator/prey ratio had no significant overall effect on attack of tadpoles by fed fish but there was a preference for the smaller tadpoles during the first six hours of the experiment. Unfed fish also consumed more of the tadpoles they attacked than did the fed fish.
Field surveys were carried out on 10 permanent water bodies in north-west Sydney to examine any correlation between the abundance of G. holbrooki and frog species richness or abundance. The most abundant species was C. signifera. Regression analysis showed a negative relationship between density of G. holbrooki and the abundance of frogs. A positive relationship between the abundance of tree frog species and the cover of aquatic vegetation was also found. While this study did not investigate other factors contributing to declining frog populations, the results of the laboratory experiments and field surveys are consistent with the hypothesis that G. hoibrooki is contributing to declining frog populations.