Factors influencing predation on eggs and tadpoles of the endangered Green and Golden Bell Frog Litoria aurea by the introduced Plague Minnow Gambusia holbrooki

The Green and Golden Bell Frog Litoria aurea has declined dramatically in New South Wales. One hypothesis suggests that this decline is largely due to predation by the Plague Minnow Gambusia holbrooki, which was introduced into Australia as part of efforts to control mosquitoes. Other frog species, such as Striped Marsh Frog Limnodynasres peronii and Peron's Tree Frog Litoria peranii have not apparently suffered from this predation to the same degree. Laboratory experiments were undertaken in order to evaluate the extent t o which predation on eggs, fry and tadpoles of L aurea by Gambusia is influenced by the presence of rocks (as potential shelter), the presence of other frog species (as alternative food) and a number of variables that reflect the ratio of the number of prey available to the amount of food that would be required to satiate the fish. These experiments showed that the proportion of available L aurea eggs or tadpoles eaten increased with both decreases in the number of prey available and increases in the satiation requirements of the fish. Less expected, however, were the results that the extent of predation on L aurea did not depend on whether rocks were present, nor on whether other frog species were present. Similarly, predation on both the Lim. peronii and the L peronii was not influenced by whether these species were presented to the fish on their own or in the presence of one or other of the other two frog species. Gambusia was observed to employ different predatory strategies when attacking tadpoles of different sizes. Tadpoles were most often attacked from the rear, usually by a school of fish. Larger tadpoles were rarely eaten after being killed. Smaller tadpoles and fry were eaten mid-ventral region first, often only the viscera and yolk sac being consumed. L aurea eggs were torn open and sometimes the yolk was consumed. Preliminary experiments on the extent of predation on L aurea eggs, fry and tadpoles by Empire Gudgeon Hypseleotris compressa , Firetailed Gudgeon H. galii , Pacific Blue-eye Pseudomugil signifer and Red-fin Perch Perca fluviatilis were also carried out. These experiments suggest that these fish will, if sufficiently hungry, eat the eggs, fry and tadpoles of L aurea but that the extent of such predation may be generally less than with Gambusia under similar circumstances. Further research in this area is necessary before management implications are drawn. The experimental studies of predation by Gambusia demonstrate that, under some circumstances, it is a voracious predator of eggs, fry and tadpoles of L aurea . This is consistent with the increase in abundance and distribution of Gambusia being a major factor in bringing about the decline of L aurea and indicates that steps should be taken to limit further spread of this fish into areas where L aurea is found. However, the few observations of L aurea apparently surviving and breeding in water bodies occupied with Gambusia suggest that it may be possible to develop management strategies that reduce any impact of Gambusia on this frog species in places where the fish is already present.