Diet of the Exotic Mosquitofish, Gambusia holbrooki, in an Australian Lake and Potential for Competition with Indigenous Fish Species

Abstract

The exotic mosquitofish, Gambusia holbmoki Girard, has become established in fresh­water systems on sand islands of high conservation significance off the coast of southeast­ern Queensland, Australia. These freshwater systems support indigenous fish species with very restricted distributions and the mosquitofish has been listed as a threat to at least three rare species. This paper records the diet of G. holbrooki in an oligotrophic dune lake and explores the potential for competition for food between the mosquitofish, the Ornate Rainbowfish, Rhadinocenlnts ornatus (Melanotaeniidae), and the Firetail Gudgeon, H.ypseleotris galii (Eleotrididae). There was a high degree of similarity in mean population diets of R. ornatus and G. holbrooki (overlap index=0.83), moderate dietary overlap be­tween G. holbrooki and H. galii (overlap index=0.49) and low dietary overlap between R. omalu.s and H. galii (overlap index=0.18). In some months, the mosquitofish switched to feeding on aquatic invertebrates normally consumed by gudgeons, and levels of dietary overlap between these species increased markedly. The capacity of G. holbrooki to feed opportunistically on a wide variety of aquatic prey, confirmed again in this study, could ex­ert a significant pressure on small populations of both indigenous fishes. Of the two spe­cies studied here, the restricted Ornate Rainbowfish appears to be particularly vulnerable to the effects of dietary interactions with the mosquitofish.