A manipulative field experiment reveals the ecological effects of invasive mosquitofish (Gambusia affinis) in a tropical wetland

The mosquitofish (Gambusia affinis: Poeciliidae) is one of the world's most widespread invaders, but our ability to predict the consequences for native species in the tropics is limited by a paucity of research and a lack of knowledge of how environmental factors influence mosquitofish impacts. We undertook a field experiment using cages to manipulate mosquitofish densities in a Hong Kong wetland during the warm wet season and the cool dry season. We measured fish effects on invertebrates and periphyton, and tested whether the results were affected by vegetation type (sedge vs. water lily). Mosquitofish reduced abundance and richness of invertebrates, and altered assemblage composition, but had no effect on periphyton biomass. Effects on invertebrate abundance were consistent between seasons, but mosquitofish reduced invertebrate richness only during the wet season, when effects on assemblage composition were more obvious. Vegetation type had no influence on the experimental results. This study adds to the few tropical, or Asian, field studies of the ecological impacts of mosquitofish, and is, as far as we are aware, the only such study measuring their seasonal variability. While we detected some cage effects during our field experiment, they were insufficient to obscure the strong influence of mosquitofish on invertebrate abundance and composition.