Southern Cross University, LISMORE, AUSTRALIA
It is well accepted that the presence of exotic ichthyofauna is a key threatening process for native fish assemblages. One of the identified threatening processes is interference competition and aggressive contact between native and endemic species. At present the exotic species most likely to affect native fish through interference competition is the introduced poeciliid Gambusia holbrooki. Aggressive interaction between gambusia and native fishes are not uncommon, and are well reported in the literature. However, these aggressive responses have rarely been quantified. We aimed to quantify the number of attacks mediated by gambusia on several native fishes over a set period and determine if aggressive behaviour between gambusia and native fish is density dependent. To test this hypothesis we used five ratios of gambusia to native fish (1:4, 2:3, 1:1, 3:2 and 4:1). The native species used were Pacific blue eyes Pseudomugil signifer, ornate rainbowfish Rhadinocentrus ornatus, and Duboulay's rainbowfish, Melanotaenia duboulayi. All experiments were conducted in 17 litre aquariums with standardised environmental parameters. The total number of fish per tank was maintained at twenty. All fish were introduced to the tanks twenty minutes before trials commenced and were kept separate via a non-transparent gate. Number of attacks per ten-minute period were recorded and each ratio was replicated several times. Attacks by gambusia were recorded at all comparative densities for all species. The results for all species followed the same pattern. Over 300 attacks in a ten-minute period were recorded in several of the trials. Interestingly, attacks increased as the number of gambusia decreased. The highest number of attacks was recorded when there was only four gambusia present. These data suggest, at least under captive conditions, that interference competition is density dependent between gambusia and the three native fishes tested.