A test for competition between two Gobiomorphus species (Teleostei: Eleotridae) in artificial channels

Abstract– The occurrence of competition between common ( Gobiomorphus cotidianus ) and upland ( G. breviceps ) bullies was tested in summer in artificial channels. Although these species have different life histories, they are morphologically and ecologically similar, and frequently cooccur in New Zealand's fresh waters. Mortality during the test period averaged 11.3±2.4% and 17.7±1.9% for the common bully in triplicate control and experimental channels, respectively; in contrast, upland bully mortality was 2.3±1.9% in the control and zero in the experimental channels. Common bullies showed no significant differences in mean length, weight, condition, and specific growth between the control and experimental tests. For upland bullies, all of the above parameters were significantly less in the controls than the experimentals, an outcome that was indicative of intraspecific competition. Moreover, there was a significant linear trend in the distribution of upland bullies, with their numbers being highest at the upstream end of the channels in both the control and experimental tests. Common bullies showed no such trend, although significantly higher proportions of fish were present towards the central positions in the experimental channels, suggesting there was some displacement of them by upland bullies. It is concluded that interspecific competition is not of major importance between populations of these two eleotrids in small streams, although the abundance and upstream penetration of common bullies may be affected to some extent by interaction with upland bullies.