The ones that got away: resource overlap and competition between a non-native fish (Oreochromis nioticus) and Thai native species, and implications for ecological risk assessment.

Abstract

Effects of non-native fish in tropical freshwater systems are commonly presumed but poorly understood. I present an approach to assess potential adverse changes in the native fish assemblage resulting from the presence of feral Oreochromis niloticus, which has an expansive naturalized range. This research is organized around an ecological risk model, consisting of a fault-tree of events. I present my findings for exposure events (site, habitat and dietary overlap) and effects events (interference competition and change in growth rates), which may lead to adverse changes in native assemblages in streams in the Eastern River system in Thailand. My research addresses two assumptions regarding the ecological risk assessment of O. niloticus in Thailand, that native piscivores, Channa spp., limit population establishment and hence ecological risk, and an economically important native species, Trichogaster trichopterus, is declining due to competition with O. niloticus. I found that Channa spp., T. trichopterus, and O. niloticus were syntopic and had high dietary overlap. When present, O. niloticus were more highly abundant than other species; which suggests their successful establishment. Native fish assemblages at O. niloticus sites were not significantly different from those at un-invaded sites, and non-native species were the only strong indicators of sites with non-natives present. Therefore, Channa spp. did not limit establishment of O. niloticus, and T. trichopterus could be negatively affected via competition with juvenile O. niloticus. I then tested whether O. niloticus adversely affect T. trichopterus through competition in laboratory experiments to determine the mechanism and outcome of competition between individuals and species in single and mixed species treatments at varying densities. Dominant fish in each tank grew more than subordinates, and O. niloticus were the dominant species in 67.7% of all mixed species tanks. The growth rate of T. trichopterus declined significantly in the presence of selectively bred O. niloticus, but the growth rate of O. niloticus was not noticeably affected by the presence of T. trichopterus. My study documents resource overlap, resource competition, and the effects of competition on growth. As such, it provides valuable insight into the ecology and conservation of native aquatic biodiversity in tropical freshwater systems.