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.
University of Minnesota Ph.D. dissertation. March 2009. Major: Conservation Biology. Advisor: Professor Anne R. Kapuscinski. 1 computer file (PDF); viii, 132 pages, appendices 1-3.
Cooper, Anne M..
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..
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