Depauperate freshwater fish communities in Sabah: the role of barriers to movement and habitat quality

Up to 30 species of fish were generally found in medium-sized rainforest st reams in Sabah, Malaysia. However, depauperate fish communities were found in two streams above large (>8 m) waterfalls (five and nine species) and in two very small streams located in deep forest (five and six species). A fi fth stream with small cascades and waterfalls had a faunal fist of 12 speci es. Species resident above waterfalls were predominantly herbivores while f ishes in the two forest streams were from a variety of trophic groups. Abun dance and biomass of fishes above waterfalls were significantly lower than all other sites. A translocation experiment was performed in one stream to distinguish between the hypotheses that fish communities above waterfalls w ere determined solely by colonization ability and were unsaturated with spe cies or that they were limited by habitat quality or food. Seven species (7 75 individuals) were translocated into one isolated section and four specie s (570 individuals) into another. Species that were translocated included t rophic groups that were not represented by resident species. Twelve months after translocation, only four and one translocated species were collected in the two sections, all at greatly reduced densities. After 20 months, the number of species were three and two, respectively. One translocated speci es, Rasbora sumatrana, had increased in abundance from 12 months and juveni les were present in the population. Individuals of other translocated speci es appeared to be remnants of stocked populations. Abundance and biomass of resident species fluctuated widely between years. Interpretation of result s was complicated by a large flood which substantially changed habitat cond itions about a month after translocations were performed. It is suggested t hat two different factors were responsible for depauperate communities: mov ement barriers for waterfall sites and physico-chemical conditions and/or h abitat availability at other sites. (C) 1998 The Fisheries Society of the B ritish Isles.