Distributional ecology of freshwater fishes in tropical rainforest streams of Borneo

Abstract

A survey was conducted to study the distribution of freshwater fishes in the middle and headwater streams of the Belalong-Temburong watershed, Brunei Darussalam, northern Borneo. The watershed is covered by a mixed dipterocarp primary rainforest. The number of species found at any one locality ranged from zero to twenty five; the number of localities occupied by any one species ranged from one to twenty six. Distribution and abundance of fish were not related to water quality, which was good at all the sampled sites. Drainage area, elevation, gradient and stream order were all significantly correlated with the number of species collected at a site. Species richness was highest in large watersheds, at low elevation, low gradient and in streams of high order. In most streams, species addition occurred with increasing drainage area, increasing stream order, decreasing altitude and decreasing gradient, but species replacement was frequent as well, particularly when waterfalls were encountered. Species belonging to the family Balitoridae dominated channels above waterfalls, whereas those belonging to the family Cyprinidae were predominant in streams below waterfalls. In deep channels, different fish species occupied different zones of the vertical stratum. Shallowing of channels corresponded to the compression of this stratum, resulting in lower species richness.