On the rocks in Borneo: Habitat use in an ensemble of grazing fishes

I used direct observation to locate and quantify habitat use by three Gastromyzon spp. (Balitoridae) and Paracrossochilus acerus (Cyprinidae) in a rainforest river in Brunei Darussalam, island of Borneo. Gastromyzon spp. = torrent loaches = "sucker fish" are morphologically specialized for fast, rocky streams. Abundance order was P. acerus >> G. cranbrooki > G. aeroides = G. punctulatus. All four are diurnally active and retreat beneath rocks at night. Up to three species were seen grazing on a single rock. I located individuals and random points and measured depth, water column velocity and substrate. Multivariate analyses (PCA, DFA, CART) defined a simple gradient from slower/deeper to fast/shallow with correlated substrate attributes; species order was P. acerus: G. cranbrooki: G. aeroides: G. punctulatus. Species overlap was substantial. Resource experiments (rock scrubbing,cages) were terminated by high flows but suggest discrimination between high- and low-periphyton rocks. This stream varies day to day from low and clear to high and turbid with much bed load movement. I speculate about the relative strengths of physical control, resource supply and interactions among these small grazing fishes.