Species richness and spatial variation in fish assemblage structure in two rivers of the Wet Tropics of northern Queensland,Australia

Synopsis Fish assemblages were sampled at 22 sites within the Mulgrave and South Johnstone Rivers of the Wet Tropics World Heritage Area, north Queensland. Flow regimes of these rivers are highly predictable, by Australian standards, due to low annual and seasonal variability. A gradual downstream change in fish assemblage structure, correlated with gradual change in habitat, substrate and type of in-stream cover, was observed in the Mulgrave River but not the South Johnstone. A reduced species richness was observed in the South Johnstone relative to the Mulgrave probably due to the effect of two high gradient sections located in the former river and a more diverse array of habitat types present in the latter. Both rivers contained more species than other tropical Australian rivers of greater size. Possible reasons for this included the constant and predictable flow regime and the greater diversity of habitats found in rivers of the Wet Tropics compared to other tropical Australian rivers.To whom correspondence should be addressed