SPATIAL AND TEMPORAL VARIATION IN FISH ASSEMBLAGE STRUCTURE IN THE MARY RIVER, SOUTH-EASTERN QUEENSLAND - THE INFLUENCE OF HABITAT STRUCTURE

Fish assemblages at eleven sites within the Mary River were sampled over a 15 month period by back-pack electrofishing. The study took place during a period of abnormally low flows but included two high flow events one of which approached record levels. Spatial and temporal variation in fish assemblage structure was pronounced. Spatial variation was related to position within the catchment and correlated with habitat attributes related to substrate composition and water depth. The absence of a strong effect due to water velocity was probably due to overall low water flows throughout the study period. Temporal variation at some sites, particularly headwater sites, was correlated with changes in the amount and type of cover available to fishes. Cover was significantly correlated with the mean species richness of each site. Flooding had very little effect on both fish assemblage structure and the physical structure of the habitat but resulted in substantial reduction in the amount and type of cover available to fishes. The flow regime of the Mary River was highly variable and consequently both the timing and duration of high and low flows was unpredictable.