Scale-dependent variation in composition of fish fauna among sandy tropical estuarine embayments

Variability in tropical estuarine sandy bottom fish faunas was investigated along a 200 km section of the northeast coast of tropical Australia at 3 nested spatial scales: among bays, among adjacent estuaries within bays, and among replicates within estuaries. Sampling was confined to a single habitat type, shallow sandy habitats, and a single class of estuaries, those with extensive areas of shallow sand in their lower reaches. This approach was employed to minimise the biases inherent in sampling estuaries containing varying proportions of the disparate habitat types that occur in tropical estuaries. The results showed: (1) the compositions of samples from all 9 estuaries in November 1999 were quite distinct from those in July 1999, July 2000 and July 2001; (2) there were no clear differences among estuaries during November 1999, but strong differences among estuaries every July, which were greater than differences among years; and (3) there was a greater difference at the scale of estuaries than at the larger spatial scale of ‘bays’, such that the differences among estuaries within bays were consistently greater than differences among bays. This pattern was consistent between seasons and among years in the same season. The clear estuary-to-estuary variability suggests that fauna of adjacent estuaries are likely to be no more similar than the faunas of estuaries hundreds of kilometres apart. This implies that no single estuary can be assumed to be a control site that can be used as a baseline against which to judge change in a different ‘impacted’ estuary