The diversity (richness and equitability) of two warm-temperate estuarine fish assemblages in Patos Lagoon (PL), Brazil, and York River (YR), USA is described using bottom trawl data. Comparisons of species richness (S) were made using the rarefaction technique which enables comparisons of S among samples (N) of uneven size. In order to compare K-dominance curves (equitability curves) of different samples with drastic differences in number of species (i.e., in order to consider equitability separately from S ), the number of species was rescaled to percentage of the total number of species in the sample (range = 0 to 100%), creating Lorenz’s curves. A new index, the adjusted equitability index [AEx%=100-(sumDx%)], based on the combination of the rational of the Lorenz’s curves and Berger-Parker (“d”) index of equitability, was calculated, where Dx% corresponds to the cumulative percentage contribution of x% of the species in the fish assemblage, and x% is equal to 100%/Si, been Si the lowest species richness (S) in the assemblages compared. Both techniques are useful to study large temporal and spatial variability of biodiversity independently of sample size and effort.
Fish species in bottom trawl collections of PL and the YR were very similar on size distribution and primarily represented by post-larvae, young-of-the-year and sub-adults ranging in length between 10 and 250 mm or larger. The total number of species (65 and 63, respectively) collected over a year of bottom trawling in the PL and YR estuaries, was about the same. In both locality the sampling intensity was high, but at the YR (n=495) were taken twice as many samples than at PL (n=253), and also more than 15,000 individuals were collected on YR. Based upon sample-size independent measurements of species richness (rarefaction curves) the expected number of species projected out to an hypothetical 23,000 fish caught was considerably higher in PL (54 species) than in the YR (45), and also the equitability was higher in PL (AEx%=62%) than in the YR (24%). Within broad limits, the PL and YR structural assemblages patterns were correlated with temperature changes, although the intensity of seasonal changes differed between them. The temperature regime of PL was clearly more moderate than that in the YR. PL was more diverse in terms of equitability and expected species richness than YR at any particular point in time. The lower winter temperature in the YR were associated with a pronounced seasonal species emigration from the estuary, a pattern which was not observed in the more thermally moderate PL. This observation corroborate with the hypothesis that latitude and temperature related phenomenon play a critical role in determining species diversity on estuaries.