Size Spectra of Fish Assemblages: Longitudinal and Temporal Variation in a Small Neotropical Reservoir

Size spectrum approaches have been increasingly used as an indicator of aquatic ecosystem structure. In this study, we estimated the size spectrum slopes of fish assemblages at three different longitudinal zones of a recently formed reservoir in Southeastern Brazil, at two different points in time: in the first and in the second year after closing the dam. We show that the size spectrum slope, given by the maximum likelihood estimator of the Pareto type I distribution, has not changed from the first to the second year only in the lotic, more preserved zone. The lentic zone, representing the more altered habitat, had a much steeper size distribution in the first year, implying in relatively higher dominance by small fish, but then showed the greatest change in slope in the second year towards the values typical for the lotic zone. The transition zone showed intermediate trends in slope.  These results can be explained by concomitant changes in phytoplankton production, water transparency and occurrence of piscivorous fish observed between the two years, suggesting that the fish size distribution can readily track changes in environmental conditions following major perturbations.