Identification of variables that influence fish populations is one of the main challenges in ecology. To explore this, data were collected quarterly from February 2000 to November 2001 using seines, along the shore of four isolated lagoons of the upper Paraná River floodplain. Serrapinnus notomelas was selected to assess the effect of abiotic and biotic variables using indirect gradient analysis. Abiotic variables were summarized by principal components analysis (PCA) and then the scores of the axis retained for interpretation were correlated with abundances of S. notomelas. Variables that best explained S. notomelas abundance were surface area of the lagoon, total suspended solids and Secchi depth (these last two, indirectly linked to predation). The most relevant biotic variable that determined population size of S. notomelas was predation.
population regulation; abiotic variables; competition; predation
