Diet composition of Plagioscion squamosissimus (Heckel, 1840), a fish introduced into the Tietê River system.

Plagioscion squamosissimus is a species from the Amazon basin that was introduced into the Tietê River system. The present study aimed to analyse the feeding habits of this species in Bariri Reservoir and to verify the possible occurrence of ontogenetic changes in its diet composition. The samples were gathered in four periods of the year: February, June, September and November 2003. The fish were gathered with different fishing net meshes in three different reservoir portions. The alimentary items found in the stomachs were identified to the lowest possible taxonomic level and had abundance, occurrence frequency, volume and biomass determined. The Alimentary Index (IAi) was calculated for each alimentary item consumed by 'corvina' for each studied period. Comparisons among the diet of different size classes of P. squamosissimus were done using the similarity coefficient of Jaccard and the Cluster Analysis (UPGMA). The Friedman proof was performed to verify if there is a significant ontogenetic variation in the species diet and changes in the consumption of different alimentary categories by P. squamosissimus among the sampled periods. P. squamosissimus presented a piscivorous feeding habit, although other items were also consumed. The biggest values of IAi were obtained for the alimentary item fish in the months of June (0.47) and November (0.39). The item Ephemeroptera (Campsurinae) was the most representative in February (0.30) and June (0.45). Despite the fact that P. squamosissimus consumed an ample spectrum of alimentary items, the ontogenetic changes were evident through the exploration of aquatic insects by the younger classes and by a diet mainly composed of fish in adult individuals. The alimentary plasticity of P. squamosissimus evidenced in this study might have contributed to the success of this species in Bariri Reservoir.