Spatial heterogeneity in roach (Rutilus rutilus) diet among contrasting basins within a lake

Habitat and diet variation of roach (Rutilus rutilus) were studied in Lake Banyoles (Spain), an oligotrophic karstic lake with strong spatial heteroge neity in food resources. In addition to seasonal variation in diet, roach d isplayed an ontogenetic shift from zooplanktivory (dominated by the cladoce ran prey Daphnia longispina) to benthivory (based on detritus, plant debris , amphipods, algae, and other invertebrates). Strong patterns of spatial he terogeneity in roach diet were detected for this lake. Firstly, there was a winter aggregation of smaller fish in the deepest basin, where benthos was scarce but a patch of copepods had been previously described. These fish d isplayed a high condition and fed on zooplankton, particularly those copepo ds. Secondly, meiobenthos, mainly ostracods and other microcrustaceans, was more important in the diet in the shallowest basin. The ontogenetic diet s hift was also mediated by spatial heterogeneity, because in basins where be nthos was scarce, larger roach continued to feed on zooplankton.