Geographic variation in the nutritional ecology of nominally herbivorous fishes in the southwestern Atlantic Ocean

Spatial and temporal niche partitioning are the basis for the coexistence of species. Determining niche partitioning among species over broad geographical scales requires multiple approaches, as local environmental features may vary spatially. We analysed the trophic ecology of 3 nominally herbivorous fishes—the doctorfish Acanthurus chirurgus , the grey parrotfish Sparisoma axillare , and the brassy chub Kyphosus vaigiensi s —to identify the drivers of variation in resource use at different locations along the Brazilian coast: Natal, Abrolhos Archipelago, and Arraial do Cabo. We used 2 complementary approaches—dietary (at 2 scales of magnification) and stable isotope analyses (SIA)—to investigate variation in the nutritional ecology of the study species. We observed interspecific dietary variation at each location. Doctorfish showed the highest dietary variability, with significant contributions from diatoms and cyanobacteria; grey parrotfish mainly ingested detritus, cyanobacteria, and green filamentous algae, while brassy chub mainly ingested brown algae but with some supplementation from red and green macroalgae. Interspecific isotopic niche overlap varied among locations following variation in the isotopic niche width of species. Trophic positions (TP) varied unexpectedly, with grey parrotfish displaying a relatively higher TP in one location and the lowest in the others. The use of complementary approaches, providing short- and comparatively long-term information (diet and SIA, respectively), and a detailed description of dietary items are essential to understand niche and resource partitioning when studying the nutritional ecology of nominally herbivorous fishes.