Spatial and size feeding niche partitioning of the rhomboid mojarra Diapterus rhombeus (Cuvier, 1829) in a tropical Brazilian Bay

We examined the stomach contents of 287 individuals of rhomboid mojarra Diapterus rhombeus collected between June 1999 and May 2000 in three zones (inner, middle and outer) of a Brazilian tropical bay. Fish were grouped in five size-class (Total Length(1) <= 80 mm; TL2 = 81-110; TL3 = 111-140; TL4 = 141-170; and TL5 >= 171). The aim was to assess whether intraspecific spatial and size-feeding niche partitioning occurs and to describe the strategy used to explore the available resources. Crustacea and Polychaeta were the most important trophic categories and Harpacticoida (Index of Relative Importance, IRI = 66.9%) was the most preferred prey throughout all zones and size classes. The smallest individuals (TL <= 140 mm) occurred exclusively in the inner zone and fed mainly on Harpacticoida, Calanoida, sedentary Polychaeta and Nematoda, while the largest individuals (TL > 140 mm) fed mainly on Harpaticoida and Ostracoda. In the outer zone, the largest individuals feed mainly on Harpacticoida and Caprella. A high within-phenotype contribution to the niche width and a generalist zoobenthivore feeding strategy were detected. Partitioning the food items along space and growth seems to be the mechanism used by this species to decrease intraspecific competition.