What matters for intraspecific diet changes: the dietary differences between different areas or the increase in body size? The case of the searobin Prionotus punctatus in a tropical bay

Resource use diversity occurs when a population is composed of ecologically heterogeneous individuals who use only a subset of the population’s resource availability. The aim of this study was to examine the diet of juveniles, sub-adults and adults of Prionotus punctatus (40 to 190 mm Total Length - TL) to assess the importance of factors driving intraspecific changes, such as spatial differences in the diet or individual body size changes. The intraspecific trophic strategy was also described. We examined the stomach contents of 210 individuals collected in two zones of a tropical bay (outer and inner) grouped into three size classes (< 90 mm TL, juveniles; 90–140 mm TL, subadults; > 140 mm TL, adults). The Prey-Specific Index of Relative Importance (PSIRI) indicated that Peneidae, Amphipoda (excluding Caprellidae), Mysidacea, Teleostei and Copepoda were the most important food items. The change in the diet during fish growth differed between the two bay zones. In the inner zone, the juveniles fed mainly on Copepoda and Amphipoda (excluding Caprellidae), whereas the adults fed mainly on Teleostei and Peneidae. In the outer zone, the juveniles fed mainly on Mysidacea, whereas the adults fed mainly on Peneidae and Brachyura, with Amphipoda (excluding Caprellidae) being an important prey for individuals in all size classes. Significant differences were detected in the diets among size classes (Pseudo-F = 5.52; P = 0.003) but not between the two zones (Pseudo-F = 2.20; P = 0.113) according to PERMANOVA. Niche breadth decreased during ontogeny, and the niche overlap among the size classes was low (0.80) in both zones. Together, these observations suggest that the feeding niche in the larger-sized individuals of this species tends to overlap irrespective of the dietary differences between different areas, which can indicate some degree of narrowing in morphological and behavioural features. The increase in body size rather than spatial dietary differences seems to be a major determinant for intraspecific changes in feeding habits. Therefore, diet partitioning along growth, rather than dietary spatial change, seems to be the main mechanism used by this species to decrease intraspecific competition.