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 Magda F. Andrade-Tubino & Rosana R. Milagre & Francisco G. Araújo Received: 23 June 2018 /Accepted: 3 January 2019 # Springer Nature B.V. 2019 Abstract Resource use diversity occurs when a popu- lation is composed of ecologically heterogeneous indi- viduals who use only a subset of the population’ s re- source 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 Rela- tive 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 juve- niles 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 ontog- eny, and the niche overlap among the size classes was low (<0.60), except for subadults and adults (>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 dif- ferences between different areas, which can indicate some degree of narrowing in morphological and behav- ioural features. The increase in body size rather than spatial dietary differences seems to be a major determi- nant for intraspecific changes in feeding habits. There- fore, diet partitioning along growth, rather than dietary spatial change, seems to be the main mechanism used by this species to decrease intraspecific competition. Keywords Triglidae . Feeding habits . Bays . Feeding niche . Food resources Introduction Fish diet studies generate information that allows for an understanding of inter- and intraspecific interactions and reveals the relationships between fish and the environ- ments in which they live (Elliott et al. 2002; Potter et al. 2015). The fish diet is a result of the interactions of several factors, such as the preference of the species for food, the spatial and temporal availability of the Environ Biol Fish https://doi.org/10.1007/s10641-019-0843-6 M. F. Andrade-Tubino : R. R. Milagre : F. G. Araújo (*) Laboratório de Ecologia de Peixes, Universidade Federal Rural do Rio de Janeiro, BR 465, Km 7, Seropédica, RJ 23897-030, Brazil e-mail: gernsonufrrj@gmail.com