Trophic ecology of the juveniles of two jack species (Caranx latus and C. hippos) in contrasted tropical estuaries

Jacks are highly prized tropical marine fish. Most of them complete their whole life cycle at sea but some use estuaries at the juvenile stage before moving back to coastal waters and joining the adult exploited stocks. Little is known about jacks’ trophic ecology in estuaries, although their juveniles' ability to successfully exploit available resources in these productive environments may strongly affect stock recruitment success in the species concerned. In this study, stomach content and stable isotope analyses were combined to investigate diet and food niche overlap of juveniles from two sympatric species of jacks (Caranx latus and C. hippos) in three contrasted estuaries (Suape, Sirinhaém and Santa Cruz) spread along the northeastern Brazilian coast. Overall, although the juveniles of C. latus exhibited a more piscivorous diet than those of C. hippos, the two species had very similar isotopic niches, with mean δ13C and δ15N values of −19.35 ± 2.10‰ and 11.03 ± 1.11‰ and of −19.10 ± 1.82‰ and 10.21 ± 1.21‰, for C. hippos and C. latus respectively. In all the estuaries sampled, both species mostly ate fish (Ni = 20.1–46.2%, Wi = 60.1–75.1%, essentially Gobiidae and Clupeidae) and crustaceans (Ni = 26.0–65.9%, Wi = 23.3–38.2%, mainly Penaeidae shrimps). As a result, the overlap between their global estuarine isotopic niches was >68%. However, diet composition for the two species varied among estuaries, indicating that their juveniles partly adapt their food preferences to local prey availability. Notably, prey preferences differed significantly between the two species only in the Santa Cruz estuary, where δ15N values were the highest for both species. Conversely, interspecific differences in δ13C ratios were greater in the Suape and the Sirinhaém estuaries, likely reflecting a wider diversity in the carbon sources sustaining local food webs. Thus, combined differences in juvenile diet and food web structure at each location resulted in much-reduced local isotopic niche overlaps between the two species (from 27% in Suape to 57% in Santa Cruz). These results have important implications for resource and ecosystem management in northeastern Brazil and call for systematic cross-site comparisons when evaluating fish ecology and resource partitioning within estuarine systems.