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 (N_i = 20.1–46.2%, W_i = 60.1–75.1%, essentially Gobiidae and Clupeidae) and crustaceans (N_i = 26.0–65.9%, W_i = 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.

千斤顶是珍贵的热带海洋鱼类。他们中的大多数人在海上完成了整个生命周期，但有些人在少年阶段使用河口，然后又回到沿海水域并加入成年的被捕捞种群。尽管千斤顶幼鱼在这些生产环境中成功开发可用资源的能力可能会极大地影响有关物种的种群招募成功，但对它们在河口的营养生态学知之甚少。在这项研究中，结合了胃内容物和稳定同位素分析，研究了来自两种同属千斤顶（Caranx latus和C. hippos）的幼鱼的饮食和食物生态位重叠。）分布在巴西东北部沿海地区三个相反的河口（苏阿佩，西林海姆和圣克鲁斯）中。总体上，尽管所少年C. LATUS表现出更食鱼饮食比那些C.河马，这两个物种具有非常相似的同位素龛，平均δ ¹³ C和δ ^15个的-19.35±2.10 N个值‰和11.03±1.11 ‰和-19.10±1.82‰和10.21±1.21‰，为C.河马和C.侧腹分别。在所有采样的河口中，两个物种都以鱼类（N _i  = 20.1–46.2％，W _i  = 60.1–75.1％，主要是戈壁科和克鲁皮科）和甲壳类动物（N _i = 26.0–65.9％，W _i  = 23.3–38.2％，主要是对虾科。结果，它们的全球河口同位素生态位之间的重叠度> 68％。但是，这两种物种的饮食组成在河口之间有所不同，这表明它们的幼鱼在某种程度上使食物偏爱适应当地猎物的供应。值得注意的是，猎物的喜好两个物种只在圣克鲁斯河口，其中δ之间显著差异¹⁵的N值是最高的两个物种。相反，在δ间的差异¹³在Suape和Sirinhaém河口，碳比率更高，这可能反映了维持当地食物网的碳源的多样性更加广泛。因此，每个地点的少年饮食和食物网结构的综合差异导致两种物种之间的局部同位素生态位重叠大大减少（从Suape的27％到Santa Cruz的57％）。这些结果对巴西东北部的资源和生态系统管理具有重要意义，并要求在评估河口系统内的鱼类生态和资源分配时进行系统的跨站点比较。