Use of δ¹⁵N and δ¹³C in reconstructing the ontogenetic feeding habits of silky shark (Carcharhinus falciformis): reassessing their trophic role in the Eastern Tropical Pacific Ocean

The silky shark is important in commercial fisheries, but their population has declined in recent years. One of the major challenges for species under this scenario is to understand their trophic interactions as they grow and to apply this knowledge for their management and protection. Based on the analysis of δ¹⁵N and δ¹³C in muscle and vertebral tissue, the aim of this study is to assess the feeding habits (possible changes in resource and habitat use) of C. falciformis related to ontogeny (neonatal, juvenile, and adult stages), which may lead to the reassessment of the species’ trophic role in the Eastern Tropical Pacific Ocean (ETPO). The isotopic composition of muscle (n = 60) and vertebrae (n = 44) was analyzed over 3 years (2003–2005). Significant differences were detected in δ¹⁵N_muscle between neonates and juveniles (H = 6.26, p < 0.05) and in δ¹³C_muscle between juveniles and adults (H = 6.78, p < 0.05). Significant differences were detected also in δ¹⁵N_vertebrae (F = 4.19, p < 0.05) and δ¹³C_vertebrae (F = 6.59, p < 0.05) among ontogenetic stages, particularly regarding females (H = 34.77, p < 0.05), which may indicate: 1) that they consume different prey types as a result of changing energy requirements linked to their ontogeny (e.g., reproduction), and 2) different movement patterns. If the estimated trophic position of C. falciformis was 3.7, then the species could be classified as a tertiary predator. The data generated in this study indicate ontogenetic changes in the feeding behavior of C. falciformis, which is critical to understanding the role of this predator in the community structure of the ETPO.