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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 δ15Nmuscle between neonates and juveniles (H = 6.26, p < 0.05) and in δ13Cmuscle between juveniles and adults (H = 6.78, p < 0.05). Significant differences were detected also in δ15Nvertebrae (F = 4.19, p < 0.05) and δ13Cvertebrae (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. [ABSTRACT FROM AUTHOR]

Ecologists examine diet composition in order to assess the spatial and temporal variations in interactions between species, the impact of different species traits on the ecological network structure, and the long-term effects of the removal of different species by small-scale fisheries. In this study, our goal was to compare the diets of silky sharks (Carcharhinus falciformis) and scalloped hammerhead sharks (Sphyrna lewini) off the south-west coast of Mexico in order to infer their diet preferences and spatial distributions. We sampled 164 S. lewini (96 stomachs had food, 68 were empty) and 183 C. falciformis (30 stomachs had food, 153 were empty) in Puerto Madero, Chiapas in 2011. The large number of empty stomachs may be the result of using longline fishing gear, which causes high stress resulting in regurgitation. Based on the index of relative importance (%IRI), the fish Chloroscombrus orqueta (IIR = 27.7%) was the most important species in the diet of S. lewini, while the squid Dosidicus gigas (IIR = 34%) was the primary prey of C. falciformis. Levin's index (Bi) and Shannon's index (H′) confirm that both sharks are generalists, as in other regions. The trophic levels of S. lewini (TL = 4.1) and C. falciformis (TL = 4.2) are characteristic of tertiary consumers; meanwhile, the Morisita–Horn index indicates low interspecific overlap between all categories. These results confirm that these two sharks have different foraging preferences or movement patterns; thus, there is no trophic overlap between species as they play unique roles in the ecological network off the south-west coast of Mexico. [ABSTRACT FROM PUBLISHER]

The Pacific sharpnose shark Rhizoprionodon longurio is caught seasonally by inshore artisanal fisheries in the Mexican Pacific. Our study focuses on the feeding ecology of this shark species in the southern Gulf of California. The prey species obtained from stomach contents were identified and quantified, and variations between sexes and maturity stages were determined. A total of 98 stomachs were analysed during two periods (2000–2001 and 2003–2004); 64% of stomachs contained food. The trophic spectrum was composed of four cephalopod species, three crustacean species, and 13 pelagic and benthic fish species. According to the index of relative importance (%IRI), the fish Echiophis brunneus (IRI = 14.4%), Opisthopterus dovii (IRI = 12.2%) and Scomber japonicus (IRI = 9.6%) were the main prey items. Based on diversity values, IRI values and diet breadth, R. longurio is an opportunistic predator. The trophic position of R. longurio was above four in all categories, which indicates that this shark is a tertiary consumer. [ABSTRACT FROM PUBLISHER]