Diet comparison between silky sharks (Carcharhinus falciformis) and scalloped hammerhead sharks (Sphyrna lewini) off the south-west coast of Mexico

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.