Sharks are apex predators in marine ecosystems and play an important role in regulating prey populations at lower trophic levels (e.g., fish, invertebrates, reptiles, mammals and birds) (Ellis et al., 1996).The bull shark, Carcharhinus leucas (Müller and Henle, 1839), and the tiger shark, Galeocerdo cuvier (Péron and Lesueur, 1822), are common worldwide in tropical and subtropical coastal areas (Compagno, 1984). The bull shark grows to a large size (over 340 cm) (Compagno, 1984) and frequently inhabits shallow water (max. depth 30 m) (Compagno, 1984). It is one of the few shark species that is physiologically capable of spending time in fresh water (Pillans and Franklin, 2004) and has been reported in rivers and lakes (Thorson et al., 1973;Thomerson et al., 1977;Montoya and Thorson, 1982; Simpendorfer et al., 2005). The tolerance of bull sharks for low-salinity conditions allows them to frequent areas close to shore and near river mouths, where they may interact with other shark species (Carlson et al., 2010). The tiger shark reaches a larger size than the bull shark (over 550 cm) (Compagno, 1984), with a wide tolerance for different marine habitats at depths ranging from the surface to 140 m. This species

We analysed the stomach contents of 69 silky sharks Carcharhinus falciformis, 44 blacktip sharks Carcharhinus limbatus and 24 whitenose sharks Nasolamia velox caught in the Ecuadorian Pacific from August 2003 to December 2004. Prey included bony fishes, elasmobranchs, molluscs, crustaceans and turtles, with bony fishes being the most important to the diets of all three sharks, suggesting they are piscivorous predators. Based on the index of relative importance, the C. falciformis diet includes Thunnus albacares, Thunnus sp. and Auxis thazard, as well as some squid, fish and turtles. Similarly, the C. limbatus diet was dominated by T. albacares, Exocoetus monocirrhus, A. thazard, Katsuwonus pelamis, members of the Ophichthidae family and other elasmobranchs. Meanwhile, N. velox consumed mainly Dosidicus gigas, Larimus argenteus, Cynoscion sp. and Lophiodes spilurus. There is little competition for food between these tertiary carnivores: C. limbatus prefers prey from coastal-oceanic habitats; C. falciformis consumes mostly oceanic prey and N. velox focuses on prey from coastal habitats. The lack of information on the biology of sharks in Ecuador hinders the development of appropriate management and conservation plans to protect shark resources. This study increases our knowledge and understanding of sharks in Ecuador, thus contributing to their conservation.

As apex predators, sharks are known to play an important role in marine food webs. Detailed information on their diet and trophic level is however needed to make clear inferences about their role in the ecosystem. A total of 335 stomachs of smooth hammerhead sharks, Sphyrna zygaena, were obtained from commercial fishing vessels operating in the Ecuadorian Pacific between January and December 2004. A total of 53 prey items were found in the stomachs. According to the Index of Relative Importance (%IRI), cephalopods were the main prey (Dosidicus gigas, Sthenoteuthis oualaniensis, Ancistrocheirus lesueurii and Lolliguncula [Loliolopsis] diomedeae). Sphyrna zygaena was thus confirmed to be a teutophagous species. The estimated trophic level of S. zygaena was between 4.6 and 5.1 (mean ± SD: 4.7 ± 0.16; males: 4.7; females: 4.8). Levin's index (BA) was low (overall: 0.07; males: 0.08; females: 0.09), indicating a narrow trophic niche. We found that sharks <150 cm in total length consumed prey of coastal origin, whereas sharks ≥150 cm foraged in oceanic waters and near the continental shelf. The analyses indicate that S. zygaena is a specialized predator consuming mainly squids.

Estupiñán-Montaño C., Galván-Magaña F., Hacohen-Domené A., Estupiñán-Ortíz J.