Feeding habits and trophic level of the smooth hammerhead shark, Sphyrna zygaena (Carcharhiniformes: Sphyrnidae), off Ecuador

Abstract

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.

The Multi-Annual Residency of Juvenile Smooth Hammerhead Shark in an Oceanic Island Nursery

“…Juveniles may also benefit from increased opportunities to feed on the vast mesopelagic resources that occur in the region and frequently invade the coastal pelagic habitat during their nighttime diel vertical migrations (DVMs). This combination would offer an expanded trophic niche during their juvenile phase as well as a more heterogeneous diet as juveniles grow and approach their adult oceanic lifestyle, after which they will essentially depend on mesopelagic resources, especially squid (Galvań-Magaña et al, 2013;Estupiñań-Montaño et al, 2019).…”

Retrospective stable isotopes of vertebrae reveal sexual ontogenetic patterns and trophic ecology in oceanic whitetip shark, Carcharhinus longimanus

“…Variance in isotopic compositions from vertebrae reflects the integration of metabolism, growth, protein composition, and feeding (Estupiñán‐Montaño et al., 2019). Feeding is the key determinant of δ 13 C and δ 15 N values in the tissue, which are influenced by the exogenous supply of isotopes (MacNeil et al, 2005).…”

“…The signatures of the zooplankton (δ 13 C = −20.1 ± 0.7‰ SD and δ 15 N = 5.3 ± 0.8‰ SD; Estupiñán-Montaño et al, 2019) sampled in the study area were used as an isotopic baseline with TP base = 2. The δ 15 N lim and k values of 21.93 and 0.14, respectively, were derived from a meta-analysis of experimental isotope data (Hussey et al, 2014).…”

“…(Estupiñán-Montaño et al, 2019). Feeding is the key determinant of δ 13 C and δ 15 N values in the tissue, which are influenced by the exogenous supply of isotopes(MacNeil et al, 2005).…”

“…Stable carbon and nitrogen isotope ratios (δ 13 C and δ 15 N, respectively) in metabolically inert tissue, such as shark vertebrae, are efficient intrinsic markers for elucidating ontogeny/sexual variations in diet and/or habitat use of many shark species, such as the white shark Carcharodon carcharias (Estrada et al, 2006;Kerr et al, 2006;Kim et al, 2012;Wolf et al, 2009), blue shark Prionace glauca (Curnick et al, 2019;Estupiñán-Montaño et al, 2019), salmon shark Lamna ditropis (Carlisle et al, 2014), and three hammerhead shark species Sphyrna mokarran, S. lewini, and S. zygaena (Loor-Andrade et al, 2015;Raoult et al, 2019). This approach is based on the fact that shark vertebrae are related to lifetime information deposited in the growth bands reflecting the entire life histories of individuals (Estrada et al, 2006).…”