Trophic inference in two sympatric sharks, Sphyrna lewini and Carcharhinus falciformis (Elasmobranchii: Carcharhiniformes), based on stable isotope analysis at Malpelo Island, Colombia

Abstract

Background. Elasmobranchs can play important roles in marine communities. But, relatively little is known about their diet, and movement. Sphyrna lewini (Griffith et Smith, 1834) consumes fishes, cephalopods, rays, and crustaceans. Carcharhinus falciformis (Müller et Henle, 1839) feed on fishes, cephalopods, crustaceans and sea turtles. To date, there are no studies available on the trophic ecology of sharks in Malpelo Island. The aim of this study was to describe the trophic ecology of S. lewini and C. falciformis, using stable isotope analysis of δ13C and δ15N, to better understand the role of both shark species in the Malpelo Island ecosystem. Materials and methods. In January, February, and November 2013, specimens of Sphyrna lewini and Carcharhinus falciformis illegally caught at Malpelo Island were confiscated at the port of Buenaventura, Colombia. For each shark specimen, total length and sex were registered. Samples of muscle tissue were taken from the nape of all specimens. Each muscle sample was lyophilized for 24 h and analysed with lipid and urea extraction and without extraction. For each shark specimen, a subsample of ~1.0 mg was used for isotopic analysis. Results. A total of 14 Sphyrna lewini (Griffith et Smith, 1834) and 12 Carcharhinus falciformis (Müller et Henle, 1839) were analysed. δ13C values were similar between S. lewini (–16.3 ± 0.1‰) and C. falciformis (–16.5 ± 0.1‰). Sphyrna lewini showed a wider trophic niche than C. falciformis, with low trophic overlap (5%) between the two species. The δ15N values of S. lewini (15.9 ± 0.11‰) were higher than those of C. falciformis (14.9 ± 0.09‰). In C. falciformis, δ13C values were similar in both sexes (–16.5 ± 0.1‰), while δ15N values were significantly different between males (14.6 ± 0.1‰) and females (15.0 ± 0.1‰). The trophic position of S. lewini was 5.25 ± 0.12, and that of C. falciformis, 5.48 ± 0.18, which suggests that both shark species occupy a high position in the marine food chain. Conclusion. Both shark species co-occur at Malpelo Island, but they do not share food resources and feeding areas, and they probably feed far from the island, using it as a resting and cleaning area. This indicates the need for more research to increase biological and ecological knowledge of both species, particularly within marine protected areas and their influence areas throughout the Colombian Pacific.

Keywords

• Trophic ecology
• carbon isotopes
• nitrogen isotope