Feeding behavior and trophic interaction of three shark species in the Galapagos Marine Reserve.

Diego Páez-Rosas,Paul Insuasti-Zarate,Marjorie Riofrío-Lazo,Felipe Galván-Magaña

doi:10.7717/peerj.4818

Abstract

There is great concern about the future of sharks in Ecuador because of the lack of biological knowledge of most species that inhabit the region. This paper analyzes the feeding behavior of the pelagic thresher shark (Alopias pelagicus), the blue shark (Prionace glauca) and the silky shark (Carcharhinus falciformis) through the use of stable isotopes of carbon and nitrogen (δ13C and δ15N), with the aim of determining the degree of interaction between these species in the Galapagos Marine Reserve. No interspecific differences were found in use of oceanic vs. inshore feeding areas (δ13C: Kruskal–Wallis test, p = 0.09). The position in the hierarchy of the food web where A. pelagicus feeds differed from that of the other species (δ15N: Kruskal–Wallis test, p = 0.01). There were no significant differences in δ13C and δ15N values between males and females of the three species (Student’s t-test, p > 0.05), which suggests that both sexes have a similar feeding behavior. A specialist strategy was observed in P. glauca (trophic niche breadth TNB = 0.69), while the other species were found to be generalist (A. pelagicus TNB = 1.50 and C. falciformis TNB = 1.09). The estimated trophic level (TL) varied between the three species. C. falciformis occupied the highest trophic level (TL = 4.4), making it a quaternary predator in the region. The results of this study coincide with the identified behavior in these predators in other areas of the tropical Pacific (Colombia and Mexico), and suggest a pelagic foraging strategy with differential consumption of prey between the three species. These ecological aspects can provide timely information when implementing in conservation measures for these shark species in the Tropical Pacific and Galapagos Marine Reserve.

Highlights

Global increase in fishing effort has led to a decline of nearly 90% in oceanic fish populations, with elasmobranchs being one of the most affected groups (Stevens et al, 2000; Myers & Worm, 2003)
The trophic interactions of elasmobranchs are sometimes difficult to determine using traditional methods, so the use of alternative techniques such as stable isotopes become an opportunity to infer from another perpestive the trophic ecology of these species
Samples of muscle tissue were taken from a total of 91 adult sharks belonging to the three species mentioned above (A. pelagicus, P. glauca or C. falciformis) (Table 1)

Summary

Introduction

Global increase in fishing effort has led to a decline of nearly 90% in oceanic fish populations, with elasmobranchs being one of the most affected groups (Stevens et al, 2000; Myers & Worm, 2003). How to cite this article Páez-Rosas et al (2018), Feeding behavior and trophic interaction of three shark species in the Galapagos Marine Reserve. The lack of biological knowledge supporting the regulation and conservation of these resources, has led to several species of sharks being listed as endangered or vulnerable by the IUCN (Clarke et al, 2006). Most sharks are top predators, controlling trophic relationships and energy flows within the ecosystems they inhabit (Myers et al, 2007; Heithaus et al, 2008). These predators are typically considered as generalist consumers, and many have adopted strategies to exploit persistent and profitable resource regions (Au, 1991; Compagno, Dando & Fowler, 2005). The trophic interactions of elasmobranchs are sometimes difficult to determine using traditional methods (diet through stomach content or behavior using tagging and direct observations), so the use of alternative techniques such as stable isotopes become an opportunity to infer from another perpestive the trophic ecology of these species

Methods

Results

Discussion

Conclusion