Greenland Shark (Somniosus microcephalus) Stomach Contents and Stable Isotope Values Reveal an Ontogenetic Dietary Shift

Julius Nielsen,Rasmus Hedeholm,Peter Grønkjær,Kim Præbel,Helene Overgaard Kiilerich,Jørgen Schou Christiansen,Peter Bushnell,John Fleng Steffensen

doi:10.3389/fmars.2019.00125

Abstract

Current knowledge on the feeding ecology of the Greenland shark (Somniosus microcephalus), a potential top predator in arctic marine ecosystems, is based on small sample sizes as well as narrow size ranges of sharks. Therefore, potential size-related feeding patterns remain poorly documented. Using stomach content data (N=88) and stable isotope values of white muscle tissue (N=40), this study evaluates the diet in sharks ranging in size from 81 cm to 474 cm (total length). The importance of prey categories (‘Fish’, ‘Mammal’, ‘Squid’, ‘Crustacean’ and ‘Other’) was evaluated based on the reconstructed prey biomass of the stomach contents. Stable isotopes values of δ13C and δ15N ranged between -14.4‰ to -19.9‰ and 11.8‰ to 17.2‰, respectively. The importance of each prey category was estimated by the Index of Relative Importance (IRI). Our findings suggest that the smallest Greenland sharks (200 cm) mainly feed on higher trophic level prey such as seals, epibenthic and benthic fishes including gadoids (Gadidae), skates (Rajidae), righteye flounders (Pleuronectidae), lumpfish (Cyclopteridae), wolffish (Anarhichadidae), and redfish (Sebastidae). Redfish were, however, only found to be important in the largest sharks sampled (>400 cm). In addition to demonstrating ontogenetic shits in their feeding preferences, this study supports that Greenland sharks are capable of active predation on fast swimming seals and large fishes.

Highlights

During the past 10 years, the Greenland shark (Somniosus microcephalus) has been subject to increased scientific attention due to its role as a long-lived, highly migratory, and relatively abundant top predator in arctic marine food webs (MacNeil et al, 2012; Campana et al, 2015; Nielsen et al, 2016)
Ontogenetic dietary shifts have been documented for other large predatory sharks such as sevengill (Notorynchus cepedianus), white (Carcharodon carcharias), and tiger sharks (Galeocerdo cuvier) (Lowe et al, 1996; Ebert, 2002; Estrada et al, 2006), and preliminary data suggest that small Greenland sharks have a lower trophic position compared to larger sharks (Fisk et al, 2002)
Sampling was obtained by the Greenland Institute of Natural Resources (GINR) annual fish surveys (RV Pâmiut, RV Sanna), the TUNU IV and V Expeditions (RV Helmer Hanssen) (Christiansen, 2012), the commercial trawler FV Sisimiut, and targeted Greenland shark expeditions with RV Porsild and RV Sanna

Summary

Introduction

During the past 10 years, the Greenland shark (Somniosus microcephalus) has been subject to increased scientific attention due to its role as a long-lived, highly migratory, and relatively abundant top predator in arctic marine food webs (MacNeil et al, 2012; Campana et al, 2015; Nielsen et al, 2016). Previous studies have investigated the diet and trophic interactions of Greenland sharks by analyzing stomach contents, stable isotopes, and fatty acids (Fisk et al, 2002; Yano et al, 2007; McMeans et al, 2010; Leclerc et al, 2012; McMeans et al, 2013; Nielsen et al, 2014). These studies are mostly based on relatively few individuals (N < 50) from 200 to 400 cm in length, with limited information on larger individuals. Dietary differences across Greenland shark size becomes relevant to investigate

Methods

Results

Conclusion