Indexed on: 29 Apr '16Published on: 29 Apr '16Published in: Polar Biology
Herein, we review and synthesize results from a series of research projects that were conducted to evaluate the role of Greenland sharks (Somniosus microcephalus) in the marine ecosystem in Kongsfjorden, Svalbard, Norway. A total of 76 sharks were caught on baited lines during the summers of 2008 and 2009 for these investigations. All of these animals, including the largest shark, a female weighing 700 kg, were sexually immature. Approximately half of the gastrointestinal tracts (GITs, N = 33) examined contained seal tissue (42.3 %), and some also contained minke whale (Balaenoptera acutorostrata) tissue (18.2 %). Atlantic cod (Gadus morhua), Atlantic wolffish (Anarhichas lupus) and haddock (Melanogrammus aeglefinus) were the dominant fish species consumed by the sharks. These fish species were found in 39.4, 18.2 and 18.2 % of the GITs, respectively. Many of the fishes were swallowed whole, including an Atlantic wolffish weighing 8.6 kg. Satellite pop-up tags deployed on 20 of the sharks showed that they travelled in the water column from the surface to depths greater than 1500 m, encountering temperatures from −1.5° to 7.4°. Accelerometers deployed on six of the sharks showed that they swim extremely slowly, with average speeds of 0.34 m/s and burst speeds of only twice this value. Various types of circumstantial evidence, including the condition of the seals and fishes found in the sharks’ stomachs, indicate that they are not only scavengers, but also active predators of both fish and mammalian prey. Given the swim speed of these sharks, we suggest that the only way they could successfully capture a healthy seal is via cryptically approaching seals that are asleep in the water. Greenland sharks clearly play a significant role as large predators in the Kongsfjorden marine ecosystem, a fact that has been largely overlooked until recently.