The Greenland Shark (Somniosus microcephalus)
Authors: Julius Nielsen and Martin Nielsen, March 2012
The Greenland shark (Somniosus microcephalus) grows up to 7 m in length and is thereby one of the largest fish in the world. It is distributed from the northern part of the east of USA, in Greenland, Iceland and Norwegian waters all the way to northern Russia (Campagno 1984). In spite of the size of the shark and the large distributional range, knowledge of this fish is very sparse. In Greenland the shark is considered a pest due to its disturbing presence near long line fishery and it is considered a “lazy” shark, which primarily feeds as a scavenger. The later has caused its ecological role to be considered as little and therefore it is often neglected in studies of Arctic ecosystems (Hobson et al. 2002). It is known that the Greenland shark feeds on a variety of animals such as crustaceans, cephalopods, snails, ecnoderms, fish and mammals (Yano et al. 2007), but new studies conducted in 2011 on Svalbard (Norway), revealed that the majory of the Greenland shars food consisted of seals and fish, which both were captured alive (Leclerc in prep.). On Svalbard and Sable Island (Canada) it is assumed that the Greenland shark performs top-down control on populations of ringed seal (Phoca vitulina) (Lucas & Natanson 2010, Leclerc in prep.). This challenges the interpretation of the shark as primarily being a scavenger. Analysis of stomach contents has shown that the sharks predates on fish such as Greenland halibut (Reinhardtuis hippoglossoides), red fish (Sebastes sp.) and Atlantic cod (Gadus morhua) (Yano et al. 2007, McMeans et al. 2010), which all are commercially important species in the northern Atlantic region. An active foraging behaviour leads to a direct interaction with other trophic levels making the shark a top down regulating predator in its distributional area. Therefore the Greenland shark should be considered as having a higher ecological significance than previously thought (Leclerc in prep, Fisk et al. 2002).
A commercial interest for Greenland sharks has arisen. The skin is used for clothing, and the fins are exported as food products. The catches are small compared to the historical catches of Greenland shark. It is believed that more than 50.000 sharks were caught in the first half of the 20th century (Anon. 1942) due to their liver which is rich in oil. Today the Greenland shark is listed on IUCN redlist as “near threatened” (IUCN redlist) and fundamental knowledge about the growth, age, migration patterns and population structure is not existent. Such knowledge is though absolutely essential to ensure a sustainable utilization. Researchers from Greenland Institute of Natural Resources, DTU AQUA and Copenhagen University will during the summer and autumn 2012 initiate projects with the aim of investigate age, growth patterns and population structure of Greenland sharks. Standard age determination methods such as counting growth rings and increments in othollits, which is used for bony fish, is not possible for large elasmobranchs such as Greenland sharks. Instead the researchers will use two independent methods, which has not been available previously when age determining Greenland sharks. These methods are aspartat racemization and carbon-14 dating. The former has been used to successfully age determining fin whales and Greenland whales (>100 years) (Garde et al. 2007) and the later method, has been used to age determine ocean quahoq (Arctica islandica). Ocean quahog is today considered to be the oldest animal on earth due to this method which found that some ocean quahoqs were approx 400 years old. Large individuals of Greenland sharks are considered by many scientists to be more than 200 years old, since its growth is extremely slow. This is partly based on the fact that the shark is coldblooded and lives in a very cold environment. Therefore its metabolism is expected to be very slow. This is confirmed by the only re-catch there have been of a tagged Greenland shark. This showed that the tagged shark only grew 8 cm in 16 years (Hansen 1963). With an maximum length around 7 meters the age is expected to be very high.
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