Cruise reports
Antarctic 07/08
Ecosystem survey of the Barents Sea (BESS
A Greenland shark of about 2.5 m is set out after rapid sampling and tagging.
Photo: Julius Nielsen
Hunting a giant Arctic fish
In recent weeks, the Institute of Marine Research (IMR) has put considerable effort into the annual Arctic Ocean survey in the area north of Svalbard. With RV "Helmer Hanssen" as the platform, the marine ecosystem was upended and samples and measurements were taken of everything from the chemical composition of microscopic amphipods to jellyfish, fish fry and larger fish such as cod and redfish.
Julius Nielsen, University of Copenhagen
Published: 19.09.2015 - Updated: 21.09.2015
A new feature this year was to carry out targeted fishing for one of the ocean's mysterious giants, namely the Greenland shark. With a maximum size in excess of five metres, the Greenland shark is among the largest carnivorous sharks and the biggest fish native to Arctic waters. Moreover, the Greenland shark is regarded as being one of the world's longest living animals and in, all likelihood, can live well over 100 years.
Risen interest
Scientific interest in the Greenland shark has risen in recent years. This is due to the notion of this shark having changed from being a lazy scavenger to now being regarded as a potential predator at the top of the food chain that actively captures fast-swimming fish such as cod and Greenland halibut, as well as seal. Consequently, the Greenland shark presumably has a far more important impact in the ocean than previously thought and, as such, it is of great interest to include it in the surveys IMR performs.
Although the Greenland sharks did not provide much resistance, handling them can be difficult so they were placed on a metal stretcher.
Photo: Jenny Bortoluzzi
The collection of data on Greenland sharks is accomplished using large hooks baited with seal blubber or fish. They are mounted on a demersal longline and fishing takes place for around 12 hours at a depth of 300–500 m. Although it is expected that there are large quantities of Greenland shark in these parts of the Arctic, everyone were surprised when the first line was taken on board. A full 20 Greenland sharks were caught with just 40 hooks. The largest specimen was 3.8 m long and weighed 470 kg. The Greenland sharks that were not injured were tagged and set free so they can be identified if they are recaptured elsewhere in the North Atlantic. Specimens were also taken on board to undergo more detailed investigations, including the stomach contents, age determination and tests to map Greenland shark's role in the Arctic ecosystem.
Cod, Greenland halibut and seal
The stomach content of the Greenland sharks caught during the Arctic Ocean survey contained a lot of cod, Greenland halibut, wolffish, seal and in one instance the remains of a polar bear. The possibility of the Greenland shark actively hunting polar bears must be considered unlikely. In all likelihood, the remains stem from a drowned animal. Nevertheless, it is not impossible that a swimming and weakened bear, which may be bleeding, can be attacked and eaten by a Greenland shark. Overall, the stomach content from this Arctic Ocean survey confirms the existing image of the "lazy" Greenland shark as an active predator, but further analysis should be conducted and thus the sampling helps to reduce the mystery of this great shark species.
The Greenland shark's distinctive circular gap.
The Greenland shark's choice of food and hunting strategy have for many years been surrounded by great mystery. This is due to the fact that Greenland sharks appear to be lazy when they are taken as bycatch on long lines, on gill nets or in trawls. As a consequence of this, it is extremely difficult (if not impossible) to imagine that the Greenland shark would be able to capture the fast-swimming animals often found in their stomachs. Despite the species' opportunistic food choices (i.e. it is omnivorous), the researchers do not believe that eating dead animals is the Greenland shark's primary hunting strategy. True enough, a Greenland shark has not been observed in the act of hunting. Consequently, this conception is based on evidence (indirect proof) by analyses of stomach contents. The composition of prey from previous studies from Canada, Greenland and Svalbard shows that cod, Greenland halibut and seal are the species' primary prey. When recently eaten specimens of these species are found in the stomachs, one would expect many scavenging small animals (amphipods, snails, starfish, etc.) if the prey was found in a dead state on the seabed.
Active predator
Observations of carnivorous small animals are not rare, but not nearly sufficient to assume that the Greenland shark only eats dead seals and fish. The stomach analyses indicate that the species' feeding strategy is more complex. In addition, observations have been made of living white whales and seals with distinctive "round" bite marks, which almost certainly can only stem from Greenland shark. This suggests that the Greenland shark is an active predator and, as such, far more important in the marine ecosystem than so far assumed.
Parts of seal bodies found in shark's stomach.
Collection of data on the Greenland shark during this year's Arctic Ocean survey is part of a larger Greenland shark project undertaken by PhD student Julius Nielsen from the University of Copenhagen. The project is also working closely with the Greenland Institute of Natural Resources Nature Institute of Greenland, UiT – The Arctic University of Norway, Den Blå Planet - Danmarks Akvarium and Andørja Adventures. The latter will be the platform for the first targeted satellite tagging project of sexually mature Greenland shark (over four metres in length), which will be implemented in October this year.