Researchers track the mysterious Greenland shark
Off the coast of Sweden swims the Greenland shark - a fish that can grow to more than five metres long and weigh over a tonne. To find out more about this mysterious species, researchers from the Swedish University of Agricultural Sciences (SLU), in collaboration with the University of Gothenburg, are catching and tagging the sharks.
Joakim Larsson is not only a professor of environmental pharmacology at the University of Gothenburg, he is also a marine biologist and a keen angler. During a fishing trip almost three years ago with Joakim's boat ‘Darwin’, a large Greenland shark was hooked by his son Max.The shark was 405 centimetres long and weighed an estimated 700 kilos, making it the largest fish ever caught on a rod off the Swedish coast. The experience sparked the idea for a new research project.
"Since 2017, I and many other anglers have been assisting a project where researchers from SLU are tagging tuna with advanced electronic tags to find out more about their ecology. After our shark catch, the researchers at SLU asked if I would be interested in doing something similar with them on Greenland shark," says Joakim Larsson.
New information through tagging
Two large sharks were tagged in the first attempt in summer 2022. Since then, the team has caught around 20 sharks that have been tagged with transmitters. From the information gathered, the researchers hope to learn more about shark movement patterns and behaviour.
"Previously, it was thought that the Greenland shark was a species that was rare in our waters, but I think they were simply looking too shallow. There is no targeted fishing for Greenland shark, but now that we have adapted the location, equipment and fishing technique for dogfish during the project, we have caught and tagged sharks in almost every attempt, sometimes several per trip," says Joakim Larsson.
To catch the sharks, the researchers use a rod and reel, which, according to Joakim Larsson, is the safest way to catch them. They use a special circle hook that normally sets in the corner of the shark's mouth and thus causes as little damage as possible. Unlike most bony fish, sharks have no swim bladder, so they can be pulled up from great depths without being harmed.
When they surface, they are measured, samples are taken for DNA analysis and they are tagged with various types of electronic tags, including satellite transmitters that are programmed to release from the shark after a predetermined time.
"The transmitter then floats to the surface and sends its position and stored data on the depths travelled by the shark. Depending on the type of tag, you can also get data on, for example, acceleration, which can provide additional information about their movement patterns and hunting behaviour," says Joakim Larsson.
There is much that is unknown about the life of the mysterious shark. Studies suggest that they can live up to 400 years and that they do not reach sexual maturity until they are around 100 years old. This means that the species is likely to be very sensitive to the impact of commercial fishing. However, the shark is not a sought-after food fish because its meat is toxic, but it is sometimes caught as by-catch in trawls in northern waters. In Iceland, there is a tradition of fermenting the meat for several months to make it edible, but the market for Greenland shark is still very limited.
"Håkarl, as it is called in Icelandic, has a penetrating, very strong odour and taste of ammonia," says Joakim Larsson, who has tried it himself.
Living at great depths
Greenland sharks usually live at great depths, preferably around 1 000 metres. Skagerrak is considerably shallower, but apparently deep enough for sharks to thrive in the deepest parts. The shark moves slowly and many older individuals are thought to be blind due to an eye parasite. Despite this, parts of fast-swimming fish, seals and even small whales have been found in their stomachs.
"In the past, it was believed that the Greenland shark was largely a scavenger. With the help of the electronic tags we have fitted to the sharks, we have collected data that will better shed light on how they search for food," says Joakim Larsson.
The research project, led by the Swedish University of Agricultural Sciences in Umeå and including Gustav Hellström, Thomas Brodin, Petter Lundberg and Daniel Palm, is now compiling data for a scientific publication. Joakim Larsson does not want to reveal in detail what they have found until the material is published.
"I can say that it is exciting information that at least surprised me a lot," he says.
Text: Karl-Johan Nylén
The shark's scientific name, Somniosus microcephalus, can be freely translated as ‘the sleepy one with the small brain’. The Greenland shark is the second largest predatory shark in the world after the great white. It can grow to more than five metres in length and weigh over 1 500 kg. It usually lives at great depths in cold water and has been observed as deep as 2 600 metres. The shark is an omnivore that eats a wide variety of fish species, as well as seals, seabirds, cephalopods and crabs.