The Greenland shark can live for several centuries and is one of the most mysterious inhabitants of the Arctic. A new study now shows that these animals can cope remarkably well with accidental capture on longlines, provided they are subsequently released humanely.
Greenland sharks (Somniosus microcephalus) grow extremely slowly and reach sexual maturity late in life. This makes bycatch all the more problematic, for example in the black halibut fishery in Arctic waters.
A research team led by Yuuki Watanabe therefore studied twelve Greenland sharks in remote fjords of the Canadian Arctic. Blood samples were taken from the animals and transmitters fitted before they were released. This enabled the researchers to monitor both their physical stress response and their behaviour following capture.
Surprisingly, the sharks exhibited comparatively low stress responses. After their release, they initially swam faster than usual, but after around ten hours their behaviour had returned to normal. The survival rate over the first 30 days was at least 90 per cent.
One explanation could be the Greenland sharks’ unusually calm nature. They appear to put up less of a struggle against the hook than many other shark species and thus expend less energy. The low temperatures of Arctic waters may also reduce the stress they experience.
Nevertheless, by-catch is not harmless. Due to their extremely slow growth, additional losses to the population can have serious consequences. However, the results show that a swift and gentle release can significantly improve the chances of survival for these exceptionally long-lived inhabitants of the Arctic.
Heiner Kubny PolarJournal