Harbour seal stuns scientists with epic swim
After a week of silence, hope was fading. Then the transmitter lit up – on the harbour seal tagged near Koster Island by professor Karin Hårding and her team of researchers.
And what it revealed was astonishing.
"We were very surprised," says Karin Hårding, professor of zoological ecology at the Department of Biology and Environmental Science, University of Gothenburg.
In mid-October, she and her research colleagues fitted transmitters to two female harbour seals on Ursholmen, just southwest of the Koster Islands. The seal team travelled to the island by boat from the Tjärnö Marine Laboratory with the aim of finding out more about the harbour seals' habitat and challenges.
"Even though harbour seals initially recovered in numbers after the seal deaths, we have seen a gradual decline in the population over the last ten years. They are not producing as many pups as they should. We have a number of studies showing that they are eating more small fish and hardly any fatty herring or cod, as they did in the past. They are struggling to meet their daily energy requirements because the herring is missing," says Karin Hårding.
Seal expeditions in the cold and dark
The work is part of the ongoing Marhab project, which aims to preserve and improve marine ecosystems in the Skagerrak and Kattegat. During the autumn, two trips were made from the Tjärnö Marine Laboratory to Ursholmen, where researchers climbed around on the rocks in the dark and cold and set out nets equipped with floats and light lower edges. When the seals get caught, they twist the net together and are kept at the surface, explains Karin Hårdning.
"We have also brought a vet with us to ensure that it is as painless as possible for the seals, and we have an effective protocol when they are measured and weighed on land, so that they can be released by us seal biologists as quickly as possible."
The transmitters stick to the seals' fur and fall off after a while, at the latest when they moult in August, but often earlier. They send data that show how the seal swims both on the surface and how deep it dives, and over which areas it moves.
"This is information that we can then compile with other data such as the depth of the sea, the nature of the seabed and what fish are in the area. In this way, we can learn a great deal about harbour seals, how they live and how they are doing," says Karin Hårding.
First to Grebbestad and then to Norway
One of the female seals swam into Dynekilen, just north of Strömstad. She has remained there since mid-October, diving and swimming around, sometimes climbing onto a rock to rest.
The other swam back and forth between Koster and Grebbestad five times. Then she disappeared.
"It was quiet for eight days, and we thought the transmitter had fallen off prematurely. But then lots of data started coming in," says Karin Hårding.
The results were a big surprise. The harbour seal had swum straight across the Skagerrak to Arendal in Norway. As the crow flies, it is about 130 kilometres.
"It is an enormous swim across open and deep sea. She has been swimming for eight days without being on land. We know that seals can swim long distances, but they usually follow the coast," says Karin Hårding.
"This is the first time we have transmitters on harbour seals from Koster, and everything we are learning now is new and important. How seals move affects how diseases such as seal death can spread and is important for management. It can also influence decisions on hunting and shows that Norwegian and Swedish authorities need to cooperate so as not to count the same seal twice.
What happens now?
"We have four transmitters left to deploy, but we will do that in the spring when we have better weather. Now it has become far too cold and stormy, and we had to return to Tjärnö. But in the spring, we will stay until we are done," says Karin Hårding.
Text: Mikael Andersson
The most common seal on the Swedish West coast.
Adults grow to about 135–155 cm in length and weigh between 50 and 90 kg. Males are larger than females.
Found along the entire west coast of Sweden and give birth on remote islands such as those around Koster, Väderöarna, Onsala, and Hallands Väderö, where they can nurse their pups undisturbed on land.
Feeds mainly on fish and sometimes octopus.
They are still hunted, even though the population is slowly declining.
Mass deaths occurred in 1988 and 2002, caused by a virus that affected all harbour seal colonies along the European coastline, including both the Kattegat and Skagerrak seas. On the Swedish west coast, 65% of all harbour seals died in a single summer.
