Written by Gitte Paulsbo
A pair of large, round eyes look up from the surface of the water and disappear into the depths. Diego is sharpening his whiskers. His chubby body swings gracefully along the seabed, he knows where he's going; Diego on home ground. The fish in front of him creeps between the sea plants and tries to escape, but Diego maneuvers elegantly after it. He follows the fish with his eyes, sneaks up on it and gobbles it up. It's over in a second. He rushes to the surface and settles on a rocky outcrop. Satisfied, he spreads out on the warm rock and lets the sun stroke his wet fur before he has to move on and paddle down into the fjord again.
This is where Carla Freitas Brandt comes in. She is a researcher at the Institute of Marine Research, and for the past two years she and her colleagues have been following Diego and the other seals in the Oslofjord, to learn more about what they eat and where they move. Diego has moved on for the day, but on the rock he has left a report for Carla.
- "We have collected stool samples from the seals and found ear stones from fish. These tell us what kind of fish the seals eat," says the researcher.
Diego belongs to the harbor seal species, a medium-sized seal species with one of the largest distribution areas for seals in Norway and the world. Harbor seals can be spotted from the more northerly parts of the Pacific and Atlantic oceans all the way up to Svalbard. The individuals in Svalbard are the world's most northerly harbour seal population and are totally protected. Harbor seals have been widespread in Scandinavia for over 300 years, but it has been a struggle for the big-eyed marine mammals to survive.
Sealing has left its mark on the population over the centuries, and in recent decades hunting and viral diseases have almost wiped out the population in the Skagerrak. The fishing industry has long been concerned that seals are eating up coastal cod, and has previously called for a bounty on seals in Norway.
The Institute of Marine Research wanted to find out if the seal actually is a threat to the coastal cod population, and initiated a research project where they have monitored the seal population in the Skagerrak through transmitters attached to the fur of the harbor seals.
- Around August, the seals shed their fur. We then glue the transmitters to their fur and hope that they transmit for a maximum of one year, until they change their fur the next time and the transmitter falls off," says Freitas, adding that the transmitters are small and hydrodynamic and that the harbor seals will probably not notice that they are there.
It is important that the seals behave exactly as they would without transmitters in order for the Institute of Marine Research to learn as much as possible about seal behavior.
Diego belongs to the second group of seals Carla and her colleagues have tracked. The first pool was a group of seals at Bolærne that they tracked from fall 2019 to 2020. Of the seals in Diego's pool, which were tagged at Jomfruland, only he is still sending data to the Institute of Marine Research. The rest of the seals have lost their transmitters.
So what can the stool samples report?
- We found that the seals mainly eat flatfish, island eel and sypike. Only two percent of the fish they had eaten was coastal cod. In total, this represents only five percent of the amount used for commercial fishing," says Carla Freitas.
So it's not Diego and his friends who are to blame for the disappearance of coastal cod.
An action plan was recently drawn up to clean up the Oslofjord and Skagerrak. Water quality will be improved and measures will be taken to better protect the fjord's biodiversity. Can seals tell us anything about the state of life under water?
- The fact that seals are thriving in the Oslofjord is a sign that there is enough food, at least when it comes to non-commercial species, which in turn is a good sign for the ecosystem. The fact that top predators choose to stay in an area often indicates that the rest of the food chain is doing well," says Carla.
So there is hope for the inhabitants of the fjord, but we still have some way to go. Freitas also says that there are very few seals in the Skagerrak now, compared to the rest of the Norwegian coast and our neighboring countries:
- In the Skagerrak, from the Swedish border to Lindesnes, there are around 1,000 individuals. That's not very many. In Sweden and Denmark, for example, there are tens of thousands. When you observe seals, you might be fooled into thinking that there are a lot of seals in one area, but it could be the same seals moving across several areas.
Diego absolutely loves to travel. As soon as he can, he sets off for a swim across both national and county borders at breakneck speed:
- A journey between Jomfruland and Sweden takes less than 24 hours. We've seen seals that swim that distance, rest for a while on land in Sweden, and then travel exactly the same distance back to Jomfruland in less than 24 hours. They are more mobile than we expected, and it's exciting to see that they navigate so well. It could be a combination of genetic imprinting, navigating according to the environment and learning from each other.
Diego pulls himself up on a rocky mountain in Vestfold. He squeezes in between a couple of other wet clumps of fur that are thawing in the sunlight. They close their eyes, lower their heads onto the smooth rock surface and exhale. A seagull lands on the rock and looks at the group taking life with striking calm. Perhaps it's wondering where the fish are. Diego opens half an eye and barely notices the poultry that has come to share the sunbathing area before dozing off again. Soon he will dive down to 90 meters after dinner, but first he has to finish sunbathing.
Read more about The Institute of Marine Research's research on seals in Norway.
Our skippers are good at spotting seals, and just like that, guests on KOKCRUISE get to see these fun animals. It's a real treat when it happens! For more opportunities to see seals in the Oslofjord:
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