New Research Project Studies the Health of Gray Seal Pups in the Baltic Sea

In a joint project, Swedish University of Agricultural Sciences, Swedish Museum of Natural History and University of Gothenburg will measure the condition of gray seal pups in the Baltic Sea. The project will run for three years.

Using drones, the researchers will photograph the pups in the Baltic Sea to measure their length and girth. They can then see whether the pups are skinny or fat, giving them an estimation of their health. The project complements the existing seal population survey, where researchers count seal individuals from a helicopter. 

– Monitoring of gray seal pups is very limited. We don’t know anything about the condition of the pups because this type of analysis has never been done before. The aerial survey of the seal population has also never been followed up, so we don’t know how many of the pups we previously counted that survived, says Karl Lundström, researcher at SLU Aqua.

The nursery of the archipelago

Gray seals give birth to their pups between February and March. At that time, the Stockholm archipelago turns into a noisy nursery, with pups, females and males crowding the islets and skerries in a blissful mess.

Among the sounds of shouting, calling and howling, the researchers need to watch their step as the pups are born with a white, fluffy coat that they shed two to four weeks after birth. The fur is a camouflage to protect them from predators if they are born on ice, and a cold winter like this, they can easily hide in the white snow.

This is an intense period for gray seals, especially for the females. As well as giving birth to their pups, which they then nurse for three weeks, the males wait for them in the water. They are ready to mate. Once mating is over and the females have fed their pups, it’s time to leave.

But the pups remain, as they are fasting for about a month. Then it’s time for them to start hunting their own food, which includes herring, sprat, whitefish and flounder.

Drone flights in Stockholm and Estonia

With this project, the researchers also want to compare the condition of the gray seal pups in Stockholm with the rest of the Baltic Sea. When Anja Carlsson and Karl Lundström are conducting drone flights in the Stockholm archipelago, researchers from the University of Gothenburg are conducting drone flights across the inland sea in Estonia.

– We can then compare whether there is a difference between the pups in Stockholm and pups in Estonia, how big they are and when they are born, says Anja Carlsson, seal researcher at the Swedish Museum of Natural History.

By visiting the same area several times, Anja also hopes to see when most pups are born.

– During the return visits, we can see how many pups that still have their white fur and which ones that have lost it. We can then calculate a rough estimate of when the peak of the birthing period occurs.

One centimeter of margin

The researchers use simple drones that fly at around forty meters of altitude. There, it takes overlapping images of the locations where the pups lie. The images are then stitched together to form a single large image, which shows the pups with high accuracy.

– The images are so detailed that we can measure the length and width of the pups within one centimeter, give or take. Using a special model, we can also convert length and width into weight. This gives us a picture of the condition of the pups when they are weaned from their mothers, says Karl Lundström.

Most pups are born in the archipelago

Seventy percent of the pups born in Swedish waters are centered in the Stockholm archipelago. An area affected by the extended trawl limit that came into force in February 2025. By examining the seal population in this area, the researchers also hope to see if the change affects the condition of the pups.

– The trawl limit may mean that food availability will change. If the availability of herring improves, we can see if the condition of the pups also changes over time and improves, says Lundström.

Reduced number of individuals

The gray seal population has increased throughout the Baltic Sea. But in Stockholm, the researchers have noticed that the individuals have decreased quite drastically since 2020.

While the number of individuals has decreased in the archipelago, they have increased in the Archipelago Sea. Anja Carlsson believes that increased hunting, tourism and changes in access to feed may be some of the reasons why individuals have declined and why they have chosen to move.

– Gray seals are fairly loyal to their home range. So when we no longer see the same number of individuals in places where we have seen them for years, it’s not a good sign. 

To know how dedicated gray seals are to their premises, Karl Lundström also hopes that the images will help them identify the females to see if they come back to the same areas. 

– This way we can see how dedicated they are to the places where they give birth to their pups. 

Seals shows the condition of the ocean

Seals are top consumers of the food chain. Anja Carlsson therefore believes that seals shows how the ocean is doing. According to her, it is an early warning sign if something is changing. – That’s why it’s so exciting that we’ve started this project now. It’s like a missing piece of the puzzle because it will show health, not just numbers. The health of the pups is a reflection of the health of the females, which in turn reflects the fish stocks. So any change will be seen much earlier now, she concludes.