The number of harbour seals born in the Koster Sea has more than halved since 2012. To understand why, researchers at the University of Gothenburg are now using drones and AI to follow the pups' early life.
When harbour seal populations on the West Coast was collapsing in the mid-20th century, it was mainly due to excessive hunting, sanctioned by bounties. When hunting was banned, the population gradually recovered and today harbour seals can be seen again in many places along the west coast. However, new threats to the seal have emerged. Intensive fishing by large industrial trawlers from many nations has led to the collapse of cod and herring stocks, causing harbour seals to shift their diet towards other less nutritious fish species, according to preliminary results from the University of Gothenburg. The researchers suspect that seals may now spend more and more time looking for food, which affects both growth and reproduction.
In Kosterhavet National Park, researchers from the University of Gothenburg have noted a decline in the number of harbour seals being born.
Declining trend
"There is a declining trend from year to year. In 2012, about 400 seal pups were born on the skerries and islands of the national park. Last year we counted 170 pups," says Daire Carroll, biologist and researcher at the University of Gothenburg.
Previously, researchers had to be content with counting the seals from a distance, but with the help of modern technology, they are now able to conduct more detailed studies that can provide new ways to monitor seals. By photographing the seal colonies with drones in June, when the female seals give birth to this year's pups, and then returning at the end of August, the researchers can assess pup growth down at the population level.
"We take pictures from a high altitude so as not to disturb the seals, and with the help of AI we can estimate the size and weight of the pups with a margin of error of just 3 kilos. A pup weighing 24kg at the end of August is much more likely to survive the winter than those weighing only 15-18kg. How much weight they gain in their first few months depends largely on the health of their mothers, and the availability of fish near the seal's haul-out sites," says Daire Carroll.
Developed in Slottsskogen
The AI model used was developed by the researchers in collaboration with Slottsskogen Zoo. When a harbour seal was born in the Seal Pond, the true growth of the pup was linked to images taken by a drone above the pond. In addition, harbour seals have unique patterns in their fur that make it possible to distinguish between individuals. AI models have also been trained to do this.
"It is difficult to assess the body condition of a seal with binoculars from a bobbing boat. Therefore, we have previously relied on surveys of dead seals conducted in Stockholm to be able to see growth and health problems, but it was about 20 seals per year. With our drone and AI model, we can examine several hundred animals in a day. Last year we covered about 87 per cent of all harbour seal pups in Kosterhavet National Park," says Daire Carroll.
Since the drone surveys began in 2021, the researchers have been able to follow the development of the seal population. It seems that only the females that have managed to accumulate substantial fat deposits give birth to a pup each year. The leaner seals only have a pup every other year, which is nature's way of slowing down population growth when food is scarce. Female seals rely on their own survival, waiting for the environment to improve.
People contribute
"Females are the most important individuals for the survival of the colony. Those females that are fit enough have a pup every year, but they are steadily becoming fewer. Let's see if we can see a positive trend reversal this year. We hope that fishing quotas will be lowered, and more marine reserves will be established so that there will be some good years soon and the seal population will stop declining," says Daire Carroll.
The method of monitoring seals with drones and AI has spread to other research groups. SLU is currently conducting a similar project on harbour seals on Hallands Väderö, and in Estonia researchers are investigating how grey seals are affected by the decline in sea ice in recent decades. They become more crowded without ice, which negatively affects the growth of the pups.
Thanks to the AI model, researchers can also use photos of seals taken by private individuals. Last year, Darie Carroll received over 800 photos of seals from the public in the Sälfie-ID project.
"Keep sending in your photos, thanks to the AI model we can identify many of the seals in the photos to the individual level. Thanks to the Sälfie-ID project, we can understand patterns in the behaviour of the seals and how they move along the entire coastline," says Daire Carroll.
Scientific article: Approaching a population-level assessment of body size in pinnipeds using drones, an early warning of environmental degradation
