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How do you identify a seal?
There are two ways to identify a seal. The first way is the 'capture-mark-recapture' method. This involves capturing a seal and marking it with a spray. We prefer to avoid this method. Because capturing a seal is very stressful. Stress changes the seal's behaviour and thus the results of the study.
Foto identification of seals in the Dollard
Our researchers therefore use the method photo identification. They take a photo of all seals coming out of the water. They do this immediately because the seal's fur is still wet. On the wet fur, the seal's pattern can be seen more clearly. For each photo, our researchers circle what strikes them most about the seal. Think patterns or scars. If the pattern matches photos taken before, we know it is the same seal.
To make this process faster and easier, they work with programme Photo-ID. A photo of a seal is put into the programme. The programme examines this photo and recognises the seal's patterns. Then the photo is automatically matched to one of the seals in the list.
Pattern is unique for every seal
Below is an example of how we can recognise seals. Near the arrows are prominent spots. As you can see in the photo, the fur has changed colour, but the spots are still visible. The arrow below his flipper points to three dots that are the same in both photos. The arrow to his head indicates a kind of white spot with four dots. Besides the programme automatically recognising the photos, it is important for us to double-check for accuracy ourselves. After all, we already have 400 seals in the database.
Identifying a seal
In the summer of 2021, there was a seal in De Onlanden – an area in the province of Drenthe. A special event, because you don't often see a seal in fresh water. After many hours of taking photos from a great distance, we managed to get a good picture of its tag number (this is on a small label on its rear flipper) and spot pattern. It turned out to be an old friend of us. The seal has a tag number 20-115. That's how we knew it was seal Thor. We also put the photo in the photo-ID programme. Through Bea and Marga's research, every seal that has been at the centre has been photographed.