They followed one seal that frequently travelled from Western Scheldt to Antwerp and one that travelled from the Wadden Sea and crossed over to Scotland. See the clips.
A seal in the Western Scheldt and the harbour of Antwerp
Swimming patterns of the seal in the North Sea
The researchers Sophie Brasseur, Geert Aarts and Roger Kirkwood of Imares at Texel equipped seals with transmitters. They did this to find out where they are located when they are not on the sandbank in the Wadden Sea or sunning on the ‘Razende Bol’. To their surprise the seals make long journeys. The movement patterns of the seal in Antwerp was completed in two months, the seal from the Wadden Sea swam a retour journey to Scotland in eight months.