2019 Harbour seal report: Trilateral surveys of Harbour Seals in the Wadden Sea and Helgoland in 2019

The seal counts of the entire Wadden Sea are synchronized to the degree possible between the three Wadden Sea countries, Denmark, Germany and the Netherlands, in order to obtain a single estimate for the number of harbour seals and newborn pups in the entire Wadden Sea. Seals are counted when hauling out on land and counts are scheduled to be carried out when low tide occurs around midday. The variation in the number of seals hauling out from year to year and over a longer period may be affected by different weather conditions, disturbance, distance to food patches, or a change in the age and sex composition of the population (Härkönen et al.1999). Also, the timing of birth has been shown to change over time, potentially affecting the percentage of pups counted at the same time of year over a long period (Reijnders et al.2010). It is unclear if and how this shift might also affect the moult counts. Additional studies are needed to determine if a further shift in timing has occurred