The harbour seal counts are synchronised to the degree possible between the three Wadden Sea countries, Denmark, Germany and the Netherlands, in order to obtain a combined estimate for the number of harbour seals and newborn pups in the entire Wadden Sea and Helgoland. Harbour seals are counted when hauling out on land and counts are scheduled to be carried out when low tide occurs around midday. The counts are carried out on five coordinated dates. The counts are carried out on five coordinated dates: three counts during the pupping season in June and two during the moulting season in August. In June the number of pups is counted and in August the total number of harbour seals is counted in the respective regions.The variation in the number of seals hauling out from year to year and over longer periods 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 each year over a long period (Reijnders et al. 2010). It is unclear if this shift continues in the Wadden Sea and if there is also a shift in timing of the moult. Additional studies are needed to determine when the peak in birth and moult occurs.