To obtain annual estimates of the number of harbour seals and newborn pups in the entire Wadden Sea and Helgoland, seal counts are synchronized to the degree possible between the three Wadden Sea countries: Denmark, Germany and the Netherlands. Seals are counted when hauling out on land and counts are scheduled to be carried out when low tide occurs around midday. The number of seals hauling out is 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). This may lead to variation in counts from year to year and potentially over several years. Also, during 1974-2009, the timing of birth shifted towards an earlier maximum pupping date, potentially affecting the percentage of pups counted at the same time of year over a long period (Reijnders et al. 2010).