| Summary: | The developments of the harbour seal Phoca vitulina population in the Wadden Sea are monitored via aerial surveys according to the Trilateral Seal Agreement concluded by the responsible authorities of Denmark, Schleswig Holstein, Niedersachsen and the Netherlands. A survey scheme was designed taking into account the conservation objectives formulated in the Trilateral Seal Agreement and the requirements under the EU Habitats Directive. To meet those objectives and requirements we followed the OSPAR guideline, which aims at detecting with a power of 80% and a probability of 5%, a change in abundance of minimally 10% in 10 years (amounting to a decrease of 1.05% per year). A power analysis was carried out based on the within and between variation in the annual seal counts during the moulting period in August in the period 1989 to 2001. This analysis indicates that the current monitoring programme consisting of two surveys in August, has insufficient power to detect an annual decrease of 1.05 percent at 0.05 significance. These two surveys enable only to detect 2% decrease over a 10-year period, and moreover with a significance of only 0.10. Should the variation in counts increase, e.g. because the population approaches carrying capacity, one will quickly have less than 80% power to detect changes. This implies an increase in the probability of wrongly concluding that no decrease has happened. To meet the set requirements, monitoring should be increased to at least 4 counts in August throughout the entire Wadden Sea. If a smaller area will be surveyed, e.g. by using independently only data for the sub-regions Denmark, Schleswig Holstein, Niedersachsen or the Netherlands, more surveys per year in each area would be needed to detect the same decrease with similar power. We therefore conclude that the choice is to either increase the present number of surveys or accept that the chance for detecting a decrease will be less than set by internationally proposed guidelines. In any case, there is every reason to continue ...
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