Optimizing survey design for Scandinavian harbour seals: population trend as an ecological quality element

To be successful, conservation and management programmes require accurate data on abundance and population trends. Noise caused by within- and among-year variance should be minimized to optimize the statistical power for detecting changes in abundance. A total of 30 years of monitoring data from seven distinct subpopulations of harbour seals (Phoca vitulina) in southern Scandinavia was used to investigate the relative contributions of factors affecting the power to detect trends in abundance. The power is typically doubled under the conditions tested when carrying out annual surveys compared with every second year. The power also increases substantially when carrying out replicate surveys during the annual moult. The gain in power increases steeply up to three annual replicates, but then levels off, and it is further increased when the mean of the two highest counts of three annually repeated counts is used. We propose that harbour seal haul-out sites are surveyed every year during the moult, with at least three replicate surveys per year. This would provide robust data for analyses of population trends, facilitating management and identification of potential influences of diseases and anthropogenic: activities.