Decline of Long-tailed Duck Clangula hyemalis numbers in the Pomeranian Bay revealed by two different survey methods

Abstract

Abundance of Long-tailed Ducks Clangula hyemalis wintering in the Pomeranian Bay was monitored between 1988 and 2014, using both ship-based and aerial surveys and cor-recting for distance dependent detection. Aerial surveys were conducted using an im-proved transect division and a double observer design to estimate detection probability near the transect line. As the latter probability was considerably below 1, we applied an additional correction factor for observer efficiency. After correcting for observer effi-ciency in aerial surveys, the two methodological approaches yielded similar densities, though an apparent underestimation in aerial surveys warrants further cross-validation. Density estimates from both platforms were merged for further analysis. After reaching peak levels in 1992 and 1993, Long-tailed Duck winter densities in the Pomeranian Bay declined by 82% until 2010. This decline was part of an overall decline in numbers throughout the Baltic Sea. An apparent increase since 2010, which was likely due to local ice-induced movements, indicates that habitats in the study area may still support high densities today.