Trends of harbour porpoise (Phocoena phocoena) density in the southern North Sea
In the southern North Sea, harbour porpoise occurrence increased in recent years after a phase of low abundances during earlier decades. Only very few studies on porpoise presence in the southern German North Sea exist so far. As anthropogenic activities will strongly increase in this part of the North Sea during the next years it is most important to assess population level effects. This study focuses on the analysis of temporal and spatial trends in porpoise density in this area of recent change. Dedicated aerial line-transect distance sampling surveys were conducted in the southern German North Sea between May 2002 and June 2013 to assess porpoise density and distribution. Statistical inferences on porpoise population trends were made using a Markov Chain Monte Carlo (MCMC) technique. Two approaches were chosen to test for a trend in porpoise density and an additional model focused on the change indensity of calves. During 55,820 km of survey effort 4377 porpoises including 140 calves were recorded. A significant effect of increasing spatial aggregation from the lower density areas in the south-eastern German Bight to hot spot areas in the western parts was detected. For the western part of the study area asignificant increase in porpoise density between 2002 and 2013 was detected. Seasons were significantly different with highest porpoise density in spring and successively decreasing densities in summer and autumn. From 2008 onwards high densities were also observed in summer. Calf density increased during the study period and was significantly higher in the west. On the basis of this extensive and unique dataset on porpoise occurrence in the southern German North Sea the findings clearly show that especially the south-western German North Sea serves as habitat of increasing importance for porpoises throughout the last decade. Definite reasons still remain unresolved. Changes in prey abundance or less favourable conditions in other areas could be important factors, which may also have caused a southward shift fromhigh density areas in northern waters. On this baseline, further integrative approaches might lead to a sound understanding of the effect of anthropogenic activities on the future development of porpoise populations.