Evaluating the distribution and density of harbour porpoises (Phocoena phocoena) in selected areas in German waters

Abstract

The harbour porpoise (Phocoena phocoena) is a small cetacean species occurring both in the German North Sea and Baltic Sea. In the process of designating marine protected areas in the framework of the European Habitats Directive (NATURA 2000), the German Federal Agency of Nature Conservation (BfN) identified candidate areas to be eventually proposed as Sites of Community Importance (pSCI). To evaluate the importance of these sites for harbour porpoises, their distribution and density were studied by conducting aerial surveys in the sites from May 2002 to September 2003 (further surveys are ongoing). Densities in the study areas were compared between study years as well as between the selected areas. The relative importance of sites was assessed by taking into account the overall distribution of porpoises in German waters. Surveys followed the standard line-transect methodology for aerial surveys. Only summer flights in the period from May to August were used for further analysis since the coverage by flights in autumn and winter was very low due to unfavourable weather conditions. In the German North Sea, 338 sightings of porpoise groups (440 individuals in total) were recorded in the summer of 2002, and 656 sightings (812 individuals in total) in the summer of 2003. In the Baltic Sea, sighting numbers in the same period were much smaller: 50 sightings (110 individuals) in 2002 and 34 sightings (43 individuals) in 2003. The main results showed clear aggregations and high densities of porpoises in the areas off the North Friesian islands of Sylt and Amrum, where there are high concentrations of the species in the summer months, which is their reproduction period. There seems to be a sharp gradient of density running from north to south. The highest density in both years was found in the study area Sylt Outer Reef (Sylter Außenriff), followed by the Doggerbank. Lowest densities were calculated for Borkum Reef Ground (Borkum-Riffgrund). The mean density did not differ significantly between study years in the same area. Harbour porpoise distribution in the Baltic Sea showed higher densities in the western part, namely in the Kiel Bight (Kieler Bucht) and Flensburg Fjord (Flensburger Förde), and in the eastern part close to the border of Poland. But all sightings east of the island of Rügen (study area Pommeranian Bay (Pommersche Bucht)) were only made in 2002. Thus, there is an enormous variation in the presence of habour porpoise in this area between the years. Currently surveys continue to determine how this area is used by harbour porpoises. Besides this, a clear west-east gradient in harbour porpoise density could be ascertained. The other two Baltic Sea study areas Fehmarn Belt (Fehmarnbelt) and Kadet Trench (Kadetrinne) are also used by porpoises, especially the area around the island of Fehmarn, but due to the small sizes of the areas additional investigation methods are applied, such as stationary acoustics (see chapter 12).