The harbour porpoise (Phocoena phocoena) in the central German Bight: Phenology, abundance and distribution in 2002-2004

Abstract

The harbour porpoise (Phocoena phocoena) is widely distributed in the North Sea. During both the SCANS 1994 and 2005 surveys, porpoises were commonly encountered in offshore waters, for example in the central German Bight. However, information on year-round abundance and distribution of harbour porpoises in that area on a monthly basis was lacking. Between 2002 and 2004, we undertook 26 aerial line-transect surveys in a 2,600 km2 area in the central German Bight, 100 km north of the Island of Borkum (Eastern Frisia). Data were analysed with DISTANCE software. A total of 406 porpoises were sighted. Sighting rates (=sightings/km transect) peaked in July 2002, February, May and September 2003, and in January and April 2004. Absolute densities (g (0) corrected) ranged between 0.14 and 1.54 animals/km2 (peak in April 2004). Proportion of calves varied between 3.4 and 27.3%. Our results show a highly irregular appearance of harbour porpoises in the study area with no apparent seasonal trends in occurrence but peaks in single months. We propose that the area is used as a transit route with harbour porpoise moving in from regions of high density in summer (Northern Frisia) and early spring (Eastern Frisia). © Springer-Verlag and AWI 2007.