Abundance of harbour porpoises (Phocoena phocoena) on the Dutch Continental Shelf, aerial surveys in July 2010-March 2011
The harbour porpoise (Phocoena phocoena) is the most abundant marine mammal species in Dutch waters. Nevertheless until 2010 abundance estimates for the entire Dutch Continental Shelf (DCS) were missing. Aerial surveys along designed track lines in July 2010, October/November 2010 and March 2011 provided density and abundance estimates for the DCS. The highest abundance estimate was made in March 2011 (n=85,572); approximately three times higher than in July 2010 (n=25,998) and October/November 2010 (n=29,963). Distribution patterns of porpoises differed between seasons, but a band of higher densities from the Brown Ridge to the Borkumer Reef was visible in all seasons. Calves were mainly seen in July, indicating that porpoises also reproduce in Dutch waters. The total abundance estimate in March 2011 corresponds to 48% of the southern North Sea population, which implies that a large part of the North Sea population resides in Dutch waters during that season. Such high densities may lead to increased conflict with human activities, making the instigation of local management actions more imminent.