The harbour porpoise (Phocoena phocoena) is the most abundant marine mammal species in Dutch waters. Aerial surveys along predesigned track lines in 2012-2017 provided density and abundance estimates for the Dutch Continental Shelf (DCS) in spring (Mar/Apr 2012, 2013) and summer (Jul 2014, 2015, 2017). The abundance estimates in spring (n=63,408-66,685) were in the same order of magnitude as summer (n=41,299-76,773). Distribution patterns of porpoises differed between seasons and years, but a band of higher densities from the southern part of the DCS to the area north of the western Wadden Isles was visible in all seasons. Calves were only seen in July. The total abundance estimates in spring and summer correspond to a maximum of 17-21% and 7-23% of the southern North Sea population respectively, which implies that a large part of the North Sea population resides in Dutch waters during both seasons.
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