Lutra 56(1)_Boonstra et al_2013

Harbour porpoises (Phocoena phocoena) in the Marsdiep area, the Netherlands: new investigations in a historical study area

The harbour porpoise (Phocoena phocoena) returned in Dutch waters in the late 20th century after a nearabsence of approximately three decades. Inspired by historical studies of harbour porpoises in the Marsdiep area (western Wadden Sea), mainly in the 1930s and 1940s, a study was initiated in the same area in 2011 to see if porpoises are now as common as previously recorded and if future, more detailed investigations would be worthwhile. We investigated the current spatial and temporal distribution of harbour porpoises using a combination of visual observations and acoustic recordings. In 2010 and 2011, harbour porpoises were most abundant around mid-March, and disappeared in April. They were most frequently observed off Texel, with slightly lower numbers of visual detections along the dike of Den Helder and at Huisduinen. Relatively high abundances were recorded around high tide at most observation sites, but particularly so off Texel. The observed abundance of porpoises in spring in the Marsdiep area, and the apparent influence of tidal currents on spatial patterns in sightings frequencies, is such that the Marsdiep area could provide rather unique, future study opportunities for harbour porpoises in the wild.