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|Recent trends and spatial patterns in nearshore sightings of harbour porpoises (Phocoena phocoena) in the Netherlands (Southern Bight, North Sea), 1990-2010|
Camphuysen, K.C.J. (2011). Recent trends and spatial patterns in nearshore sightings of harbour porpoises (Phocoena phocoena) in the Netherlands (Southern Bight, North Sea), 1990-2010. Lutra 54(1): 39-47
In: Rijksmuseum voor Natuurlijke Historie Lutra (Leiden). Brill: Leiden. ISSN 0024-7634, meer
Cetacea [WoRMS]; Phocoena phocoena (Linnaeus, 1758) [WoRMS]
Cetacea, Phocoena phocoena, sightings, seasonality, trends, Netherlands
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The harbour porpoise (Phocoena phocoena) returned in Dutch waters in the late 20th century after a near-absence of three decades. Gradually increasing numbers of sightings in the early 1990s were followed by a proportional rate of increase of 41% per annum over a period of 15 years until 2003. More data have been collected since and this paper reports on the most recent trends, changes in seasonality and spatial patterns in nearshore sightings between 1990 and 2010. The results show that the rapid increase came to a halt, but current sightings frequencies are on a higher level than before 2005. The highest numbers are now recorded in the northern half of Noord-Holland, whereas sightings rates in Zuid-Holland have declined. Sightings were made almost year-round, but with very low frequencies in May and June. During July-November a gradual increase was found, followed by some stabilisation in numbers during December-January and a marked further increase in February-March. The frequency in sightings crashed every year in early April. Additional (ecological) research is needed to enhance our understanding of the observed patterns and to link the nearshore sightings data with offshore population assessments during aerial surveys.