| Summary: | Between 1970 and 2005 sightings data of cetaceans in the southern North Sea were collected as part of the Marine Mammal Database of the Dutch Seabird Group. The data include incidental sightings and reports as well as results from systematic surveys and seawatching data. They are therefore difficult to correct for fluctuations in observer effort. The material was evaluated firstly to see which species occur in Dutch waters and, secondly, to see if any spatial and temporal trends in abundance can be observed. In this study the harbour porpoise (Phocoena phocoena) is excluded from the analysis, except in the discussion, where the results are put into context with data about strandings and overall abundance. The status of the 17 different species that have been recorded in the southern North Sea between 1970 and 2005 was evaluated based on the frequency of occurrence of sightings (dead and alive) in the last 36 years. Two species were listed as resident (harbour porpoise and whitebeaked dolphin (Lagenorhynchus albirostris)), because they were abundant in 36 and 34 years respectively. The bottlenose dolphin (Tursiops truncatus), recorded in 23 years, was listed as a regular visitor or passage migrant. Ten species were qualified as irregular visitors (represented in 4-18 of the 36 years of study) and four as vagrants (recorded in <4 years). The frequency of sightings of cetaceans has generally increased between 1970 and 2005, but it is unclear how the increased popularity of cetaceans by the general public (i.e. more reports) has contributed to that trend. From effort-corrected data this trend seems genuine, but offshore surveys suggest more variable results.
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