A peek into the past – harbour porpoise strandings in the Netherlands during the mid-twentieth century

Between 1931-1975 two birdwatchers visited the North Sea beach along the central west coast of the Netherlands, independent of each other, but overlapping in both time and geography. On their repeated trips they not only noted the location and length of the stretch of beach they visited, but also the number of dead harbour porpoises (Phocoena phocoena). This enables us to calculate the density, i.e. the average number of stranded porpoises per kilometre, which is considered more informative than absolute numbers. The calculated densities of both observers are comparable, as are the monthly and yearly patterns. Combined with the data from the national cetacean strandings database, the porpoise strandings during the study period show a recurrent undulating pattern, also after this period, from 1964 until the late 1980s, when the density was considerably lower.