Cetaceans stranded in the Netherlands in 2015-2019

Abstract

This report presents a validated list of stranded cetaceans in the Netherlands, as well as some cetaceans accidentally brought in on ship bulbs, between 2015-2019. During this period 2701 cetaceans representing eleven species were reported. The list also includes a few bones and skulls, among others of killer whale (Orcinus orca) and bottlenose dolphin (Tursiops truncatus), species not reported otherwise during this period. The most common species was harbour porpoise (Phocoena phocoena), with 2651 individuals. The average number of porpoises per year was 530, lower than the preceding seven-year average, but numbers fluctuated largely between years and there has been no particular trend after the strong increase in the early 2000s. The monthly pattern of strandings as recorded during the previous periods, with peaks in March and July-September, remained the same. The number of stranded porpoise is equally spread along the entire coastline. The density, expressed as the number of stranded porpoise per kilometre per year, is 0.6 for the entire coastline, or 1.2 if the extensive and less well surveyed areas of Western Scheldt, Eastern Scheldt and Wadden Sea proper are omitted. Sex ratio remained stable over the years and is in line with results from before 2015, with a preponderance of males. On the basis of length, over half were immature and 8.7% neonate. Among neonates and immatures there was a preponderance of males as well, but not in adults. In the Wadden Sea area more neonates and adults were found than in the other two subareas. The major cause of death was infectious disease, followed by predation by grey seal (Halichoerus grypus). Accidental by-catch was identified as the cause of death of 11% of the stranded animals. Of particular interest was the stranding of six sperm whales (Physeter macrocephalus) in January 2016, the largest stranding event of this species in the Netherlands. It was part of a stranding event spread out over the Central and southern North Sea, involving thirty individuals. During 2015-2019 four dead fin whales (Balaenoptera physalus) and eight minke whales (B. acutorostrata) were reported, several of which were hit by ships. There seems to be a slight increase in the strandings of fin whale and Sowerby’s beaked whale (Mesoplodon bidens) since 2000, while white-beaked dolphin (Lagenorhynchus albirostris) shows a decrease. There is no temporal trend for any of the other species since 2000.