Cetaceans stranded in the Netherlands in 2020-2023
Authors: Guido O. Keijl, Emmy Venema, Pepijn Kamminga & Lonneke L. IJsseldijk
https://doi.org/10.63269/SJL5810
Abstract: Between January 2020 and December 2023 a total of 1940 (live and) dead stranded cetaceans were reported in the Netherlands, belonging to 14 species. With 1896 individuals, harbour porpoise (Phocoena phocoena) was the most commonly reported species. Most porpoises were found in the south-west of the country (Delta), but highest densities were encountered on the Wadden Sea islands. An Unusual Mortality Event (UME) occurred between 23 August and 4 September 2021, when 197 porpoises washed ashore on the Wadden Sea islands. They possibly originated from the Dogger Bank area and appeared to have died from a bacterial infection with Erysipelothrix rusiopathiae. Due to the UME, the strandings density of porpoises was especially high in the Wadden Sea area: 1.5 (individuals / km / yr), compared to 0.7 along the mainland coast and 0.2 in the Delta area. Despite the UME, the countrywide density decreased from 0.6 during 2015-2019 to 0.4 in 2020-2023. Since the unusually high number of strandings during 2011-2013, the average number of strandings per year is steadily decreasing. The proportion of males is decreasing (59% in 1998-2007, 56.2% in 2020-2023), the proportion of neonates is increasing (12.4% versus 14.9%). About 50 fresh porpoises per year are necropsied. Causes of death have changed slightly (bycatch from 16% during 2014-2019 to 12%, grey seal (Halichoerus grypus) predation from 24% to 19%, malnutrition/starvation from 17% to 15%, infectious disease from 32% to 37%), but whether this is influenced by selecting only fresh porpoises for necropsy is yet unclear. Rare species stranded were killer whale (Orcinus orca) and bottlenose whale (Hyperoodon ampullatus). The number of species is slowly increasing, the number of individuals other than porpoise only marginally. Other species were minke whale (Balaenoptera acutorostrata), fin whale (B. physalus), humpback whale (Megaptera novaeangliae), common dolphin (Delphinus delphis), striped dolphin (Stenella coeruleoalba), and Sowerby’s beaked whale (Mesoplodon bidens), all of which seem to be increasing (as strandings) in the south-eastern North Sea. Also reported, but decreasing as strandings, is white-beaked dolphin (Lagenorhynchus albirostris). Species stranded, but not showing any long term trend, during this period are long-finned pilot whale (Globicephala melas), white-sided dolphin (Leucopleurus acutus), sperm whale (Physeter macrocephalus) and bottlenose dolphin (Tursiops truncatus). The number of stranded cetaceans left unidentified increased from 17% since 2000 to 66% since 2020, for various reasons. Naturalis Biodiversity Center has stopped registering cetacean strandings as of 16 January 2024. Strandings can now be reported at and/or requested from www.stranding.nl. On this new platform the archive of www.walvisstrandingen.nl can also be found.
