Marine mammal surveys in Dutch North Sea waters in 2019

Aerial surveys to estimate the abundance of Harbour Porpoise Phocoena phocoena were conducted on the Dutch Continental Shelf in summer 2019. These surveys followed predetermined track lines in four areas: A - Dogger Bank, B - Offshore, C - Frisian Front & D - Delta. Between 16 July and 4 August the entire Dutch Continental Shelf (DCS) was surveyed. Marine mammals were assessed using line transect distance sampling methods. Density and abundance estimates were calculated. In total, 150 sightings of 189 individual Harbour Porpoises were collected. Porpoise densities varied between 0.54-1.76 animals/km² in the areas A-D. The overall density was 0.66 animals/km². The lowest density (0.46 animals/km²) was recorded in area A – Dogger Bank. The densities in the other areas were in the same order of magnitude, ranging between 0.68-071 animals/km². In summer 2019 the total number of Harbour Porpoises on the Dutch Continental Shelf (areas A-D) was estimated at 38,911 individuals (CI = 20,791-76,822). This estimates falls in the range of abundance estimates since 2010, with a minimum of 25,998 (CI = 13,988 – 53,623 in 2010) and a maximum of 76,773 (CI = 43,414-154,265 in 2014) individuals. The confidence intervals of the abundance estimates overlap, indicating no statistically significant differences between the years. The time series, however, is relatively short to measure trends. These abundance estimates show that up to a fifth of the North Sea population, estimated at 345,000-361,000 individuals, has been present on the Dutch Continental Shelf during the summer surveys in 2010-2019. The results of these aerial surveys will feed into the OSPAR MSFD indicator on abundance and distribution of marine mammals. In total 26 sightings of other marine mammal species than Harbour Porpoises were recorded. These comprised 22 sightings of seals (Grey Seal Halichoerus grypus and Harbour Seal Phoca vitulina). The majority of the seals was observed in coastal waters off the Wadden Isles. Three single Minke Whales Balaenoptera acutorostrata were seen (feeding) in area A – Dogger Bank and B – Offshore, with another one sighted off effort in the same area. One sighting of a pod of two White-beaked Dolphins Lagenorhynchus albirostris was made in area B – Offshore. This research is part of the BO-project ‘monitoring bruinvis’.