Contaminantenonderzoek en vliegtuigtellingen van bruinvissen (Phocoena phocoena) in Nederland, 2023

This annual report presents the results of the contaminant analyses on stranded harbour porpoises in 2023 and the results of the aerial surveys during SCANS-IV in the summer of 2022. The research has two main objectives: to quantify the contaminant load of harbour porpoises in the Dutch waters of the North Sea, and to quantify the numbers and distribution of harbour porpoises in the (Greater) North Sea. In 2023, blubber samples of adult male porpoises stranded on the Dutch coast were analysed for PCBs. Average PCB concentrations in adult male porpoises between 2014 and 2023 were 23.8 mg/kg lipid weight (lw) ±12.7 Σ22PCBs (3.8-62.5 mg/kg lw). Concentrations were higher in the south of the Netherlands (Eastern Scheldt, Western Scheldt and Delta) than in the north (Wadden Sea). The SCANS-IV survey in the summer of 2022 resulted in a density of 0.55 porpoise/km2, which corresponds to an abundance estimate of 339,000 individuals (CV = 0.17) in the Greater North Sea. This abundance estimate is similar to the abundance estimates from the previous SCANS surveys in 2005 and 2016, 345,000 (CV = 0.18) and 355,000 (CV = 0.22), respectively, while the number during the first SCANS survey in 1994 (289,000 CV = 0.14) was slightly lower. The southward shift of porpoises that started between 1994 and 2005 still continues.