15% of dead Dutch harbour porpoises have ingested plastic in stomachWageningen Marine Research
Wageningen Marine Research has a long track record in study of the diet of beached harbour porpoises (pdf; 15.7 MB). Non-food items in the stomachs were recorded, but not following a strict method suitable dedicated to plastic monitoring. Following this early approach, about 7% of the animals were noted to have plastic in the stomach.
In more recently analysed samples of stomachs, the diet analyses were supplemented with the standard method used in the EU for monitoring plastic abundance in the stomachs of northern fulmars and marine turtles. This addition implies that all stomach contents are filtered over a 1 millimeter sieve and studied under the microscope. Using this standard approach, plastics were recorded in 15% of the animals.
Studies from other countries bordering the North Sea report far less plastics, at most 1 to 2%. But usually those studies only visually inspected stomachs during autopsies. In order to obtain a proper view of regional differences or trends over time, the standard method with the 1 millimeter sieve will be necessary.
The findings suggest that harbour porpoises mainly accidentally ingest plastics when foraging on fish close to the bottom. In many cases, harbour porpoises that contained plastics, had also ingested other natural bottom debris, such as shell fragments and bog-wood (semifossil wood derived from old surfaced peat layers in the bottom of the North Sea). Stomach contents of northern fulmars mainly reflect plastics pollution near the water surface, but harbour porpoises, like seals, could thus act as a relative indicator for plastic abundance near the bottom of the sea.
The research article can be downloaded here: Plastic ingestion by harbour porpoises Phocoena phocoena in the Netherlands: establishing a standardized method(pdf; 4.7 MB)