Plastic ingestion by Cory's and Scopoli's shearwaters (Calonectris spp.) from the north-eastern Atlantic Ocean and Mediterranean Sea

Plastic ingestion by seabirds reflects plastic levels in their marine environment and therefore seabirds are monitored within the EU Marine Strategy Framework Directive to track the distribution and trends of plastic pollution. We present plastic ingestion data from 529 individuals of two Calonectris species (C. borealis and C. diomedea) from corpses collected across the north-eastern Atlantic Ocean and the Mediterranean Sea. Overall, birds from the Mediterranean ingested more plastics than those from the Atlantic, and fledglings carried higher plastic loads than post-fledglings. In contrast to an earlier proposal to monitor and define plastic ingestion thresholds by the number of particles, we advocate a mass-based system. Plastic mass better reflects environmental contamination and biological harm than particle counts, which can be inflated by fragmentation and are less ecologically relevant than the actual volume ingested. Using the cleanest 10 % of individuals in the most polluted population as a reference, we suggest Threshold values of 0.0098 g for fledglings and 0.0041 g for post-fledglings. According to this definition, between 40 and 88 % of birds exceeded the Threshold, depending on the sampling location. This mass-based Threshold offers an ecologically meaningful metric, and we recommend this approach for plastic pollution monitoring in the northeast Atlantic and the Mediterranean.