Mediterranean seabirds carry very high amounts of plastic in their stomachs

The results provide important insights into how Europe monitors marine plastic pollution. The researchers propose new threshold values that can be used within the EU Marine Strategy Framework Directive (MSFD) and the OSPAR Convention.

Birds in the Mediterranean most affected

The study found that birds from the north-western Mediterranean had, on average, up to five times more plastic in their stomachs than shearwaters from Atlantic regions, such as the Azores and the Canary Islands. Although the overall amount of plastic in these species is lower than in the northern fulmar (Fulmarus glacialis) – the species WUR has monitored for decades – the regional differences are clearly visible. Recently fledged young birds were found to have particularly high amounts of plastic in their stomachs.

According to first author Jan Andries van Franeker, the differences were greater than expected: "The Mediterranean is already known as a polluted region, but the amounts of plastic we found in these birds were still remarkably high. These birds show very directly what is happening beneath the water surface, in a world we can hardly monitor ourselves."

Notably, the plastic particles found in shearwaters are much smaller than the pieces typically found in northern fulmars. This suggests that the birds most likely ingest much of the plastic indirectly, via the fish they prey on.

Plastic mass tells more than particle numbers

An important element of the study is the recommendation to assess plastic in seabird stomachs on the basis of total mass, rather than the number of individual pieces. According to the researchers, the mass of ingested plastics provides a much better indication of the actual ecological impact.

The proposed threshold values are 0.0098 grams of plastic for young birds and 0.0041 grams for older birds. The suggestion is that regions in which more than ten per cent of the examined birds exceed these values should, under the EU MSFD, not be considered to have achieved good environmental status.

Last author Suse Kühn, marine ecologist at Wageningen Marine Research, emphasises why this mass-based approach is important: "The weight determines how much space the plastic occupies in the stomach, and that is directly linked to the harm it can cause. Mass is therefore a much more robust measure for monitoring and policy."

Building on decades of WUR expertise

The new threshold values align with the long-running Fulmar Monitoring Programme, in which WUR has played an international role for many years. The experience gained in this programme forms the basis for expanding monitoring to other seabird species in more southerly regions. Because separate threshold values have been established for young and older birds, both age groups can be used in monitoring. As with the fulmar monitoring, all data are being shared with OSPAR so they can be incorporated into international monitoring programmes for marine pollution.

For Van Franeker, who has been involved in research on plastic in seabirds for many years, this work represents an important milestone: "We once started with a single species in the North Sea. To see that this work now contributes to developing a European reference framework is very special. My hope is that these insights will help contribute to cleaner seas – for birds, for fish, and ultimately for ourselves."

Building block for European policy

With the proposed threshold values, EU Member States can monitor developments in marine plastic pollution in a uniform and ecologically meaningful way. The study therefore represents an important building block for achieving European environmental objectives and for protecting vulnerable marine ecosystems.

More information

• Read the article Plastic ingestion by Cory’s and Scopoli’s shearwaters (Calonectris spp.) from the north-eastern Atlantic Ocean and Mediterranean Sea in the scientific journal Marine Pollution Bulletin.

Text: Wageningen Marine Research
Images: Shutterstock (lead photo: young Cory's shearwater
(Calonectris borealis), Gran Canaria, Canary Islands, Spain); Jan Andries van Franeker; Erwin Kompanje