Date Published: January 5, 2018
Publisher: Springer Netherlands
Author(s): Jan A. van Franeker, Elisa L. Bravo Rebolledo, Eileen Hesse, Lonneke L. IJsseldijk, Susanne Kühn, Mardik Leopold, Lara Mielke.
Stomach contents of harbour porpoises (Phocoena phocoena) collected in the Netherlands between 2003 and 2013 were inspected for the presence of plastic and other man-made litter. In 654 stomach samples the frequency of occurrence of plastic litter was 7% with less than 0.5% additional presence of non-synthetic man-made litter. However, we show that when a dedicated standard protocol for the detection of litter is followed, a considerably higher percentage (15% of 81 harbour porpoise stomachs from the period 2010–2013) contained plastic litter. Results thus strongly depended on methods used and time period considered. Occurrence of litter in the stomach was correlated to the presence of other non-food remains like stones, shells, bog-wood, etc., suggesting that litter was often ingested accidentally when the animals foraged close to the bottom. Most items were small and were not considered to have had a major health impact. No evident differences in ingestion were found between sexes or age groups, with the exception that neonates contained no litter. Polyethylene and polypropylene were the most common plastic types encountered. Compared to earlier literature on the harbour porpoise and related species, our results suggest higher levels of ingestion of litter. This is largely due to the lack of dedicated protocols to investigate marine litter ingestion in previous studies. Still, the low frequency of ingestion, and minor number and mass of litter items found in harbour porpoises in the relatively polluted southern North Sea indicates that the species is not a strong candidate for annual monitoring of marine litter trends under the EU marine strategy framework directive. However, for longer-term comparisons and regional differences, with proper dedicated protocols applied, the harbour porpoise has specific use in quantifying litter presence in the, for that specific objective, poorly studied benthic marine habitat.
The wide distribution and abundance of man-made litter, in particular plastics, has been signalled as a major threat to the oceans (UNEP 2011, 2014; CBD 2016) which affects a broad range of marine organisms through entanglement and ingestion (Kühn et al. 2015). Marine litter has long been an important issue in the Oslo-Paris Convention for the Protection of the Marine Environment of the North-East Atlantic (OSPAR) and more recently has become strongly embedded also in European Union (EU) policy. In the marine strategy framework directive (MSFD), the European Commission (EC) calls on its member states to achieve a ‘Good Environmental Status’ (GES) in all European seas by the year 2020 (EC 2008, 2010). Among the many environmental aspects of MSFD aiming at ecological conservation and sustainable use of marine resources, ‘Descriptor 10’ addresses marine litter, in which GES is defined as the situation where “marine litter does not cause harm to the coastal and marine environment”. How this should be interpreted and dealt with in terms of assessments is being addressed further by a specialist group (Galgani et al. 2010; MSFD-TSGML 2011, 2013; Werner et al. 2016), and regional planning in order to achieve GES is underway (e.g. OSPAR 2014).
Stomachs of 654 harbour porpoises found dead on the Dutch coast between 2003 and 2013 were analysed (Table 1); two necropsied individuals were from year 2003, 268 from years 2005 to 2009 and 384 from years 2010 to 2014. Overall 7% of the animals were neonates/calves, 72% were ‘juveniles’, and 21% were adults. Nutritional condition varied within all age classes, reflecting a mix of cases that suffered fairly instant mortality (those in normal to good nutritional condition) to animals that had been starving for a longer period of time. Roughly 80% of the stomachs contained food remains. Food remains were scarcely found in neonates/calves, showing many had not yet started independent feeding.Table 1Sample composition by age class and frequencies of occurrence (%FO) of main stomach contents for all harbour porpoise stomach samples, irrespective of method of analysisnAvg body length cm ± SDAvg condition score ± SD%FO litter%FO plastic%FO foodNeonate4781± 63.6± 1.40.00.021Juvenile469110± 103.6± 1.57.77.081Adult137146± 103.3± 1.48.08.083Unknown1––000All654116± 203.5± 184.108.40.2068
Over the full study period 2003–2013, and combining different methods, 7% of 654 beached harbour porpoises from the Netherlands had plastic in the stomach. However, using a dedicated protocol with microscopic inspection of all remains from the rinsing beaker and from the 1 mm sieve showed that 15% of 82 harbour porpoises (81 from the 2010–2013 period) had ingested plastic. Group comparisons using the z test as well as GLMM analyses of individual data confirmed a highly significant difference in results between the two methods. The dedicated protocol complies with the ones established in the MSFD for the formal monitoring of plastic ingestion by fulmars (MSFD-TSGML 2013; OSPAR 2015) and marine turtles (Matiddi et al. 2017). We strongly recommend this standard protocol to be applied to marine mammals in diet studies where quantification of marine litter is included.
Using a standard approach to quantify ingested (plastic) litter in stomachs of harbour porpoises in the Netherlands, a 15% frequency of occurrence of mostly small plastic litter items was observed. In a polluted area such as the southern North Sea, such frequency indicates that ingestion of litter currently represents no major hazard for this species. The low frequency of ingested litter makes the harbour porpoise a less practical species for targeted annual monitoring of change under the EU MSFD as this would require large yearly sample sizes to be processed. However, harbour porpoises, like seals, can provide a measure for plastic abundance in a poorly studied compartment of the water column, that is the water–sediment interface. Future work, in combination with ongoing diet studies, could thus indicate regional differences and long-term temporal changes in benthic litter abundance. The dataset in this paper has clearly demonstrated that such is only possible if data are collected from adequately sized samples using a protocol standardised to established MSFD monitoring efforts. The basic element of the standardisation is to not rely on visual detection of litter during autopsies or rinsing procedures, but to sieve all stomach contents over a 1 mm mesh and inspect all remains under binocular microscope.