Occurrence of Microplastics in Harbour Seals (Phoca vitulina) and Grey Seals (Halichoerus grypus) from German Waters

SIMPLE SUMMARY: It is well known that the occurrence of microplastics (MP) is ubiquitous in both water compartments and in biota. Nevertheless, knowledge on the microplastic exposure of marine mammals is scarce. This study gives first evidence of microplastic burden in seal species inhabiting German waters of the North Sea and Baltic Sea. Accordingly, 63 gastrointestinal tracts (GIT) of harbour seal (Phoca vitulina) and grey seal (Halichoerus grypus) carcasses were analysed. This investigation is urgently needed since information on microplastic burden in grey seals, especially from the Baltic Sea, are still lacking. This information is particularly important as it allows a direct comparison of this species and its sympatric species, the harbour seal, from different waters. A slightly higher incidence of microplastics (>100 µm, here categorized as fibres and fragments) was found in specimen found in the Baltic Sea. Moreover, higher microplastic loads were detected in stomachs compared to the intestinal samples, with first evidence of fragment retainment within the GIT. Nevertheless, an accumulation of microplastics in marine mammals could not be excluded for all categories since the egestion of those subjects is documented and no significant differences in life history parameters such as sex and age, nor parasite infestation were revealed. ABSTRACT: The level of knowledge on microplastic exposure in marine mammals is limited by the access to dead and alive individuals. Focusing on the Northeast Atlantic area, some studies already confirmed the microplastic presence in free-ranging marine mammals, such as harbour porpoises or harbour seals inhabiting the North Sea (NS). In contrast, knowledge on the exposure to grey seals and particularly on specimen inhabiting the Baltic Sea (BS) are scarce. This study examined 63 gastrointestinal tracts (GIT) of harbour seals and grey seals originating from German waters (NS and BS) found between 2014 and 2019. Besides the documentation of microplastic findings, this study is dealing with life history and health parameters, attempting to identify correlations with microplastic presence. This study confirmed beside the presence, the egestion of microplastics (>100 µm; MPs) in the examined seals, without correlations in parasite infestations or inflammation responses. 540 suspected MPs were identified in 62 intestinal samples (42% fibres, 58% fragments), and 228 MPs in seven stomachs (28% fibres, 72% fragments). In accordance, first evidence of the retainment of fragments in the GIT were given. However, no significant difference in MP occurrence was indicated for different sex or age groups.