Plastic in digestive tracts and gills of cod and herring from the Baltic Sea

Plastic litter is widespread on our planet and is recognized as a contaminant of high concern. Plastic ingestion and retention in gills were studied in two key Baltic fish species: herring (Clupea harengus L.) and cod (Gadus morhua L.). In total, 183 fish from the southern Baltic Sea were analysed. Plastic litter was found in digestive tracts of 12.7 and 14.8% of herrings and cods, respectively. In addition, gills were shown to constitute an important transfer route of plastic to cod (9.9% of cods) but not to herring thus likely reflecting species-specific differences in fish lifestyles and/or water filtering capacity. No more than one plastic item per individual was found except for three fish with two items in their stomachs. Dominant microplastics (<5 mm) (MPs) (88.6% of all items) composed of irregular fragments, fibres and foils (61.3%, 25.8% and 12.9% of all MPs, respectively) were followed by meso- (5-25 mm) and macro-plastics (>25 mm) (5.7% each of all items). Plastic ingestion and retention in gills did not affect fish body condition assessed by Fulton's K index, although herrings that ingested plastic particles beyond the microplastic size range (i.e. >5 mm) were characterized by the lowest condition indices. All plastic items recovered from fish exhibited clear signs of weathering and surface extended due to their roughness and fragmentation which may enhance both sorption properties for chemical compounds and/or their leaching. Nevertheless, chemical contaminant levels in herring muscles were not different than in a plastic-free group. The vast majority of fibres found in fish was identified as air-borne or procedural contamination and were excluded from further analyses. In addition, the vessel's paint dust found in fish stressed the need for strict quality assurance also during fish sampling.