Occurrence and potential effects of plastic ingestion by pelagic and demersal fish from the North Sea and Baltic Sea

Over the last decades synthetic polymers, so called plastics, became one of our most indispensable tools used in nearly all of our daily applications. The littering and loss of plastic waste is nowadays becoming a threat to wildlife since the material is not biodegradable and may persist for centuries. About 6.4 million tons of plastic enter the oceans annually, which undergo degradation into smaller fragments by weathering, wave abrasion and UV radiation during their residence on sea. Large debris, so called macroplastics (>5 mm size) represent a risk to larger marine animals mostly by entanglement. Ingestion of plastics is believed to cause a false sensation of satiation or internal injuries and blockage. In contrast, microplastic (<5mm size) is reported to be available for a variety of marine biota by ingestion and could hereby enter the food web at low trophic levels. Previous studies have shown that fish, also in the North Sea, ingests plastic particles, mostly in the microplastic size range, but effects are still uncertain. Feeding guild-specific as well as species-specific plastic ingestion by fish needs to be investigated to evaluate the susceptibility and amount of ingestion by fish. \nIn this study 290 gastrointestinal tracts of the fish species cod (Gadus morhua L.), dab (Limanda limanda L.), flounder (Platichthys flesus L.), herring (Clupea harengus L.) and mackerel (Scomber scombrus L.) from the North and Baltic Sea were investigated for the occurrence of plastic ingestion. The visually pre-sorted plastic particles were verified by identifying the polymer type applying attenuated total reflection (ATR) and focal plane array (FPA) based micro Fourier transform infrared (FT-IR) spectroscopy. Circa 5.5 % of all analysed fishes had ingested plastics. PE was the predominantly recorded polymer whereby nearly 74 % of all plastic particles were in the microplastic size range. In 3.4 % of the demersal and 10.7 % of the pelagic feeding individuals plastic ingestion was recorded showing a significant difference in the amount of ingestion between the two feeding habits of the investigated fishes. This result is likely due to the high ingestion rate of nearly 18 % of the investigated pelagic feeding mackerels. The condition factor K and the hepato somatic index (HSI) were applied to test differences in the health status between individuals that had and had not ingested plastic. No direct effect could be recorded in individuals that had ingested microplastics. A large rubber strap detected in a cod possibly led to internal blockage which resulted in a low individual K and HSI. Microplastics appeared to pass the digestive tract without any injury. Mortally affected individuals which died in the field remain undetected by our method applied and further investigations for better estimations of the frequency of plastic ingestion by fish and the possible impact it might have need to be addressed in future work.