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Microplastics not harmful to fish

According to the scientific findings of a group of researchers led by fisheries ecologist Jörn Scharsack from the Thünen Institute in Bremerhaven, the quantities of microplastics ingested by fish in the North Sea and Baltic Sea do not lead to adverse effects on fish health. According to the scientists, eating fish from the two seas also does not pose a health risk to humans in this respect.


As part of the "Plastic Litter in Seafish" (PlasM) project, which was funded by the German Federal Ministry of Food and Agriculture (BMEL), the Thünen Institute research team conducted several studies to investigate the extent to which microplastics affect the health of fish. The goal of the project was to better assess the risk posed to fish by plastics in the marine environment. It reportedly included the testing for microplastics of fish species from the North and Baltic Seas, as well as the analysis and determination of microplastics in fish in order to be able to assess the amounts of microplastics found in the sea. Impact studies with exposure experiments aimed to provide information on the consequences of microplastic ingestion on fish health.


As a first step, the research team examined plastic debris and fish from samples collected from the North and Baltic Seas with a bottom trawl net. The collected plastics were categorised by polymer type using an infrared spectroscopy procedure. Especially the polymers polyethylene, polypropylene and polyamide were detected. Whether the wild fish caught had ingested microplastics with their food was determined in laboratory tests on the digestive tracts of dab and herring.


In the fish sampled, it was only in the digestive tracts of the dab flatfish species that the researchers were able to detect fewer than ten microplastic particles per fish. Analysis of the microplastic particles using infrared imaging spectroscopy revealed that polypropylene was the most common plastic in the fish's digestive tracts.


In another experiment, the progeny of three-spined sticklebacks caught at the mouth of the River Weser near Bremerhaven served as test animals. The researchers gave the sticklebacks a diet laced with polyester fibres for nine weeks. For their study, they chose smaller microplastic fibres (< 0.3 mm), comparable to fibres that come from household washing and enter the environment as detergent effluent. Smaller fibres have often been neglected in monitoring studies, according to Scharsack and his team, so that little is known about their potential impact on the environment. For comparison, another stickleback group was fed diets with additions of natural cotton fibres, and a third group received diets with no added fibres at all. The concentration of microplastic fibres in the feed varied from 0.2 to 2 milligrams per gram of feed. With the higher microplastic concentrations, the scientists wanted to establish whether the growth or health of the fish might be affected if the pollution of the seas with microplastics should continue to increase. Subsequent examination of the fish, their internal organs, immune cells and intestinal tract showed no effects on growth or health, regardless of whether the fish had been fed diets with a little or a lot of microplastic fibres, with natural fibres, or without any added fibres at all. The microplastic fibres, like the cotton fibres, were excreted through the intestinal tract, as revealed by microscopic faecal examinations.


To find out how microplastics affect the reproduction and the early development stages of fish, the researchers at the Thünen Institute conducted another experiment in which half of the egg clutches of sticklebacks were exposed before fertilisation to microplastic concentrations three to four times higher than those found in the North Sea and Baltic Sea, while the other half were exposed during the rearing phase. Here, too, polyester fibres of PET were used. In this way, the scientists wanted to test whether microplastic fibres in the water could affect the fertilisation rates of fish eggs and the subsequent development of embryos and larvae. They studied the growth and maturation of the fish embryos, as well as their immune systems, to identify potentially harmful effects from ingesting the microplastic fibres. According to the study, the results of the laboratory experiments show that microplastic fibres in the water also do not affect the fertilisation and early development of fish embryos and larvae.


As a conclusion from their study results, the scientists at the Thünen Institute do not expect any significant harm to the fish even at relatively high microplastic concentrations in the sea and also do not see any health risk to humans due to microplastics from fish consumption.


According to Scharsack, the study results are also transferable to other fish and even to other vertebrates because the basic structures of the intestinal system are comparable. "I would go so far as to say that, in principle, they can be applied to all vertebrates. Our research certainly does not show that the increasing amount of litter in the oceans is not a problem, it is just that we don’t have any concrete evidence that the ingestion of microplastics affects the health of fish or inhibits their  development," said Scharsack.




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