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A study has found that 28% of fish living in the Thames Estuary have eaten microplastics.
Research into the UK's plastic pollution also revealed that 39% of fish in the Firth of Clyde estuary in Scotland were similarly affected.
The research was led by Alex McGoran, a London NERC DTP PhD student based at Royal Holloway (University of London) and the Museum, with contributions from others including Dr Paul Clark and Prof David Morritt.
Microplastics - pieces of plastic smaller than five millimetres - are having an enormous impact on some UK estuaries and the fish that live in them.
Alex examined the digestive tracts of 876 fish and shrimp from the Thames and Firth of Clyde. Of the 21 species investigated, 14 had microplastics in their guts.
Alex says, 'People have started to really take note of the severity of plastic pollution and our research further demonstrates why this is such pressing issue. Both rivers are extremely diverse ecosystems, home to hundreds of different species.
'To see this large number of species that our plastic waste is putting in danger is actually rather shocking.
'Our results show the need for more research into freshwater and estuarine ecosystems to be carried out so we can better understand the effects microplastics are having on their inhabitants.'
There are 155 estuaries in Britain. Although microplastics can accumulate in particularly high densities in estuaries, not much research has been completed into their effects on these environments - most research focuses on damage to the oceans.
The Thames Estuary and the Firth of Clyde are both important habitats and nurseries for fish. But both rivers are near to major sources of plastic pollution: large cities and shipping lanes.
The Thames is home to 950 species, including 112 fish species. Flatfish, especially flounder, are particularly common in rivers across Europe, and they have been used in environmental studies for decades.
The European flounder (Platichthys flesus) is an important species for measuring the health of the Thames, but past studies on the fish have not yielded happy results. In a 2017 study, Alex found that up to 75% of sampled flounder had fibres in the gut.
In this latest study, Alex found that 33% of flatfish, 19% of other bottom-dwelling fish, 14% of mid-water fish and 6% of brown shrimp had eaten plastic.
Significantly more fish from the Firth of Clyde (39%) had eaten plastic than those from the Thames (28%). The average number of plastic pieces ingested by fish, however, did not differ between the two sites.
Alex found the most common polymers recovered were nylon and polyester, both of which are used extensively in the textile industry.
Fishing gear is also often made of nylon, and polyester is a major component of wet wipes. In just one month, the citizen science project Thames21 recovered more than 5,000 wet wipes from the foreshore of the Thames.
Fibres made up the vast majority of the plastic that Alex found, rather than larger pieces like food wrappers.
Alex says, 'These fibres are so small that we can't tell where they have come from. Products and byproducts of the textile and fishing industries could be responsible for much of the pollution in the Thames Estuary and Firth of Clyde, but we don't know for sure.
'Washing machines are likely to be a major contributing factor, as tiny fibres from clothes make it through the system and into rivers. Sewage treatment works are good at filtering them out, but the sheer volume of waste water means they don't catch everything, and many treatment plants do not have filters small enough to remove microplastics.'
Alex's next project will examine microplastics in the food chain, using specimens from the Thames Estuary.
Dr Paul Clark says, 'Assuming current trends continue then the total amount of plastic produced by 2050 will be 33 billion tonnes. Therefore the amount of plastic litter polluting our beautiful blue planet will dramatically escalate over the coming years.
'Plastic pollution is on the same calamitous magnitude as climate change and deforestation. We are in need of a monumental behavioural change in human attitudes.
'What I find most depressing about plastic pollution of our aquatic environment is that it is now irreversible and its presence will persist for many generations.'
Small lifestyle changes help to reduce the plastic levels in rivers around the UK.
Don't flush sanitary products or any kind of wet wipe down toilets, even the ones which claim to be flushable.
Alex says, 'Flushable does not mean biodegradable. Flushing anything down the toilet other than human waste and toilet paper increases the chances of microplastics in the water system.'
Products for washing machines are also available that are designed to catch excess microfibres coming from clothes.
You can cut down on single-use plastic by purchasing reusable water bottles, coffee cups and lunchboxes.
Alex adds, 'There are many alternatives to plastic out there, although they may be less convenient. However, the more we avoid buying plastic products, the more companies will start to think about sustainable solutions.'