Research Shows Thames Fish Have Ingested Microplastic

This post was first published on the Thames Estuary Partnership website.

Last year, a published study by Royal Holloway, University of London (RHUL) and the Natural History Museum (NHM) found that in 2014 up to 75% of the flounder in the Thames Estuary had ingested plastic. In a second study, RHUL-NHM, in conjunction with Field Studies Council Millport, has now discovered that more species are ingesting plastic in the Thames and in the Firth of Clyde.

Large volumes of plastic are rolling along the riverbed, in just three months, nearly 8,500 pieces of litter were recovered from the Thames (Morritt et al., 2014). It is now known that bottom-dwelling fish are ingesting a lot of this debris (up to 75%, McGoran et al., 2017, RHUL), but just how many species are affected by plastics in UK catchments?

The four sample sites (Thamesmead, Erith, Isle or Grain & Firth of Clyde) used in the study and an example of the plastics ingested by the fish collected.
Photo by Alexandra McGoran. Map produced by Dr Paul Clark.

Last year RHUL-NHM conducted a study on 760 fish from 20 species (including dogfish, thornback rays, flatfish and pelagic fish) and one species of shrimp from the Thames Estuary and Firth of Clyde. Of these, 16 species had ingested plastic. The study found that 33–47% of the fish sampled had ingested plastic, mostly fibres. Over 100 brown shrimp (Crangon crangon) were collected, 6% of which were found to have ingested plastic. See below microplastic pieces recovered from the digestive tract of estuarine fish in the Thames Estuary and Firth of Clyde.

The fish sampled are a snapshot of plastic ingestion in the Thames and the Clyde and allow conclusions to be drawn about the ingestion of plastics by fish species from different sites but long-term studies are required to fully explain the mechanics of plastics in the estuary. The study adds to a growing body of evidence of plastic ingestion in the Thames, but was the first to compare the two UK estuaries.
Photos by Alexandra McGoran.

Particles removed from fish guts were analysed by spectroscopy techniques which identified 1,285 pieces of plastic, mostly Polyethylene Terephthalate (in the Clyde) and nylon (in the Thames). Both polymers are used in the textiles industry and, given that a wide range of colours were recovered, it suggests that washing machine outputs and clothing are a major source of plastic pollution in urban estuaries.

Indeed, various studies have tried to quantify the fibre output from washing machine waste water. Some studies on waste water from washing machines have suggested that up to 13 million fibres can be shed in just a 1 kg wash of polyester (Sillanpää & Sainio, 2017). Not all estimates are this high however, but even conservative estimates suggest that on average synthetic garments release 1,900 fibres per wash (Browne et al., 2011). Work in the tributaries of the Thames has demonstrated that a greater abundance of microplastic is present in sediment when there is a higher number of sewage inputs in a stretch of river (Horton et al., 2017).

The studies reported here have found that several species in the Thames and the Clyde have ingested microplastics, many of which may be coming from washing machines. The unequal distribution of plastic ingestion at different points in the food web (e.g. fish and shrimp) is an interesting area of study that requires further research. This research into the potential for trophic transfer, e.g. between prey and predator will shed light on the wider impacts of plastic pollution.

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