Tristan da Cunha is the most remote inhabited group of islands in the world. The volcanic islands rise out of the South Atlantic Ocean far from any other inhabited countries.
A museum discovery voyage in 2019 collected life from the deep ocean surrounding these distant islands.
Alex McGoran, a researcher at the Museum and PhD student from Royal Holloway University of London, studied the stomach contents 32 fish collected during the voyage.
‘Two thirds of the individuals we looked at had plastic in them and viscose was present in half of the specimens,’ says Alex, ‘I’m not surprised that plastic has made it into the depths of the ocean.’
Microplastics in the environment are readily ingested by fish, but until now little has been known about plastic and deep ocean life.
Synthetic and semi-synthetic fibres, including polyester and viscose, were the most prevalent type of plastic found. Viscose is a semi-synthetic material which has been chemically treated and modified for use in clothing such as artificial silks, as well as sanitary items such as pads and wet wipes.
On closer inspection, Alex found that even the prey inside the fish had eaten plastic fibres. For example, inside one common fangtooth fish there was a was a cock-eyed squid and a bearded sea devil which had both eaten plastic. This suggests that plastic might be being passed up the food chain.
This new study will provide a valuable insight into the potential impacts of plastics to an ecosystem known to be a significant sink of microplastics.
The sampling and research voyage to the islands was part of the Blue Belt Programme, a marine survey of British Overseas Territories funded by the UK Government.
How does plastic get inside the fish?
One way fish eat a plastic microfibres is by thinking it might be food.
‘One of the species I looked inside is a lantern fish,’ explains Alex. ‘In the daytime these fish live in the dark of the deep ocean and at night they travel up to feed on plankton when it’s safe. Microplastics can look like plankton, so they accidentally feed on the plastics and then go back down to the sea floor. ‘
Researchers think that between ten and 30 percent of fish in any sample will be contaminated with microplastics.
Ingesting microplastics can be a danger for the fish as harmful chemicals can cling to some plastics and then be absorbed by the fish. This can affect their health, energy levels and ability to reproduce. Plastics can also carry bacteria from one environment to another, possibly spreading pathogens to defenceless new hosts.
In the case of viscose, Alex says that we don’t yet know if these fibres offer a surface for chemicals to cling to. In fact, the real impact of these fibres on the environment is not yet known.
‘We have so far ignored all these cellulose based fibres like viscose, because they are natural,’ says Alex. ‘We don’t have a lot of information about how they break down, so there could be physical impacts on the fish that we just don’t know yet. These fibres could have a negative effect on the fish or might have no effect, but I think we can safely say that it is unlikely that any microplastics are having a positive effect.’
To see inside the stomach of a fish, Alex carefully removes the stomach and dissolves all the organic material inside. She then runs the stomach contents through a filter to catch the plastic fibres. She looks at these fibres under a microscope to identify the type of plastic.
Where does the plastic come from?
The plastic found in the fish might have come from landfill on St Helena, from fishing gear and ropes or from washing machines on the island.
Washing machines release, on average, of between 700 thousand and 6 million fibres per load. Wastewater treatment plants can prevent most of these fibres from being released, but fibres still enter the oceans through run off from treatment plant sludge which is used as fertiliser. Thankfully, the introduction of improved filters in washing machines and at treatment plants may reduce this source of plastic in oceans.
Alex says once plastics are in the ocean, they can travel far. You might expect high density plastics to sink to the bottom and accumulate in the sediment on the ocean floor, but vertical mixing in the ocean and ingestion by fish and other animals can bring low density plastics to the seafloor and resuspend high density items.
James Maclaine is the Senior Curator of Fish at the Museum, who was onboard the discovery voyage that collected these fish.
‘As part of the Blue Belt Programme, we carried out as much sampling, surveying and measuring as we could around Tristan and St Helena,’ says James ‘The tissue samples we collected are now being looked after at the Museum’s BioBank.
‘This project will hopefully increase our understanding of these seamounts and their biodiversity so that we are better able to protect them in future.’