Turtles eat more black and green plastic as it looks like sea grass, scientists find

Sarah Knapton
Turtles are fooled by plastic that resembles food - PA

Black plastic is already the bete noire of recycling, with lasers unable to bounce off its dark surface, causing it to pass straight through the sorting system into landfill or incineration.  

But new research has discovered it’s also more harmful to marine life.

Green turtles are more likely to swallow plastic that resembles their natural diet of sea grass, and so end up with far more black and green plastic in their stomachs.

Scientists from the University of Exeter and the Society for the Protection of Turtles examined the guts of 19 turtles found washed up on beaches in Cyprus.

Plastic was found in all turtles whose full gastrointestinal tract could be examined, with one found to contain 183 pieces.

But when they compared the different colour plastic found inside turtles to that on the beach - which acts as a proxy for the colours floating in the sea - they found the animals were 2.5 times as likely to ingested black and green plastic as other colours. 

The amount of clear plastic was also higher than other colours. 

“Previous research has suggested leatherback turtles eat plastic that resembles their jellyfish prey, and we wanted to know whether a similar thing might be happening with green turtles,” said Dr Emily Duncan, a marine biologist from the University of Exeter.

“Sea turtles are primarily visual predators – able to choose foods by size and shape – and in this study we found strong evidence that green turtles favour plastic of certain sizes, shapes and colours.

“Compared to a baseline of plastic debris on beaches, the plastic we found in these turtles suggests they favour threads and sheets that are black, clear or green.” 

Some of the plastic items found in the stomachs of turtles, with the ratio of black and green far higher than that on the beach Credit: Emily Duncan

The turtles were also seven times more likely to have eaten thin strips of plastic which resembles sea grass, the researchers found, with black strips possibly tearing off from bin bags.  

Black plastic is widely used in food products because it stops light penetrating and so keeps the contents fresher for longer. It is also cheaper because it can be made from low-value multi-coloured plastic with its dark colour masking any imperfections so is used for everyday items such as bin bags.

But the near-infra red (NIR) optical scanners used in recycling plants cannot penetrate the carbon black pigments meaning that it passes through the system. Although some plants are now starting to improve their scanning technology most still cannot recycle black packaging. 

The study could not determine what, if any, role the plastic had in the turtles’ deaths. Most had likely died as a result of becoming entangled in fishing nets.

Turtles often die after becoming entangled in discarded or lost fishing nets  Credit: BBC Blue Planet II

However researchers found  that smaller turtles tended to contain more plastic, possibly because they are less experienced (and therefore more likely to eat the wrong food) or because diet choices change with age and size.

All of these turtles contained plastic, with the number of pieces ranging from three to 183.

“Research like this helps us understand what sea turtles are eating, and whether certain kinds of plastic are being ingested more than others,” said Brendan Godley, Professor of Conservation Science, who leads the Exeter Marine research strategy.

“It’s important to know what kinds of plastic might be a particular problem, as well as highlighting issues that can help motivate people to continue to work on reducing overall plastic consumption and pollution.”

The paper was  published in the journal Scientific Reports.