Ninety per cent of the planet’s seabirds are having plastic for dinner, a new study has found.

The findings are from the first global assessment of plastic ingestion by seabirds, published this week in the Proceedings of the National Academy of Sciences.

Toothbrushes, cigarette lighters, bottle caps and even a doll’s arm are just some of the items on a seabird’s dinner menu these days, says Dr Chris Wilcox of CSIRO Marine and Atmospheric Research.

Previous research suggests there are as many as 580,000 pieces of plastic polluting each square kilometre of the ocean at any one time. And there have been increasing reports about it being found in the guts of marine organisms including turtles, fish, dolphins and seabirds.

To see how pervasive the plastic threat was to seabirds across the planet, Wilcox and colleagues compared maps showing the changing density of plastic over time with maps showing the distribution of seabird species.

They also reviewed published studies of plastic ingestion carried out in 135 species of seabirds between 1962 and 2012. For example, previous studies on Lord Howe Island have found that around 10 per cent of the body weight of some birds is plastic. By combining all this data, the scientists were able to develop a model that links the amount of plastic a seabird consumes to the amount it’s exposed to.

“As birds encounter more plastic they’ll have more plastic in their gut and conversely if they encounter less they’ll have less,” says Wilcox.

Using the model the researchers estimate that today, a “shockingly high” 90 per cent of seabirds are ingesting plastic. The researchers also found that the threat was relevant to 99 per cent of seabird species.

“If you use that model and predict forward we conclude that by about 2050, plastic will be in about all seabird species on the current trend.”

The research suggests plastic ingestion is likely to have its highest impact in the Tasman Sea, southeast of Australia, where there is a high number of seabirds and a high density of plastic pollution, says Wilcox.

Mistaken prey?

Researchers are not sure why seabirds eat plastic, but it could be because the fragments look like prey, says Wilcox.

This idea is supported by experiments on turtles and plastic bags by one of his PhD students.

“It seems likely that the turtles mistake plastic bags for jelly fish,” says Wilcox.

Alternatively, seabirds could be eating plastic because genuine food, such as fish roe, is stuck to it.

Plastic can be hard to excrete and is slow to break down, says Wilcox, which means it could take up room in a seabird’s guts, make it feel full, and stop it from eating properly.

If the plastic is sharp it can also puncture holes in the gut.

And there is also concern about the danger of toxins – either in the plastic itself, or adhered to it – that mimic hormones and have been linked to problems such as egg shell thinning.

Exponential plastic production

“The doubling time for plastic is around every 11 years. So between 2015 and 2004 we made as much plastic as we did between the time plastic was invented and 2004,” Wilcox says.

“The amount will double again between 2015 and 2026.”

The problem of plastic in the oceans calls for more control on littering and illegal dumping in developed countries and an improvement in waste management infrastructure in the developing world, says Wilcox.

But, incentive schemes are also useful in battling our “throwaway culture”, he says, citing evidence that container deposit legislation in South Australia has reduced the number of plastic drink bottles entering in the environment by a factor of three.