There is still a lot we don't know about the path of plastic waste through the ocean, though we are learning more all the time. Scientists late last year found that a single shopping bag can be torn into millions of tiny pieces, with the resulting microplastics incredibly difficult to track (though some smart people are working on it). Small fragments of plastic have now been uncovered in seal poo, serving as the first evidence of the material making its way up the food chain to marine predators.

Finding plastic in seal feces actually wasn't all that surprising to the researchers, who hail from Plymouth Marine Laboratory, the University of Exeter and the Cornish Seal Sanctuary. Earlier studies have found evidence of what is known as trophic transfer, a process where a creature like zooplankton or fish with microplastics in its system is consumed by another, and the synthetic polymers make their way into the predator.

But this has previously only been observed in smaller marine animals such as mussels and crabs. Efforts to observe the trophic transfer process play out with larger animals in the wild has been a little more difficult, because of issues like an inability to tell whether the microplastics had been ingested directly or indirectly.

So the team turned to captive seals instead. The male grey seals that live in the Cornish Seal Sanctuary are fed intact, wild-caught Atlantic mackerel. By analyzing the digestive tracts of those fish, the team found a third of them contained between one and four plastic fibers and fragments, the most common type being the polyethylene used in plastic bags and bottles.

By then analyzing the feces of the seals that ate them, the team found nearly half of the samples also contained between one and four plastic fragments. According to the team, this is the first evidence of trophic transfer occurring in marine mammals.

"Our finding that microplastics can be passed from fish to marine top predators is something we've long thought was the case but, until now, lacked the evidence to back our theory up," says Sarah Nelms of PML's Microplastics Research Group, lead author of the study. "We have shown that trophic transfer is an indirect, yet potentially major, route of microplastic ingestion for these predators. By examining scat from captive animals and the digestive tracts of fish they were fed upon, we could eliminate the possibility that the seals were eating plastic directly and be sure that any microplastics we found in their scat came via the fish."

A study from 2016 estimated there to be between 93,000 and 236,000 metric tons of microplastic just on the surface of the ocean, which the researchers say accounts for roughly one percent of the total plastic waste in the sea. Not only do we not know where most of it is, we don't really have much of an idea about the damage it could be causing.

"Our study demonstrates how microplastics can be transferred from prey to predator and therefore passed up through the food chain," said Dr Pennie Lindeque, lead of PML's microplastics research. "More work is needed to understand the extent to which microplastics are ingested by wild animals and what impacts they may have upon the animals and ecosystems."

The research was published in the journal Environmental Pollution.

Source: Plymouth Marine Laboratory, Nature