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Seals, Large Percentage of Deep-Sea Fish Contaminated by Microplastics

Scientists testing North Atlantic mesopelagic fish eaten by the ocean’s top predators found that nearly 75 percent were contaminated with microplastic fibers. Researchers have also discovered microplastics in the scat of gray seals and in the Atlantic mackerel they eat.

Written by Erica Cirino Published on Read time Approx. 4 minutes
A gray seal at the Cornish Seal Sanctuary in the United Kingdom, where the scat of seals was found to be contaminated with microplastics, as were the fish they are fed.Matt Cardy/Getty Images

Scientists estimate that there could be more plastic than fish in the ocean by weight in 2050. Many of those tiny plastic pieces are not only consumed by marine life at the sea’s surface but also end up inside a large percentage of fish living in much deeper waters of the open ocean, researchers have found.

The findings come as another group of scientists have discovered microplastic fibers in the feces of captive gray seals in the United Kingdom as well as in the digestive tracts of the wild Atlantic mackerel fed to the marine mammals, according to a study published on Thursday in the journal Environmental Pollution

Deep-sea fish are thought to make up most of the ocean’s biomass and are eaten by top predators, including tuna, salmon and sharks. They spend their days in the mesopelagic zone of the ocean at depths of 650–3,000ft (200–900m) and swim upwards at night to feed in the more nutrient-rich waters near the ocean’s surface. On these journeys to the surface, however, it’s possible they are taking in more plastic pollution than scientists had thought.

In a study published last week in the journal Frontiers in Marine Science, scientists caught 233 fish in a remote part of the North Atlantic east of Newfoundland in Canada. They found 73 percent of the fish – which included several types of lanternfish, Bean’s sawtoothed eel, scaly dragonfish and bristlemouth fish – had microplastics in their digestive tracts. “Such high numbers of microplastics in the gut contents of mesopelagic fish is of great concern,” the scientists wrote.

Lead author Alina Wieczorek, a PhD candidate at the National University of Ireland Galway, didn’t expect to find such high levels of microplastic, especially hundreds of miles from any coastline.

“The key message for us is that our pollution has now reached even the most remote areas and that the plastics may impact key ecological players such as deep-sea fish,” said Wieczorek.

A lanternfish, one of the deep-sea species found to be contaminated with microplastic fibers. (NOAA)

She was surprised by the results partly because the percentage of plastic is much higher than discovered in two previous studies. Those studies found, respectively, 11 percent and 9–35 percent of mesopelagic fish in other parts of the ocean were contaminated with microplastics.

The difference, said Wieczorek, might be explained by her study’s precise research methods. She said researchers used a more concentrated alkaline dissolution to separate plastic from organic material in fish gut samples and a finer-meshed filter to grab the smallest pieces of plastic. The scientists noted in their study that extremely small microplastics made up 20 percent of their samples, a size-class of microplastic other scientists using different research methods might have missed.

Almost all of the plastic the team recovered from both surface water samples and those inside fishes’ guts were plastic fibers, likely shed from woven materials like clothing (which can shed hundreds of thousands of fibers when washed) or ropes from fishing gear. For that reason, the team had to be extremely rigorous about ensuring that fibers from their own clothing didn’t make their way into samples. The size, shapes and colors of microplastic found in the surface waters and fish were nearly identical, suggesting mesopelagic fish may have consumed them during their daily feeding migrations near the sea surface.

Though the team hasn’t studied the effects of plastics on these fish, Wieczorek said possible effects could include weight loss, impaired feeding activity, inflammation or poisoning from the toxic chemicals microplastic bits absorb from seawater.

Plastic in these mesopelagic fish could also have broader repercussions for the ecosystem. The fish play an important role in the marine carbon and nutrient cycle by feeding near the surface at night and during the day swimming to the seafloor, where they excrete feces. Their consumption of plastic may also be speeding the transfer of plastic particles to the seafloor. Scientists have documented that now even the deepest-dwelling marine organisms have plastic in their stomachs.

When mesopelagic fish are eaten by predators, the plastics may also have the chance to move up the food chain. Other studies have shown how microplastics and the chemicals they contain appear to be transferred from organism to organism, though the extent of it is not well understood. How exactly mesopelagic fish – which eat tiny zooplankton – are ingesting plastic is unknown.

What the presence of plastics in mesopelagic fish means for human health is also highly uncertain. “We don’t know whether it’s harmful for us yet. We still have a lot to learn about human health impacts,” said Chelsea Rochman, a scientist at the University of Toronto who has also found microplastic in marine animals, in fish from the South Pacific Gyre and in a wide variety of bivalves and fish sold for human consumption in Indonesia and California. She said Wieczorek’s study shouldn’t worry people about their health “more than any other fish study,” including her own.

To get a better idea of what the possible human health implications could be, “we need to know how exactly the fish are getting exposed to the microplastics,” said Wieczorek, emphasizing a need for future research. “Is it directly from the water or through their prey species, which would mean that the plastics are transferred through the food chain?”

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