Microplastics found in stomachs of spotted seals

Microplastic pollution has contaminated nearly every inch of the natural world. Tiny plastic strands have been discovered at the bottom of the Marianas Trench in the Pacific Ocean and at the top of the Alps in Europe. Now new research shows that microplastics are also common in stomachs of spotted seals in the Bering and Chukchi Seas.
Researchers looked inside the stomachs of 34 spotted seals that were collected in Gambell and Shishmaref in 2012 and 2020. Of those, 33 contained microplastics.

“This was the first study that to directly examine marine mammals for microplastics in their stomachs within Alaskan waters,” said Alex Sletten. “But what it shows is that there’s been over a decade of consumption. So these plastics are in the environment, they’re in the prey, and seals have been eating them at least since 2012.”
Researchers know less about how or if these plastics are absorbed into the body, and how that might affect humans who subsist on marine mammals like ice seals.

“We need to look at microplastic absorption and accumulation in these seals and tissues, as well as in humans, and just continue monitoring and see how it changes,” Sletten said.
Sletten presented her work last week in a Strait Science lecture hosted by UAF’s Northwest Campus and Alaska Sea Grant. She explained that the purpose of the research was simply to establish whether microplastics were present in spotted seal stomachs. But she also wanted to investigate whether there were any meaningful differences in the presence of microplastics based on the age of the seals, the years the samples were collected and the location where the seals were harvested. She looked at pups and non-pups, samples collected in 2012 and then in 2020, and seals harvested in both Gambell and Shishmaref.
The samples were collected through ADF&G’s Arctic Marine Mammal Program. As part of this program, hunters and other participants in various communities along the Bering, Chukchi and Beaufort Seas gather tissue samples from marine wildlife like ice seals. Those samples are frozen in a lab in Fairbanks.
Microplastics are so ubiquitous that Sletten and her colleagues had to pre-filter the water they were using to process and prepare the seal stomachs to prevent adding plastics and messing up their count.
“We actually found that the water we were using as part of this process—just regular tap water—did have microplastics in it,” Sletten said.
Microplastics are small pieces of plastic that have broken down over time. They are often byproducts of plastic manufacturing, but they can also be formed through mechanical wear of plastic products. Polymers are rubbed off tires as vehicles drive on roads. Plastic strands from synthetic fabrics get pulled off clothing in the wash. These particles then enter the world’s waterways, and eventually end up in the ocean.

Ocean circulation patterns mean that plastics can travel far from their source, and in the Arctic, sea ice is a vector of microplastics.

“As ice is forming and expanding, any plastics that are in the water can actually be incorporated into the ice matrix and stored over time,” Sletten said. “So as the ice moves with microplastics trapped in it, it could potentially deposit microplastics into other areas of the Arctic as it melts.”
For large marine mammals, microplastics often enter the body through accidental ingestion or through consuming smaller prey that contains microplastics. Even prey as small as zooplankton contain microplastics.
Researchers at Oregon State University recently estimated that pregnant gray whales consume at least 9.55 million particles of microplastics and other human-made debris. They based this estimate on the abundance of particles they were finding inside a few species of zooplankton that serve as an important food source for gray whales, especially pregnant ones that have high metabolic demands.

A larger fragment of plastic that gets lodged in an animal’s gastrointestinal tract could pose health risks. If that plastic gets absorbed into the body, Sletten said there’s a wide range of health hazards that could occur.
“There’s over 10,000 different chemicals that can be used in the construction of plastic,” she said. “Over 2,400 of those are considered PBTs, or persistent bioaccumulative toxic substances.”
These toxins could affect the hormone production of organisms and their reproductive success. Lab-based studies on mice have found that microplastic consumption affected their ability to reproduce and look for food.
Sletten chose to study spotted seals because of a few factors that would likely increase their exposure to microplastics. The animals eat fish, such as Arctic cod, capelin and walleye pollock. These prey animals are at a higher trophic level, meaning they are higher up the food chain, and thus have more chances to accumulate microplastics. Spotted seals also make frequent dives to or near the seafloor, where microplastics are known to sink and build up in the sediment.

Across the 33 spotted seal stomachs that contained microplastics, Sletten found a total of 211 plastic particles. Of those, 190 could be classified as microplastics. The rest were too large.
And of those 190 microplastics, 189 were fiber strands invisible to the naked eye.
Microplastic concentration did not vary by how old the seal was, where it was harvested or when it was harvested, Sletten said.
However, the seals that had primarily bottom-feeding prey in their stomachs tended to have a higher abundance of microplastics, compared to seals that had eaten a mixture of prey from the bottom and fish from the water column.
The researchers also compared seal stomachs that contained primarily prey high on the food chain vs. low on the food chain.
“If the seals were consuming prey that were higher trophic level, such as those cod, they had an increased amount of microplastics in their stomachs compared to those that were eating shrimp or other small invertebrates,” Sletten said.

Nearly half of the microplastic strands Sletten observed were black in color. The next most common colors were blue and clear. But she didn’t investigate the exact sources of the particles. She explained that this type of research is still new, and its results are often inconsistent, but tracing the origins of microplastics would have important implications.
“There’s certain polymers that are more common for textiles versus fishing gear, and if we could narrow down…sources of plastic that are problematic in certain regions, we can look at, possibly mitigating their entry into that environment,” Sletten said.
Sletten said these methods could be applied to other marine mammals, such as other species of ice seals in Alaska.
“These species are all sentinel species for the marine ecosystems, so monitoring these species will help us understand how the ecosystem is doing as a whole,” she said.
And the research adds to the mountain of existing evidence that plastics are contributing to widespread contamination.
“Mitigating our plastic use and pollution is only going to help the situation,” Sletten said.