The beautiful, all-you-can-eat sushi platter you shared with friends last week might have included a special ingredient: plastic.

Microplastics — the remnants of plastic bags, takeout containers and straws that almost-but-not-quite disintegrate in the oceans — are found in 3 out of 4 fish, such as squid, cuttlefish and swordfish in markets around the world, say the authors of a February study.

"These fish inhabit a remote area, so theoretically they should be pretty isolated from human influences, such as microplastics," said Alina Wieczorek, lead author of the Frontiers study.

"However, as they regularly migrate to the surface, we thought that they may ingest microplastics there," she said.

Food chain pollutants

Consumers are waking up to pollutants in their food chain, and scientists are joining them to raise awareness and combat other issues like overfishing. Last week, thousands marched in the United States and 25 other countries for World Oceans Day.

Under the hood of a shark costume was Brian Yurasits, director of development at the nonprofit TerraMar Project, which educates and promotes ocean issues. Yurasits circulated with about 3,000 others at the march in the shadow of the Washington Monument and a life-size, inflatable blue whale.

Holding a sign that read, "Sharks are friends, not food," Yurasits emphasized that issues about ocean health "is more than just plastic."

"It's overfishing, climate change, invasive species and habitat loss," he said.

The youth-led Sea Youth Rise Up advocates for ocean conservation, including reduced single-use plastics such as plastic straws, water bottles and shopping bags, which the ocean breaks down into microplastics. Much of the plastic that ends up in the oceans was blown into rivers first from uncontained trash on land.

Microplastics are microscopic and smaller than plankton, a popular food choice of larger marine life. They are made of hydrocarbons, a compound found in petroleum and natural gas, and attract other pollutants, according to the National Association of Geoscience Teachers.

Because microplastics can't be digested, they build up in the fish that consume them.

"The biggest impacts aren't the ones we can see very easily," said Katie Farnsworth, a professor and marine geologist who studies coastal sediments at the Indiana University of Pennsylvania. "The biggest danger is those microplastics, because they are being eaten by things in the bottom of the food web, and then move their way up through the chain."

The plastics can give off toxins, she said, because plastic is made from hydrocarbons. And hydrocarbons, she explained, attract and bind with other pollutants that are in the ocean.

Carbon dioxide

But microplastics aren't the only threat to marine life. Ocean acidification and overfishing also imperil the health of oceans.

Ocean acidification occurs when seawater absorbs carbon dioxide from the atmosphere. Carbon dioxide is released into the air by burning fossil fuels, like oil and coal. That makes the ocean more acidic, which harms shellfish, other marine life and plants.

Ocean acidity has increased about 25 percent since the Industrial Revolution starting in 1760, the EPA reports, commonly depicted by billowing smokestacks at coal-burning factories.

Julia Dohner is a second-year Ph.D. student studying marine chemistry and geochemistry at the Scripps Institution of Oceanography in San Diego. She is also a surfer who spends lots of time in the Pacific Ocean.

"Everything I think about is through the context of carbon dioxide," Dohner said. "For me, reducing one's carbon footprint is really important. It's kind of a straightforward task, if you want to do something about the environment."

"There's been a lot of effort going into understanding how quickly our oceans are acidifying and understanding how those conditions will affect various forms in the ocean," Dohner said.

Overfishing

Overfishing also threatens ocean health. It occurs when more fish are caught than the population can replace through natural reproduction. According to the World Wildlife Federation, several important commercial fish populations, such as Atlantic bluefin tuna, have declined to the point where their survival as a species is threatened.

Regulating overfishing is nearly impossible because "fish could care less about political boundaries," said Farnsworth, meaning fishing boats follow the fish, often disregarding lines drawn around territorial boundaries.

"Regulations in one country don't help very much because you have to get treaties to get everybody in agreement," she said.

Dohner said she believes that the biggest threat the ocean faces is a lack of awareness of these issues.

Also read: Human Touch Can Rehabilitate Patients

"There's all this research going on about how our planet is changing and what it's going to look like in the future," Dohner said. "But at the end of the day, if we can't convince people such that there is tangible policy changes enacted, then what have we really accomplished?" (VOA)