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What tiny plastic particles are doing to tiny fish

By Chelsea Harvey
June 2

Yet another study is adding to scientists’ growing suspicions that tiny bits of plastic in the ocean are causing big environmental problems. A paper published Thursday in the journal Science suggests that microplastics — small plastic particles less than 5 millimeters in diameter — could be hurting the survival of European perch, and likely other species as well.

Plastic waste in the ocean is a well-documented — and growing — problem around the world. A 2015 study estimated that anywhere from 4.8 million to 12.7 million metric tons of plastic were dumped into the ocean in 2010 alone, and researchers predict that amount will continue to rise. In recent years, the problem has taken on another dimension as scientists have become aware of the issue of microplastics.

When plastic goes into the ocean or other bodies of water, it doesn’t exactly decompose — rather, it tends to break down over time into smaller and smaller parts. These tiny microplastics are believed to be especially dangerous for marine life because they’re so easy to ingest. They’ve been found in the bodies of all sorts of animals, from filter feeders like clams and mussels to small fish and birds.

The problem is that scientists are still largely in the dark about how these microplastics are affecting the animals that eat them and how these effects might scale up and impact whole populations. But these kinds of studies are critical for helping policymakers decide what kinds of regulations are warranted when it comes to how plastic is created and disposed of.

The new study, conducted by a pair of researchers from Uppsala University in Sweden, starts to address these questions in a particular species of fish known as the European perch. The European perch has historically been a common fixture in the Baltic Sea along the Swedish coast, said fish ecologist Oona Lönnstedt, a postdoctoral researcher at Uppsala University and the new study’s lead author. But in recent years, she said, citizens have noticed that the fish seems to be declining.

Lönnstedt and colleague Peter Eklöv decided to investigate the possibility that microplastics might be contributing to the fish’s struggles. So they set up a series of experiments in which they exposed perch eggs and baby perch, just 10 days old, to water containing varying concentrations of microplastics to observe its effects on hatching success, behavior and response to predators, survival rates and even body size.

The researchers chose to use microplastics made of polystyrene, a common type of plastic pollutant, with a diameter of 90 micrometers (which is about 3.5 thousandths of an inch) — a size that’s been commonly observed in nature, according to Lönnstedt. They exposed the fish to three different conditions: water containing no microplastics at all; water containing a concentration of about 10,000 particles per cubic meter, which is more or less an average amount observed in nature, Lönnstedt said; and water containing a concentration of 80,000 particles per cubic meter, which is high but has still been observed in the wild.

The researchers found that the fish fared worse under higher concentrations of microplastics in every experiment they conducted. Fish eggs that had been exposed to microplastics had a lower hatch rate — 89 percent under the average concentration and 81 percent under the high concentration, as opposed to 96 percent when no microplastics were present. Fish exposed to microplastics also tended to be smaller and less physically active.

Some of the most worrying results had to do with the fish’s response to predators. Healthy perch are able to use chemical cues in the water, which they can detect when another fish is attacked nearby, to avoid predators. However, the researchers found that fish exposed to an average concentration of microplastics displayed a weaker response when the researchers injected these chemical cues into their water — and fish exposed to the high concentration didn’t display any response at all.

The researchers further tested the predator response by actually releasing pike — which feed on perch — into their habitats, which were designed to simulate a natural environment. Fifty-four percent of the perch who were not exposed to microplastics were eaten within 24 hours, as compared to 66 percent of the perch who were exposed to average concentrations. And all of the perch exposed to high concentrations of microplastics ended up being eaten within a day.

The exact mechanisms behind all of these effects remain unclear, although Lönnstedt and her colleagues are working to unravel those secrets. It seems clear that exposure to the plastics can have a chemical effect in some situations, as in the case of the perch eggs, where the mere exposure to water containing microplastics had a negative impact on their hatching success.

Lönnstedt added that perch seem to exhibit a preference for consuming microplastics over other, real sources of food, which could be costing them energy — even starving them — over time. In fact, she and Eklöv observed that the fish who were exposed to high concentrations ate only microplastics and failed to consume any of the regular food they were provided.

“It seems like there’s a chemical cue, a smell about these plastics, that triggers a feeding response in the fish,” she said. “They think that the plastic is this high-energy resource that they need to feed on in high levels.”

This might be part of the reason they fared so poorly against predators, Lönnstedt added. “They have no energy to behave, so to speak,” she said. “They were basically very inactive in the water column and they ignored the smell of predators.”

Lönnstedt noted that perch are likely not the only fish who are susceptible to the effects of microplastics. She said that some additional research (still unpublished) that they’ve conducted since the perch experiments has suggested that other species — certain types of coral reef fish, for instance — are also affected in similar ways.

Looking at individual species is useful for understanding how particular populations may be suffering — but another important area of research is figuring out whether some types of plastics are more harmful than others, a research question that Lönnstedt is also investigating.

“What we’re actually trying to do now is pinpoint…which plastic polymer is the worst for fish,” she said. “Is it polyethylene? Is it polystyrene? Is it PVC?” Once these differences are identified, then it may be possible to phase the worst chemicals out of production, she said.

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“We can’t eliminate plastic from our society — we are a plastic society,” she said. “But we can actually find alternative products or alternative ways of using plastic.”

The possibility of actually affecting policy is the real benefit of conducting these types of studies, which are still sorely lacking when it comes to microplastics, said Chelsea Rochman, a researcher at both the University of California Davis and the University of Toronto, in a comment also published Thursday in Science.

“History tells us that the motivation for new environmental policy is much stronger when there is demonstrated ecological impact,” she wrote.

For now, the observed effects of microplastics may be limited to a relatively small body of literature — but it’s one that’s already produced some worrying results. Several months ago, for instance, research suggested that microplastics may harm the ability of oysters to reproduce. Other studies focusing on mussels, marine worms and algae have turned up similarly alarming findings.

“Ideally, environmental policy should be catalyzed by scientific evidence rather than environmental catastrophe,” Rochman wrote. In other words, continued research on the growing issue of microplastics — particularly in the context of whole communities or ecosystems — will be critical for identifying the kinds of problems that might occur on a large scale before it becomes too late to stop them.