Tiny plastic particles in the ocean are changing the lives of baby European perch in many ways. Not only do microplastics inhibit hatching and stunt growth, these also prevent baby fish from eating zooplankton and responding to potential predators. The findings are published in Science this week. 

The world’s plastic production is an estimated 300 million tonnes (330 million tons) a year, and it’s increasing by 20 million tonnes (22 million tons) annually. Much of that waste ends up in our oceans in the form of microplastics less than 5 millimeters in size. Sometimes they’re manufactured that way (like microbeads in exfoliating facewash), but large, non-biodegradable pieces often break down into tiny bits over time. 

Because microplastic pollutants accumulate in shallow coastal habitats, juvenile fish likely encounter high concentrations of this debris in their nursery habitats. Yet, exactly how microplastics impact aquatic animals during their vulnerable early stages is still unknown. 

To investigate, Uppsala University’s Oona Lönnstedt and Peter Eklöv collected fertilized egg strands of European perch (Perca fluviatilis) from the Baltic Sea. Back in the lab, they exposed them to three different concentrations of 90-micrometer polystyrene particles in glass aquaria containing estuarine water and a supply of brine shrimp. "The microplastic particle levels tested in the current study are similar to what is found in many coastal habitats in Sweden and elsewhere in the world today," Lönnstedt explained. 

Exposing developing embryos to microplastics decreased hatching success: 96 percent of the clean-water fish hatched, but fish in the high-concentration treatment had a hatching rate of 81 percent. Additionally, fish reared in high microplastic concentrations were much smaller than fish reared in average concentrations. 

This stunted growth rate might be due to the amount of microplastics they ingested. On average, larval fish from the high microplastics group consumed 7.15 particles, while fish in the average concentrations group ate just 1.4, but in addition to brine shrimp. The stomachs of fish in the most plastic-filled water contained only plastic particles (pictured above). "This is the first time an animal has been found to preferentially feed on plastic particles and is cause for concern," Eklöv said in a statement. 

What’s more, larvae exposed to microplastics didn’t know to escape from predators. Prey typically have an innate response to what’s known as damage-released chemical alarm cues: The smell of injured fish signals the presence of a threat, which triggers fear responses like freezing in place. Exposed larvae didn’t exhibit antipredator behaviors, not even when their natural predators, juvenile pike, were released into the water. They were all dead within 24 hours. Meanwhile, 46 percent of the control fish were still alive after a day. 

^{Pike (Esox lucius) feed on perch that have ingested microplastic particles. Oona Lönnstedt}

^{Image in the text: Adult perch in the Baltic Sea. Peter Eklöv}