A recent study adds to the growing concerns surrounding plastic pollution in oceans. Published by researchers from Uppsala University in Science, the report found juvenile fish that consumed microplastic altered their behavior, making them more likely to be killed by predators. Their research also had an unexpected finding--the fish preferred microplastics more than their natural food sources. 

There has been a steep decline in the population of European perch (Perca fluviatilis)--the main fish analyzed in the study--in the Baltic Sea. 

“We know something is happening during their recruit stages because the adults will go in and reproduce their eggs, but then the recruits begin to disappear during their larval stage,” explains Dr Oona Lönnstedt, the study’s lead author. 

Since the Baltic Sea is polluted with microplastic waste, the researchers suspected that it may be linked with the decreased perch population. They collected samples of fertilized perch eggs from the Baltic Sea and and simulated an environment that was similar in an aquarium to see what impact polystyrene, a type of microplastic debris, had at each stage of development.  Compared to a control group, the larval fish exposed to microplastics were smaller, swam shorter distances, and were more likely to spend time motionless. It also affected their olfactory senses, which are used to detect predators.

Juvenile pike prey on larval perch. (Image by Oona Lönnstedt, used with permission)

As a result, the fish that consumed microplastics were more likely to be killed by predators. When the groups were exposed to a juvenile pike, one of their natural predators, all of the perch in water with high levels of microplastics were eaten within 16 hours. In comparison, about half of the control group were still alive after 24 hours. This could mean that many perch may die before they have a chance to reproduce. 

This not only endangers their survival, it may also affect other marine life. There has also been a decline in the number of pike in the Baltic Sea. The researchers believe this may be linked to their consumption of juvenile perch that have ingested microplastics and it is likely the plastic contaminants bioaccumulate in the food chain. 

Lönnstedt says it was a massive surprise to find the fish preferentially eating the microplastics. Unlike filter feeders such as mussels and oysters, who passively take in plastic particles, the perch were actively seeking it. Though uncertain of the cause, the researchers believe it may be a chemical cue that triggers a feeding response similar to one found triggered by junk food. “I usually liken it to kids eating fast food--McPlastic--they think it is a high energy resource that will keep them full but it is actually not,” Lönnstedt tells me.

Lönnstedt has started to examine the feeding preferences of other species of fish and has found damselfish also choose microplastics over their own natural food sources. In addition to polystyrene, the study’s authors plan to expand their research to include other types microplastics.