Article: Lönnstedt, Oona M., and Peter Eklöv. “Environmentally relevant concentrations of microplastic particles influence larval fish ecology.” Science 352.6290 (2016): 1213-1216.
From plastics found in seafood and table salt, to plankton, whales, and corals eating plastic, we’ve talked a lot about plastics and microplastics here on Oceanbites. Humans produce about 300 million metric tons of plastic each year. Much of that plastic turns into waste and ends up in our waterways and oceans (Fig. 1). Plastic doesn’t break down easily, though it may break into smaller pieces, and persists for a long time in the environment. Plastics cause problems for marine creatures by either leaching toxic chemicals into the water or by inhibiting digestion when consumed. Many recent studies have highlighted that marine species are now consuming plastic, whether passively (filter feeding) or actively. However, few studies have looked at the ecological implications of eating plastic and what a plastic diet means for a marine creature. Eating plastic can’t be healthy, right? For large, adult fish, eating a bit of plastic might not be a death sentence, but small, larval fish already have an uphill battle for survival. It is easy to imagine that an all plastic diet would impact the growth and behavior of larval organisms. I mean, there was a reason our parents forced vegetables upon us as kids, right? Researchers Oona Lönnstedt and Peter Eklöv recently investigated how eating plastic can impact the growth, survival, and behavior of larval fish off the Swedish coast. What they found was pretty alarming.
Microplastics are defined as bits of plastic smaller than 5mm (Fig. 2). Microplastics are found across the world’s oceans. Microplastic concentrations off the Swedish coast, the study site for this research, range anywhere from 150 particles per m3 to over 100,000 particles per m3. Fish living along this coastline are likely to encounter and ingest quite a bit of microplastic as a result. Researchers first investigated what the chemical changes associated with microplastic concentrations would do to the hatching success of a common coastal fish. They collected eggs of the Eurasian perch (Perca fluviatilis, Fig. 3) and placed them in one of
three types of water, varying in microplastic concentration: no plastics, average concentrations of plastics (~ 10,000 particles per m3), and high concentrations of plastics (~ 80,000 particles per m3). The concentration of microplastic significantly impacted the hatching success of perch. Under conditions of no microplastic 96% of fish eggs hatched whereas under high microplastic concentrations, 81% of fish eggs hatched. The hatched fish then continued to grow in their microplastic treated environments. At 10 days old, fish behavior was observed by tracking the activity of fish and the distances they traveled. Fish grown with no microplastics in the water were more active, traveled farther, and spent less of their time motionless and inactive (Fig. 4).
At 2-weeks, a subset of fish from each microplastic treatment were tested for their response to predatory cues. For larval fish, the ability to detect predator cues and act to avoid being eaten is critical to survival and growth into adulthood. Larval fish were exposed to a chemical cue that signified the presence of a predator and their response was observed. Fish grown with no microplastic reduced their activity and movement, as a way of hiding from the predator. Fish grown with microplastics did not alter their movement and remained as active as they were normally. This means that fish exposed to microplastics aren’t able to pick up predator cues likely affecting their ability to survive.
The researchers underscored this point by exposing another subset of larval perch to an actual predator, a juvenile pike (Esox lucius), and observing how many individuals survived over a 24-hr period. Fish reared without microplastics had a 46% survival rate, but when grown under high microplastic concentration had a 0% survival rate (Fig. 5). Researchers also found evidence of stunted growth in larval fish grown under high microplastic concentration, where fish were about 10% smaller on average.
Perhaps the most interesting result of this work was discovered when researchers looked at the diets of the larval fish grown under these environments. In all treatments, fish were given the same set amount of brine shrimp to feed on. Fish grown under average microplastic concentration had ingested both plastic and brine shrimp, but fish grown under high microplactic concentration had only ingested plastic, despite the presence of a real food source (Fig. 6). This highlights that larval fish were actively choosing to feed on microplastics instead of their typical zooplankton food (Fig. 7)!
We know that plastics can cause major problems for marine creatures, but this study is one of the first to highlight the impact that growing up surrounded by plastics can have some seriously negative ecological effects. This work showed that average and high concentrations of microplastics can reduce the hatching success of larval fish, makes them more inactive, blocks their ability to detect predators, reduces their survival rate, AND makes them smaller! The most disconcerting finding, however, is that these larval fish were ACTIVELY consuming the plastic over their normal food, meaning fish are only going to be making the problem worse for themselves. There is no doubt we need to do a better job of cleaning up plastics and reducing our reliance on them, particularly if they are going to continue to negatively impact marine life.