Fish larvae favour microplastics to natural diet

Plastic pollution is a key theme of this year’s World Oceans Day, and a worrying new study finds fish larvae can actually turn to eating microplastics instead of natural sources.

8th June 2016
Juvenile pike (left) and a larval perch that has filled its stomach with microplastic waste particles (right) © Oona Lönnstedt

Juvenile pike (left) and a larval perch that has filled its stomach with microplastic waste particles (right) © Oona Lönnstedt

Plastics, or at least the first cheap and viable mass production techniques of creating plastics, were invented by Leo Baekeland in 1907, which means that next year, unless they’ve been recycled, we’ll still be able to pick out any of these original scraps that could still be floating around the world’s oceans.

Many large plastic products, such as plastic bags, break down into microplastic (smaller than 5mm in diameter) after they enter lakes and oceans due to ultraviolet radiation, physical forces and being broken down by water. Once there, they sink to the bottom and become part of the seabed, where their effects on the ecosystem are scarcely known and clean up is almost impossible.

Furthermore, very little is currently known about the effects of microplastics on aquatic organisms. However, a recent study by a team of biologists at Uppsala University in Sweden has found that the development of perch larvae is harmed by microplastic pollution.

Fish food

Larvae are particularly vulnerable to aquatic pollutants as their internal systems are yet to fully develop and aren’t capable of filtering out any harmful particles. The researchers collected fertilized perch egg strands from the Baltic Sea and reared them in three different mesocosms (controlled outdoor tanks), the first containing an average microplastic concentration (10,000 particles/m3), the second a high microplastic concentration (80,000 particles/m3) and the third a control, containing no microplastics whatsoever.

All the perch larvae in the experiment were fed the same amount and species of crustacean larvae, and juvenile pike, a natural predator of perch larvae, were introduced. The survival of the perch larvae was monitored every two to six hours over a 24-hour period.

They discovered that perch larvae reared in tanks containing microplastics showed reduced activity and stunted growth, making them particularly vulnerable to predators, resulting in increased mortality rates. In fact within 24 hours, 66 per cent of those reared in average microplastic concentrations were consumed by the pike, which rose to a staggering 100 per cent in tanks containing high microplastic concentrations.

But it wasn’t just reduced activity and stunted growth that reduced their chances of survival, as lead author of the study, Oona Lönnstedt, explains: “Fish exposed to microplastic particles ignored the smell of predators which usually evokes innate antipredator behaviors in naïve fish.”

Worryingly, it was also discovered that larval perch with access to microplastic particles ignored their natural food source and ate only plastic. “This is the first time an animal has been found to preferentially feed on plastic particles and is cause for concern,” says co-author Professor Peter Eklöve.

Keystone species

Both perch and pike are keystone species, which, similar to a keystone supporting a stone archway, means they play a vital role within their ecosystem and have far-reaching ecological effects. A reduction in their numbers would lead brown bears, which would normally feast on the fish, looking for alternative food sources such as deer and moose, impacting their populations in turn. 

These findings highlight the far-reaching effect of plastic pollution on the planet’s ecosystems and will pose further questions as to how we manage our plastic waste for future generations.

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