Uh oh! Baby fish prefer plastic to real food

This choice changes their behavior — and can lead to their death
Jun 2, 2016 — 2:15 pm EST
Larval perch

Larval perch with a belly full of tiny plastic beads. The fish preferred this junk food over the nutritious prey on which it usually feeds. 

Oona Lönnstedt

Let a baby perch choose between a nutritious meal and a piece of plastic, and this little fish will go for the plastic. That's the disturbing conclusion of a study to be published June 3 in the journal Science.

It's disturbing because eating plastic stunts the animals’ growth. It also alters the behavior of the baby fish. This new behavior makes them easier for predators to find — and gobble down as lunch.

Researchers aren't sure why baby perch choose plastic over real food. But their new data might explain why some fish populations are decreasing, says Peter Eklöv. One of the study's authors, he works at Uppsala University in Sweden. There he probes how predator fish and their prey interact.

"It's kind of scary what the results show,” says Eklöv. “We really need to do something about this." His team is now testing species of baby fish other than perch. And early results show these fish, too, prefer plastic over real food.

People have been concerned for some time about rising plastic pollution in oceans and lakes. Sunlight, waves and wind break down plastic into tiny fragments. Plastic lint flows out of washing machines and down the drain. Eventually it can flow into rivers and the sea. Microbeads added to some face cleansers are taking the same path into the environment.

Previous studies showed fish and other aquatic organisms can ingest these tiny bits of plastic. That plastic can then move up the food chain as seabirds, seals and other marine predators eat those fish.

A second risk: Plastic can pick up toxic chemicals. These include PCBs, pesticides and flame retardants. When ingested, plastic can release these pollutants, poisoning the dining predator.

Eggs and hatchlings affected

The Swedish researchers used beads made from polystyrene ­— a type of plastic used in packaging. Their new study now shows for the first time that baby fish with access to this plastic preferred it to their natural food: zooplankton. Those zooplankton are tiny creatures that drift in the sea and are an important food for small fish.

The Uppsala team started its new research by studying perch. These are a type of freshwater species. They also can live in semi-salty, or brackish, water. One such place is the Baltic Sea.

The zooplankton that baby perch normally eat are the same size as are many of the tiny plastic bits found polluting lakes and oceans around the world.

Eklöv and Oona Lönnstedt, a marine biologist, collected perch eggs from the Baltic Sea. They put roughly equal numbers of them in three fish tanks containing zooplankton.

Into two tanks, the researchers added tiny polystyrene beads. They added low levels to one tank. The concentration was equal to what has been seen in some coastal ocean waters. A second tank got far higher levels of the beads. The last tank got no plastic.

Almost all of the eggs — 96 percent — hatched in the tank free of plastic. That rate is normal. In the tank with low level of plastic beads, 89 percent of the eggs hatched. In the tank with high levels of plastic beads, only 81 percent hatched.

Growth and behavior impaired in survivors

Hatchlings in the tank with no plastic grew normally. Those in the tanks containing plastic did not.

Larval fish growing in water with the low level of plastic beads grew at a slow rate. Those exposed to higher levels of the plastic grew more slowly still. 

That's because when fish feed on plastic, they can't digest it, notes Eklöv. "It gets stuck in their digestive tract and blocks it," he explains. Now the fish can't digest anything, even zooplankton. Eventually the young fish can starve to death.

Unless, that is, a predator fish eats them first.

In a second experiment, Eklöv and Lönnstedt added a chemical to each fish tank. It smelled like a pike, a fish that eats baby perch. Perch know when a pike is nearby based on this scent.

Baby perch raised in water free of the plastic reacted normally to the pike scent. Their movements froze. These fish didn’t want to risk attracting attention.

Perch exposed to the lower level of plastic were less likely to freeze. Those exposed to high levels of polystyrene beads did not alter their movements at all. They swam normally.

Next, the researchers put real pike in the tanks with the baby fish. Almost half of the perch that had been raised in plastic-free water were still alive after 24 hours. That was considered normal.

Fish reared with the low levels of plastic weren’t so lucky. Fewer than one-third lasted 24 hours before being eaten. Fish exposed to the high level of plastic were all goners. None survived 24 hours before being gobbled up by the pike.

The researchers aren't sure how plastic affects a fish's ability to pick up the scent of predators. But they know the result is deadly.

Chelsea Rochman is an aquatic ecologist at University of Toronto in Canada. She says scientists know without a doubt that small pieces of plastic are littered across the planet. But they know very little about their impacts on wildlife.

"This study provides some of the first demonstrated evidence of impacts that are relevant to nature," she says. “They show that microplastic [tiny plastic] debris decrease the ability for a fish to hatch from its egg.” That means it will not survive, she adds.

The new data "also show a decrease in the ability of fish to escape a predator,” she points out. And that, she notes, “can ultimately reduce the size of the fish population."

Power Words

(for more about Power Words, click here)

Baltic Sea   A part of the north Atlantic Ocean that is enclosed by Scandinavia, Finland and Europe’s Baltic countries.

brackish   A term for water that contains a mixture of saltwater and freshwater.

debris    Scattered fragments, typically of trash or of something that has been destroyed. Space debris, for instance, includes the wreckage of defunct satellites and spacecraft. 

digest   (noun: digestion) To break down food into simple compounds that the body can absorb and use for growth. Some sewage-treatment plants harness microbes to digest — or degrade — wastes so that the breakdown products can be recycled for use elsewhere in the environment.

digestive tract   The tissues and organs through which foods enter and move through the body. These organs include the esophagus, stomach, intestines, rectum and anus. Foods are digested — broken down — and absorbed along the way. Any materials not used will exit as wastes (feces and urine).

ecology   A branch of biology that deals with the relations of organisms to one another and to their physical surroundings. A scientist who works in this field is called an ecologist.

flame retardants    Chemical coatings added to products, such as pyjamas, plastics, foam and furniture, to suppress or delay how fast they might burn in a fire.

freshwater     A noun or adjective that describes bodies of water with very low concentrations of salt. It’s the type of water used for drinking and making up most inland lakes, ponds, rivers and streams, as well as groundwater.

larva   (plural: larvae) An immature life stage of an insect, which often has a distinctly different form as an adult. (Sometimes used to describe such a stage in the development of fish, frogs and other animals.)

marine    Having to do with the ocean world or environment.

marine biologist     A scientist who studies creatures that live in ocean water, from bacteria and shellfish to kelp and whales.

pesticide     A chemical or mix of compounds used to kill insects, rodents or other organisms harmful to cultivated plants, pet or livestock, or unwanted organisms that infest homes, offices, farm buildings and other protected structures.

plastic    Any of a series of materials that are easily deformable; or synthetic materials that have been made from polymers (long strings of some building-block molecule) that tend to be lightweight, inexpensive and resistant to degradation.

pollutant    A substance that taints something — such as the air, water, our bodies or products. Some pollutants are chemicals, such as pesticides. Others may be radiation, including excess heat or light. Even weeds and other invasive species can be considered a type of biological pollution.

polychlorinated biphenyls (PCBs)    A family of 209 chlorine-based compounds with a similar chemical structure. They were used for many decades as a non-flammable fluid for insulating electrical transforms. Some companies also used them in making certain hydraulic fluids, lubricants and inks. Their production has been banned in North America and many countries throughout the world since around 1980.

polystyrene    A plastic made from chemicals that have been refined (produced from) crude oil and/or natural gas. Polystyrene is one of the most widely used plastics, and an ingredient used to make a widely used white, rigid foam (often sold under the name Styrofoam).

population    (in biology) A group of individuals from the same species that lives in the same area.

predator   (adjective: predatory) A creature that preys on other animals for most or all of its food.

prey   (n.) Animal species eaten by others. (v.) To attack and eat another species.

toxic   Poisonous or able to harm or kill cells, tissues or whole organisms. The measure of risk posed by such a poison is its toxicity.

zooplankton   Small organisms that drift in the sea. Zooplankton are tiny animals that eat other plankton. They also serve as an important food source for other marine creatures.

NGSS: 

  • MS-LS1-5
  • MS-LS1-7
  • MS-LS2-1
  • MS-LS2-4
  • MS-LS2-5
  • HS-LS1-1
  • HS-LS2-2
  • HS-LS2-6
  • HS-LS2-8
  • HS-ESS3-6

Citation

O. Lönnstedt et al. Environmentally relevant concentrations of microplastic particles influence larval fish ecology. Science. Vol. 352, June 3, 2016. doi: 10.1126/science.aad8828.

Further Reading

J. Raloff. "Home, plastic home." Science News for Students. July 16, 2013.

 

S. Ornes. “Cleaning clothes dirties oceans.” Science News for Students. October 5, 2011.

A. Martinez. “Swirling seas of plastic trash.” Science News for Students. June 22, 2011.