Pierson Hill

A little Prozac makes guppies most peculiar.

Exposed guppies freeze up, hide out, and move around less after a simulated attack. Such behaviors do not bode well for fish populations.

A little bit of an anti-depressant makes wild guppies less active, camp out more under plants and freeze up for longer after something scares them, according to a new study.


The latest research out of Australia adds to mounting evidence that one the world’s most widely prescribed drugs—fluoxetine, most commonly known by the trade name Prozac—is getting flushed into waterways and making fish act different. And different isn’t good: The drug appears to be changing habits designed to keep fish alive and reproductive.

Fluoxetine, used to treat depression and anxiety, is the third most prescribed drug in the U.S., with about 28 million prescriptions annually. The drug is frequently found in waterways by way of people excreting the drug or flushing pills.

Credit: Jake Martin

Researchers collected wild adult guppies from Queensland, Australia, and put 60 fish in tanks with a low dose of fluoxetine, 60 in with a high dose and 60 in drug-free water. After 28 days they started testing their behavior, specifically simulating a bird strike and seeing how they responded. They used a fake heron, a bird that preys on guppies, and had it strike into the center of the tank and then immediately retract.

"There is no doubt those are environmentally relevant treatment levels, and there’s no doubt there are changes in behavior."-Bryan Brooks, Baylor UniversityFish exposed to fluoxetine remained stationary (called “freezing”) for longer and spent more time under plant cover. Both freezing and heading to plant cover are anti-predator behaviors for fish and are crucial to their survival.

“There is no doubt those are environmentally relevant treatment levels, and there’s no doubt there are changes in behavior,” said Bryan Brooks, a professor of environmental science at Baylor University who was not involved in the study.

The findings, published in the journal Aquatic Toxicology, were sex-specific, with dosed females spending more time under plant cover and dosed males becoming less exploratory. These may seem small changes but they’re important for tiny fish often preyed upon. “Time and energy used in remaining stationary is diverted from other fitness-related activities, such as foraging and finding mates,” said lead author Minna Saaristo, a research associate at Monash University in Australia, via email.

Saaristo said that male guppies benefit from being “bold and exploring new territories” to find mates, so any changes to their behavior could harm the overall fish population.

Anti-depressants, such as fluoxetine, work by increasing serotonin function in the brain. This system of the brain is similar across vertebrates, Saaristo said. Drugs that affect human brains can and will affect fish brains.

“These drugs are intended to alter behavior,” Brooks said.

More than a decade ago Brooks and colleagues were the first to find anti-depressants in fish. They reported fluoxetine and other drugs in every liver, brain and filet tissues they tested from fish in a north Texas stream.

Since then multiple studies have shown fluoxetine is not only accumulating in fish but altering behavior.

In a landmark 2013 study, University of Wisconsin-Milwaukee researchers found that fluoxetine-exposed male minnows were anxious, anti-social and aggressive—in some cases killing females. The lab work suggested that the drugs altered how genes in the fish brains were expressed, or turned on and off. The minnows were exposed when they were a couple of months old and still developing.

Last March University of New England researchers reported that male Siamese fighting fish were less aggressive and bold after fluoxetine exposure.

Brooks said two of the current study’s strengths were using guppies—which are not routinely studied—and dosing at levels that are commonly found in waterways.

Brooks said he is encouraged to see such studies specifically looking at fish behavior. Pharmaceuticals are still tricky compounds that can “fall through the regulatory cracks,” he said. Traditionally water contaminants—think pesticides and metals—have been tested for toxicity but not the subtle behavioral changes Saaristo and others are reporting.

It’s important to look for behavioral changes to aquatic creatures, Brooks said, because our waters increasingly have pharmaceuticals such as fluoxetine, and certain endocrine disrupting compounds, that may not prove acutely toxic but alter behavior and development.

Saaristo said the next step for her research group is to tease out if there are indeed fish population-level impacts from the fluoxetine exposure.

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