Antidepressants are making their way into freshwater ecosystems and changing the way fish behave while hunting for food, according to biologists.

Researchers have found that fluoxetine, the main ingredient in Prozac, can disrupt the foraging behaviour of mosquitofish (Gambusia holbrooki), a species of freshwater fish found in the US and Australia.

Like most small freshwater fish, mosquitofish tend to hunt in social groups – shoals – to forage more efficiently and reduce the risk of predation.

Pollutants

In their study, published in the journal Biology Letters, the researchers found that fluoxetine had no apparent impact on solitary fish.

But in the case of mosquitofish shoals, exposure to high doses of the drug affected their overall food consumption and altered their foraging behaviour.

The team said its findings suggest that “social context may be an important, but underappreciated, factor influencing the ecological impacts of chemical pollutants on wildlife”.

Fluoxetine, along with many other drugs, is often passed through urine and makes its way into freshwater bodies such as lakes and rivers because water treatment systems cannot filter it out.

These psychoactive pollutants have been shown to influence behaviour in animals – with a recent study by King’s College London reporting that cocaine contamination in the River Thames was making eels hyperactive.

Confounded

Previous studies have shown how antidepressants affect individual animals but few have questioned whether “the impacts seen in social isolation are reflective of those in a social context”, the researchers said.

To find out whether fluoxetine affected group dynamics, the team from Monash University in Melbourne, Australia, exposed 206 fish in the lab to fluoxetine levels found in freshwater bodies.

The low-fluoxetine group was given 30 nanograms per litre (ng/L) of the drug while the high-dose group was exposed to 300ng/L of fluoxetine.

The team said it only used female mosquitofish in the experiments to “control for any effects of sexual behaviour that may have confounded the results”.

Risk

This is because previous research by the team has shown that when exposed to fluoxetine, male fish end up spending much more time pursuing female fish for sex than they would normally.

After being exposed to fluoxetine for 28 days, the behaviour of the solitary fish remained unchanged by the drug.

However, the groups of fish that were exposed to high levels of fluoxetine were less aggressive and consumed less prey compared with shoals exposed to none, or low levels, of the drug.

The researchers wrote in their paper: “Our results suggest that behavioural tests in social isolation may not accurately predict the environmental risk of chemical pollutants for group-living species and highlight the potential for social context to mediate the effects of psychoactive pollutants in exposed wildlife.”

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Nilima Marshall is a PA science reporter.