Male fish exposed to levels of the common antidepressant fluoxetine at levels equivalent to those found in many freshwater ecosystems spend more time chasing females than if the water is pure, a study has found. Although this effect is yet to be observed in the wild, it is feared polluted waters may encourage unsafe behavior.

Many cities' wastewater are becoming contaminated with drugs, both illegal and prescription, flushed through our bodies. Treatment plants don't remove all these molecules, and disturbingly large amounts are being detected in some ecosystems, leading to phenomena such as eels high on cocaine.

Fluoxetine, marketed as Prozac, is a particular concern, with one study finding it makes crabs more aggressive. Monash University PhD student Jake Martin has been testing the effects on mosquitofish (Gambusia holbrooki), a model species often used to predict more widespread fish responses.

In Science of the Total Environment, Martin reports that when male fish were kept for five weeks in fluoxetine-laced water and then put into tanks with females, they spent more time chasing their new tank-mates than controls raised in clean water. The finding is slightly surprising, since Prozac is not reported to increase sexual activity in humans. Indeed, there are disputed claims it may have the opposite effect. Nevertheless, Martin noted to IFLScience that the drug acts on the serotonin system, known to be important in reproduction in both humans and other animals.

Given that male mosquitofish are notorious among zoologists as sexual harassers, with females expending a lot of energy to avoid their attentions, a further increase in this behavior is quite something.

Martin told IFLScience one fear is that if fluoxetine makes male mosquitofish even more sex-obsessed, they may make themselves more vulnerable to predators. Feeding time may also be reduced. He is also concerned about the effect on the females, who already suffer from the efforts they are forced to engage in to avoid male attention, even under ordinary conditions.

To limit the possible variables, Martin raised the females in the trials in unpolluted water, but hopes to do future testing to see whether their behavior changes when exposed to the drug too.

Martin wasn't just interested in fluoxetine's effect on mosquitofishes' libidos. In the same paper, he reports on whether the fish raised in drugged waters were bolder when placed in a new environment. On this count, Martin found no significant difference, but in a previous paper, he reported fish raised in fluoxetine-laced waters were more willing to pass close to a simulated predator.

Martin also reported fluoxetine disrupted a clean-water correlation between exploratory behavior and time spent chasing females. He told IFLScience: “This may throw selection pressures out,” justifying further study.