Our increased use of antidepressants is having a knock-on effect on nature: scientists have found evidence of these drugs in the brain tissue of 10 different species of fish in the Great Lakes region.

Not only does it threaten the biodiversity of the lakes, the researchers say, it can also cause harmful changes to the behaviour of the fish.

The international team of researchers says there's no danger to humans from eating these fish, but that more needs to be done to get these antidepressant chemicals out of our wastewater before they have a chance to damage marine life beyond repair.

"These active ingredients from antidepressants, which are coming out from wastewater treatment plants, are accumulating in fish brains," says lead researcher Diana Aga from the University of Buffalo. "It is a threat to biodiversity, and we should be very concerned."

"These drugs could affect fish behaviour. We didn't look at behaviour in our study, but other research teams have shown that antidepressants can affect the feeding behaviour of fish or their survival instincts."

That previous research suggests, for example, that some fish become less aware of predators when hit with antidepressant chemicals, although the reaction to the drugs varies between species.

The team looked at ten different species of fish in the Niagara River, a crucial channel connecting Lake Erie to Lake Ontario.

In all of the species – smallmouth bass, largemouth bass, rudd, rock bass, white bass, white perch, walleye, bowfin, steelhead, and yellow perch – they found evidence of antidepressants.

The highest concentration of drugs was found in a rock bass: 400 nanograms of norsertraline per gram of brain tissue. Norsertraline is produced in the body by sertraline, the active ingredient in Zoloft.

These chemical levels aren't as high as the ones artificially generated in marine species for related research, but they're still very worrying for scientists.

What's more, the concentrations of drugs in the brains of the fish were significantly higher than the concentrations in the river water, suggesting the fish are picking up more and more of the chemicals over time.

For the researchers, it's important that wastewater plants do more to filter out chemicals like those from antidepressants, which appear in the urine of the people taking them.

These plants generally concentrate on killing bacteria and removing solids and faeces, but need to address drugs that are often being ignored, says the team behind the study.

"These plants are focused on removing nitrogen, phosphorus, and dissolved organic carbon but there are so many other chemicals that are not prioritised that impact our environment," says Aga.

"As a result, wildlife is exposed to all of these chemicals. Fish are receiving this cocktail of drugs 24 hours a day, and we are now finding these drugs in their brains."

As for exactly how these chemicals might damage fish or change their behaviour, or what the knock-on effect might be to the broader ecosystem, scientists just don't have enough information to go off yet – but their concerns are very real.

With antidepressant use rising steadily in the US and other countries, the problem needs to be addressed before more damage is done. Let's hope people can get the help they need and our rivers and lakes can be kept free of the chemical fall-out.

"The risk that the drugs pose to biodiversity is real, and scientists are just beginning to understand what the consequences might be," says one of the team, Randolph Singh from the University of Buffalo.

The research has been published in Environmental Science & Technology.