The persistent discharge of even small concentrations of female sex hormones into lakes and rivers can completely decimate wild fish populations, according to a new study by Canadian biologists.

The study, released Monday by Fisheries and Oceans Canada, found minnow populations in an experimental lake in northwestern Ontario began to collapse after prolonged exposure to small amounts of synthetic estrogen, similar to that found in birth control pills.

The study found male fish, including larger species like trout and suckers that have longer lifespans and feed on minnows, began producing egg proteins and that early stage eggs were even found in the testes of some of the fish.

The hormones also impacted the potency of male sperm, while female fish were found to produce more egg proteins, said researcher Karen Kidd.

"We knew male fish were becoming feminized because of the estrogens that are in sewage effluent," said Kidd, noting it's a phenomenon that's turned up in earlier studies.

"What we didn't know was what that means for the fish population. Can these males still successfully reproduce or is their reproduction affected by these estrogens? Are the populations sustainable or not?"

The seven-year study involved adding five to six nanograms of estrogen per trillion litres of water — the equivalent of a few grains of sand in an Olympic size swimming pool — and studying the effect on the fish population.

Kidd said the ratio is based on what's been found in the natural environment.

Since estrogen is a chemical that will degrade within weeks of entering lakes and rivers, compared to stronger pollutants that can linger for decades, Kidd said fish populations will generally recover once the hormone is removed.

Those with already limited lifespans like minnows, however, face more dire consequences with even limited exposure, she said.

"The answer is certainly not in reducing the use of birth control pills, it's making sure our wastewaters are treated effectively," Kidd said, noting water treatment is a municipal responsibility that varies widely across the country.

"We're always putting these effluents into our rivers, so fish are always getting exposed even though the compounds are not persistent."

While secondary wastewater treatment can get rid of up to 95 per cent of the estrogens released by sewage plants, Kidd said a lot of waste receives only primary treatment, or in some cases none at all.

John Steele, a spokesman with Ontario's Environment Ministry, said wastewater treatment facilities are required to abide by certain provincial, and to some extent, federal requirements, but it's ultimately municipalities that decide what's permitted to go into sewers.

Since many sewers contain both sanitary and storm waste, Steele said it's not uncommon for effluent to be discharged untreated to avoid flooding in periods of severe wet weather.

The provinces are working with the federal government to establish a set of standards for wastewater discharge, he added.

Kidd said most of Canada's water guidelines focus on more powerful chemicals and suggests the government ought to take a closer look at the harm posed by things like estrogen.

"We know from studies like ours, and others, that even though they are not persistent, they can still have a very dramatic effect on fish health," she said.