MONTREAL -- There is "little likelihood" Montreal's plan to dump eight billion litres of raw sewage into the St. Lawrence River will have an effect on fish reproduction if it's done before the winter months and monitored properly, officials with Environment Canada said Friday.

Caroline Blais, a director at the federal agency, said the expert panel hired by the federal government concluded the risks associated with waiting are worse than the city's plan to dump the sewage immediately into the river.

"The experts' evaluation is that the risks of not going ahead could result in impacts that are larger than if we didn't proceed in the way Montreal wants to go," she said.

Blais said the agency, however, has not yet made a final recommendation to Environment Minister Catherine McKenna regarding whether or not to authorize the discharge of the untreated sewage.

McKenna is expected to make a decision by Nov. 9, the day Environment Canada's suspension lifts on Montreal's plan to dump the dirty water.

Montreal Mayor Denis Coderre's announcement in early October that the city "had no choice" but to release the sewage sparked public consternation and national attention, particularly due to the fact it came during a federal election campaign.

Coderre has said the dump is necessary because the city must temporarily close a large sewer that feeds sewage to a treatment facility and alternative solutions would be too costly.

The temporary closure is due to the city's plans to relocate a snow chute and conduct critical work on the sewage infrastructure.

In mid-October, the discharge plan was suspended by the previous Conservative government, which appointed an independent panel of experts to determine if the waste water would be acutely toxic.

Coderre quickly rebuked the Tories, repeatedly insulting and criticizing them for being anti-science and for suspending the work in order to seem more environmentally conscious during an election campaign.

The panel's report, released Friday, concludes that the fall is the best time to dump the sewage because doing so in other seasons could disrupt fish-spawning cycles.

The scientists warn, however, "efforts must nonetheless be made to ensure that sediment does not accumulate ... in order to avoid a longer-term increase in toxicity."

Montreal's sewer system is aging, the panel noted, and a failure to upgrade the infrastructure -- which the city said it plans on doing during the temporary closure -- could cause a rupture and an unplanned discharge of sewage during fish-spawning months.

"Not performing the previously described work ... at the very least, could lead to an equivalent discharge of untreated waste water into the St. Lawrence River" during months that are much more dangerous to the ecosystem, the scientists noted.

The panel added that Montreal should implement several additional mitigation measures aside from what the city is already planing to do.

They include monitoring the plume of discharge more extensively in order to better plan for emergency clean-up measures, and considering using a tanker ship at specific discharge sites to filter out large debris and solids from the water before releasing it into the river.

A spokesman for the city of Montreal said in a statement Friday "the document reveals the urgency of the need for repairs to the sewage system."

Philippe Sabourin added the mitigation measures the experts suggested "will be studied and taken into consideration."

The St. Lawrence River is one of the most important waterways on the continent and drains more than 25 per cent of the freshwater reserves on the planet.

Roughly 15 million Canadians and 30 million Americans live along the river's drainage basin, which includes the Great Lakes.