Environmental officials were on the scene Friday morning in Nuns' Island, where hundreds of dead fish were found in a small artificial lake.
Catherine Kellner lives nearby and noticed the fish while walking by the lake a couple of days ago.
"The lake was still frozen over, and the dead fish were trapped in the ice floating, and thousands of seagulls were just feasting, and then along the borders of the lake was an unusually high number of dead fish," said Kellner.
The lake has since thawed completely, and municipal workers scooped the fish out of the lake with nets Friday and hauled them away in garbage bags.
Verdun borough mayor Jean-Francois Parenteau said Friday this is a recurring problem.
"In the wintertime, the water supply is too low, and there's a formation of a layer of ice on the top of the lake preventing the water from receiving oxygen," said Parenteau.
He said the fish suffocate from the lack of oxygen.
Parenteau said this year more fish than normal died because the cold winter created a thicker ice cover.
He says the borough plans to build a pumping station this summer to bring oxygenated water from the St-Lawrence river into the lake, which should fix the problem.