Health Canada has granted fish farms in Newfoundland and Labrador emergency approval to use a pesticide to help control sea lice.
Salmon farmers on the island's south coast have had major issues with the parasites because of warm water temperatures.
The Newfoundland Aquaculture Industry Association said the use of the pesticide Salmosan is a necessary move to help fight outbreaks that can take a serious toll on caged fish.
Miranda Pryor, the association's executive director, said there's no need to worry about the pesticide's use.
"These are very safe. They're very safe for the fish," she said.
"They're very safe to be used in the marine environment and they're certainly safe from a consumer perspective."
Mildred Skinner, Fish, Food and Allied Workers union representative, said lobster fishermen on the island's south coast fear it will harm their catches.
"The problem is we don't know the impact of this stuff," she said. "You know it was restricted by the government for some reason."
Matt Abbott, marine coordinator of the Conservation Council of New Brunswick, said studies show that Salmosan does in fact harm many sea creatures.
"Things designed to kill a crustacean like sea lice can certainly have an impact on other crustaceans, be they lobster, be they shrimp, krill, crab," he said.
Abbott said salmon farms should move towards raising fish in contained tanks on land.