Three P.E.I. rivers hit with fish kills in past week

Anoxic events occur when oxygen is depleted from the water

Eric McCarthy
Published on August 4, 2016

The beach along the Montrose River is carpetted with sea lettuce. Rotting sea lettuce in the water is seen as the likely cause of an anoxic event last weekend that led to the discovery of dead fish in the river. Anoxic events have been reported in four Island rivers over the past week, and fish deaths were discovered in at least three of them.

©Eric McCarthy/TC Media

ALBERTON - Anoxic events have led to fish deaths in at least three P.E.I. rivers in the past week, says conservation officer Wade MacKinnon.

Anoxic events occur when oxygen is depleted from the water.

MacKinnon, who is manager of investigation and enforcement with the province’s Public Safety and Justice department, said that since last weekend there have been reports of such events occurring in the Montrose, Dock and Trout rivers in West Prince and a section of the Southwest River near Margate. He said dead fish have been observed in at least three of those systems. He couldn’t confirm whether there were dead fish in the Trout River, near Coleman.

All of the anoxic events as well as the fish kills occurred in the tidal portion of the waterways, MacKinnon said. He indicated none of those events are believed to be directly linked to pesticide use.

Anoxic events, he said, are often caused by nitrates in the waterway, which stimulate the growth of sea lettuce and usually occur near where the watershed is more densely populated or where intense agriculture activity occurs.

“We’re not seeing (anoxic events) in many areas that don’t meet those conditions,” he said.

There were no reports of trout being killed by the anoxic events, which MacKinnon says pretty well rules out pesticide runoff. He said trout are usually up in the fresh water portions of the streams where the water is cooler this time of year. Had a pesticide flowed through the stream, there would have been dead trout found, too, he said.

MacKinnon estimates the biggest of the fish kills occurred in the Southwest River and in its Doiron and Tuplin Brook tributaries. A kayaker reported seeing thousands of dead fish, including mummichogs (gudgeons) on Tuesday.

MacKinnon agreed mummichogs are known to be hardy little fish, but he said if they get trapped in tidal pools during low tide, then they can’t escape from the oxygen-depleted water and they die.