Heat, lack of oxygen likely caused fish to die in a west-end Toronto pond: province
Provincial results reveal there are no indications of a chemical spill in the area
Provincial officials have deemed the sudden die-off of hundreds of fish in Toronto's Topham Pond as a naturally occurring event due to high temperatures and a lack of oxygen in the water.
It comes after park management and residents last week discovered the pond in Eglinton Flats park teeming with the remains of several small and large fish.
While the Toronto and Region Conservation Authority (TRCA) is waiting to see water quality results, it agreed with the Ontario Ministry of Environment, Conservation and Parks (MECP) that the fish likely died from a lack of oxygen and changing temperatures.
"Similar incidents have previously occurred elsewhere within TRCA's jurisdiction, particularly in shallow ponds, and are often associated with low oxygen levels or changes in water temperature," read a TRCA statement.
It added that water quality samples had been collected and that the TRCA would continue to identify the exact factors behind the fish die-off.
In a statement, the MECP said its own test results found no indications of a chemical spill in the area that killed off the fish.
It said it had supervised the city this week during a clean-up of the pond's dead fish and had collected additional samples to validate the TRCA's upcoming results.
The ministry expects its own results to be available later in July.
David Clark with Toronto Urban Fishing Ambassador, which according to its website promotes recreational fishing in Toronto and the Greater Toronto Area, says he spoke with the MECP to understand what he witnessed at the pond.
In an email, Clark said he took pictures of an oil-like substance on the pond's surface, which the ministry said is the result of decaying organic matter in die-off events.
He said the ministry also agreed with him that the presence of phragmites, an invasive grass that causes damage to wetlands and beaches in North America, was likely making the situation worse.
Clark is recommending officials look at ways to improve pond conditions, for example by removing phragmites that impede water circulation, "as well as floating the idea of a fountain or two to improve pond circulation and oxygenation."
Clark added he would contact the Ministry of Natural Resources about restocking the pond with fish after proactive measures are taken to make sure this does "not happen again."
Shallow, nutrient-rich ponds at risk, expert says
Ponds that are shallow and have high levels of nutrients — resulting in more vegetation — are more likely to see large amounts of fish die, said Don Jackson, a fish ecologist and professor at the University of Toronto.
As water temperature increases, he said the metabolism of everything living in the water also increases. Meanwhile, the ability of water to hold oxygen goes down.
Oxygen can get further depleted overnight as vegetation, such as aquatic plants and algae, go from producing oxygen during the day to consuming oxygen at night time, he said.
"You can reach conditions in ponds at 3 o'clock, 4 o'clock in the morning before sunrise where oxygen can approach zero levels," Jackson said in an interview with CBC Radio's Metro Morning on Friday. "It can get really stressful for lots of organisms."
He said reducing fertilizer runoff and picking up pet waste can lower nutrient levels in the pond, limiting vegetation growth and helping prevent oxygen levels from dipping too low.
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With files from Kirthana Sasitharan, Dale Manucdoc and Metro Morning