Death by road salt. Death by paint discarded by negligent contractors. Death by ash from a prescribed burn.
These are three of the theories residents in a local Facebook group came up with for why dozens of dead fish have appeared in one of High Park’s ponds.
But none of them are the most likely culprit, according to four aquatic experts who spoke with TorontoToday.
What likely killed these fish is suffocation — the result of a natural phenomenon known as “winterkill.”
What’s winterkill?
It’s a smaller, southeastern pond in High Park where the dead fish were spotted by a local resident, who alerted others in a neighbourhood Facebook group last weekend.
On Monday, when TorontoToday visited the park, more than three dozen dead fish were spotted near on the eastern edge of the pond, which is directly south of Deer Pen Road.
Many of the fish were frozen into the ice, while others bobbed in the water or had sunk to the bottom of the pond. The deceased fish included inch-long goldfish and six-inch bullhead catfish.
Trent University biology professor Graham Raby said the fish most likely died as a result of a lack of sufficient oxygen in the pond during the winter.
Aquatic life has a tough time surviving through the winter in ponds because there is less oxygen in the water compared to lakes or rivers, he said.
“The fish and other organisms … gradually use up all the oxygen in the pond until at some point you start getting lots of fish dying,” he added.
Aquatic life in the High Park pond likely had a particularly tough time over the past few months because of the long winter, said Toronto Metropolitan University biology professor Andrew Laursen.
“There was ice cover for a longer period of time than we've had in recent years. [This meant] there's no way for oxygen to exchange across the air-water surface,” he said.
Asked about the fish, a spokesperson for the Toronto Region Conservation Authority cited the same suspected cause of death, and noted that staff stopped by the pond today to assess the situation.
“Once the ice melts, the presence of dead fish becomes evident,” the spokesperson said.
Why do fish die in some ponds and not others?
On Monday, TorontoToday was only able to spot the dead fish in one part of the pond — and did not see evidence of dead fish in some of the other ponds in High Park.
To the untrained eye, this may seem to suggest a nefarious cause of death, though experts said it’s not surprising.
Deeper ponds or those fed by another river or stream tend to have more oxygen, said University of Toronto professor of ecology Donald Jackson.
It’s possible that the eastern edge of the pond is particularly shallow or gets the least amount of additional water compared to the other ponds or parts of the pond, he said.
Laursen added that both of the park’s ponds closest to Parkside Drive tend to be low-oxygen environments, because they are part of the city’s stormwater management system.
When significant rain falls in Toronto, these ponds help capture some of the runoff. This is a positive for flood management, but means a lot of additional dirt and leaves flow into the ponds.
During the winter, bacteria eat these leaves and other organic matter. This process consumes oxygen, further depleting available air for the fish, said Laursen.
What about road salt?
Though experts said winterkill is the most likely cause of death of the High Park fish, some residents said they believed that overzealous road salting may have played a role, too.
Jackson said this may be a possibility, noting that salt levels found in Toronto region bodies of water have sometimes been far beyond what fish can tolerate.
“Some salt can be good for fish, but the level of salt that we have going into our bodies of water — sometimes it’s approaching stuff that’s closer to the ocean — and our fish don’t like that,” he said.
Mississauga’s Credit Valley Conservation Area provides real-time monitoring of chloride levels in nearby water bodies. Jackson noted that at some points this year after a thaw, he’s seen the chloride levels far beyond the maximum set by the federal government.
Laursen agreed road salt may have played a role in the mortality of the fish, noting that when animals are under more stress they consume oxygen less efficiently.
Other possible causes?
In response to the initial Facebook post on Saturday, one resident suggested a prescribed burn may have been the culprit for the demise of the local aquatic life.
Prescribed burns are small fires intentionally set by trained staff in the park to help restore the rare black oak savannah tree, according to the High Park Nature Centre.
A nature centre staff member told TorontoToday, however, that the park’s prescribed burn for this season hasn’t yet taken place, and thus cannot be the cause of the fish death.
The final theory raised in the Facebook group was that contractors or careless residents had put paint down the sewer, killing the fish.
Jackson said depending on the quantity and type of paint, it’s possible this played a role, while Laursen said he didn’t think it was very likely.
Regardless of the cause of death, Trent University biology professor Paul Frost said he doesn’t expect the deceased fish to remain in the pond for long.
“Maybe a raccoon will come along,” he said.
“The great blue herons aren’t here right now, but they would probably feast on that.”
