Countless birds and fish are dying in a retention pond that was once a haven for wildlife, residents in Winnipeg's Mandalay West neighbourhood say.

Garvin De La Mothe and his girlfriend often visit the pond in Santa Fe Park at the corner of Dr. Jose Rizal Way and Adsum Drive to watch birds frolic​ in the water. On Aug. 16, they were startled to see dozens of dead or dying birds coated in green algae, and dead fish floating at the water's surface.

"What I saw was probably 20 or 30 birds — Canadian geese, mallards [and] song birds," said De La Mothe, who lives just a block away from the pond. "Literally thousands of minnows."

The couple reported the deaths to 311. They said workers came to collect the animals a couple days later.

"It really affected my girlfriend and myself," De La Mothe said.

Garvin De La Mothe and his girlfriend saw a mass death of birds and fish on Aug. 16 at the Santa Fe Park retention pond. (Jeff Stapleton/CBC)

By Thursday most of the dead animals had removed. CBC saw three dead birds covered in flies on the pond's gravel shoreline.

A video De La Mothe took with his phone last Thursday shows hundreds of silver minnows and larger fish dead alongside waterfowl​.

"Now I call it the death pond," he said.

Birds sent for rehabilitation

Animal advocate Choo Rosenbloom saw the dead birds as well. After seeing a gull in distress, she ran to grab a box she thought would cradle it safely.

"I went to the car, came back to the pond and he was belly up," she said. Within minutes, the seagull died.

Rosenbloom did recover two pelicans and three other seagulls, which she took to the Prairie Wildlife Rehabilitation Centre in Winnipeg. She hopes they will recover — but isn't optimistic.

"Then you go down there and you see the three pelicans — large, majestic birds — unable to swim, just gasping for breath," she said. "It's really heartbreaking. That's why I was trying to catch them all."

Cause of deaths remains murky

The cause of the die-off is still unknown.

A spokesperson for the City of Winnipeg, Michelle Finley, says early data points to natural causes but work is still being done to investigate.

The city sent samples of the dead animals to the provincial chief veterinarian's office for tests.

A spokesperson for Sustainable Development says staff are "working diligently to determine what has led to the deaths."

One possible cause is botulism, which can occur naturally in ponds, the provincial spokesperson says.

"Botulism outbreaks are common at this time of year. Unfortunately, little can be done to address this until weather conditions change with cooler temperatures and/or rain that would raise the water level in the pond.

"In extended periods of hot weather, blue-green algae can develop, and this has also been known to cause fish and bird mortality."

A dead bird washes up on the shore of the retention pond in Santa Fe Park on Aug. 23, 2018. (Jeff Stapleton/CBC)

Lisa Tretiak, president of the Prairie Wildlife Rehabilitation Centre, says it isn't abnormal to find one or two dead birds in a pond, but as many as a dozen at a time is peculiar.

"There is something very specific within this pond that is killing these animals," she said.

Tretiak believes the toxic algae may be causing the deaths because both fish and birds are being affected.

The centre responded Wednesday night to a request to pick up animals the city had rescued from the pond. On Thursday the centre posted that three of the birds had died. ​

The province and city is urging people to avoid contact with the water, keep pets away and avoid touching dead birds.

Feeding birds an issue

Multiple residents CBC spoke with on Thursday said wildlife may be dying in the lake in part because people regularly feed birds in the area.

De La Mothe says he has witnessed people throwing everything from dinner leftovers to popcorn and bread crumbs at the shore — all food unhealthy for birds to eat.

​In Finley's written statement, she said the city urges residents to avoid feeding wildlife.

"Feeding wild birds at retention ponds is not recommended as it causes the number of birds in the area to grow at an unsustainable rate and can lead to an increase in avian diseases," she wrote.

Any injured or sick wildlife should be reported to Manitoba's Department of Sustainable Development, she said. Dead animals should be reported to 311.