In the warm waters flowing from an Ontario nuclear plant, fish are gathering by the thousands in Lake Huron — a striking scene now visible in rare detail.

Using a $250,000 underwater drone — one of only about 10 of its kind in the world and the only one operating in fresh water — documentary filmmakers Yvonne Drebert and Zack Melnick are capturing footage from parts of the Great Lakes that are often too deep or remote to observe directly.

CBC News was given rare access to footage from once such dive ahead of a livestream event planned for Earth Day to promote the premiere of their new documentary Hidden Below: the Great Lakes, a series featuring rare wildlife from the largest freshwater ecosystem on Earth.

In this case, the drone will focus on the warm water outflow from Bruce Nuclear Generating Station, near Tiverton, Ont., and the fish that use it as an open water refuge and feeding ground.

‘Fish city’

Bruce station is North America’s largest nuclear plant and releases water as part of its reactor cooling system. The area, with its elevated temperature, is well known by locals as a place that attracts fish.

The rare images of this phenomenon — captured in cinematic 4K by Melnick and Drebert — show scenes reminiscent of the ocean with millions of silver-bodied gizzard shad flashing in the sunlight, while at least a dozen other species live alongside them, including bigmouth buffalo, walleye, bowfin, salmon, trout, catfish and carp.

“No one really had a chance to look under the water there and it was pretty jaw-dropping,” Drebert said.

Melnick said they called it “fish city,” describing the unusually dense gathering of fish.

“If you were to ask us what’s one spot to introduce people to the wonder and the magic of that freshwater world, we would say to come here to these warm waters.”

Scientists say the warmer water in the plume creates an abundance of food for the fish, increasing nutrient cycling and accelerating algae growth. The plants attract fish who feed on them and in turn, the predators that follow.

The illusion of abundance

“Just about all of the Great Lakes have these thermal plumes,” said Nicholas Mandrak, a University of Toronto biologist who is one of the country’s leading freshwater fish scientists and will also provide live commentary during the Earth Day stream of the so-called “fish city.”

He said the most significant thermal plumes come from power plant cooling systems, but also include industrial discharges, wastewater treatment outflows and the mouths of rivers.

While some might see millions of fish congregating in the warm waters as a sign of abundance, scientists say it may actually reflect an ecosystem that has fundamentally changed.

“The fact they’re concentrating at thermal plumes because there’s a lack of food in the main lake, is an unhealthy sign of what the mussels have done.”

By “mussels,” Mandrak is referring to zebra and quagga mussels — two invasive species from Europe. Individually, the mollusks about the size of a pistachio, but both species have multiplied into the billions, causing widespread ecological and economic damage in the Great Lakes since their introduction in the 1980s.

The tiny, clingy shellfish strip plankton from the water, shrinking the available food. They form in thick, dense mats that smother habitat and clog municipal and industrial water intakes.

“If the food levels were still high through the Great Lakes like they were before the mussels, we would probably see less concentration of fish in these these plumes,” Mandrak said.

Bruce Power, the company that runs Bruce Nuclear Generating Station, did not answer questions about how big or warm the outflow is by deadline.

Sapna Sharma, a York University professor who studies environmental stressors on lakes, said that because most Canadian fish species are adapted to colder water, some may be suffering and the warmer conditions also allow invasive species to gain a toehold in northern ecosystems.

“Our native fisheries require cold water, well-oxygenated conditions,” she said. “The Saugeen Ojibway Nation lives on the shores of Lake Huron and they’ve been fishing for lake whitefish since time immemorial and having warmer waters is negatively impacting the lake whitefish that the community relies on.”

A 2020 report prepared for Saugeen Ojibway Nation said the causes of Lake Huron whitefish declines remain uncertain, but cited research that suggests warm spring conditions were linked to poor survival of young whitefish in one nursery area.

The study said multiple stressors, including invasive species and habitat change, are also reshaping the fishery.

Sharma said the Great Lakes are an extraordinary resource that supports agriculture, transportation and provides fish and drinking water for more than 40 million people, so it’s important not to take them for granted.

She said to see something unusual, such as the so-called “fish city,” raises a red flag.

“It raises questions about what’s behind it — especially whether warm water discharge from nearby industry is changing the ecosystem, and what that means for the people who depend on it.”