Mass fish die-off this winter in Banff 'not uncommon in nature'

For the second winter in a row, hundreds if not thousands of fish have died as a result of winterkill in Vermilion Lakes in Banff National Park.

Winterkill, which is a relatively common natural event in lakes in Alberta and elsewhere, is caused when fish don’t get enough oxygen as light can’t penetrate the surface and they perish beneath the ice.

“Fish kills like this one are not uncommon in nature,” said Mark Taylor, aquatics team lead for Banff National Park.

“We were only looking at four holes that were open in the ice so we can only see so much, but it’s definitely hundreds, if not thousands of fish.”

The dead and dying fish – white suckers, which can grow to 30-50 centimetres, and the smaller goldfish-sized brook stickleback – were first reported on Jan. 26.

“They were mostly gasping and still alive, but the writing was on the wall,” said Taylor.

“When you go back and look, then there were more mortalities.”

On March 22, a closure was put in place in an area adjacent to Vermilion Lakes Road between Vermilion Lakes 2 and 3 because the decaying fish may attract hungry bears.

Bear No. 122,  the patriarch of Bow Valley bears known as The Boss, was first reported out of his den just days earlier on March 18 and was roaming the valley bottom in search of early season food.

Split Lip, marked No. 136 has also now been spotted. He is No. 122's biggest competition for dominance.

“In January, it was deemed not a concern. There were scavengers around, like I am sure the ravens and coyotes were around,” said Taylor.

“But then there must have been a sighting at Vermilion or in the valley and that would have triggered more concerns about bears come March.”

Generally, contributing factors to winterkill can include low water levels, early ice-up or a late ice-out that contributes to extended total days with ice cover.

Additional factors can include how shallow the lake is, thickness of ice as well as snow cover, extended periods of cloudy weather, or larger than normal amounts of decomposing plants or algae on the bottom of a lake, among other factors.

In this case, Taylor said there was a lengthy cold snap prior to the discovery of winterkill, which he believes may be a contributing factor.

“That’s kind of the traditional way that a fish-kill happens, is really cool temperatures that create really thick ice, and either the thick ice or the snow blanketing on top of the ice is blocking light from penetrating into the water column,” he said.

“But I think there’s also potentially other contributing factors as well, because a cold spell, as you know, isn’t exactly unique here.”

In addition, Taylor said there have been anecdotal reports of aquatic plants in Vermilion Lakes seemingly doing better in recent summers than previously, noting this has actually been documented at Johnson Lake.

One theory, he said, is related to the record high summer temperatures.

“We had two summers in a row with heat domes, and then you get a hyper-abundance of vegetation that then has to decompose come winter, which will use up more oxygen under the ice potentially than a winter that followed a more regular summer,” he said.

“There’s some research that shows that that is possible, that if you have heightened amounts of macrophytes, then it can consume more oxygen in the following winter.”

Taylor said another contributing factor may be related to the depth of the lake, which is very shallow at about 1.5 metres.

“Those lakes are always more susceptible to fish kills than a more regular deep lake, where these fish kills are not an issue. They have enough oxygen” he said.

“It really is only a problem with shallow lakes and wetlands.”

Canmore’s Bob Sandford, senior government affairs liaison in global climate emergency response at the United Nations University Institute for Water, Environment and Health, said Vermilion Lakes are what limnologists characterize as polymictic lakes – lakes that are too shallow to develop thermal stratification.

“This means that their waters can mix from top to bottom throughout the ice-free period. You see this occurring every day during the summer,” he said.

“Continuous turnover means the waters are richly oxygenated. The hitch comes, however, with temperature increases.”

Sandford said it has been established by scientists that generally lakes in Canada are warming at twice the global rate, and points to warm temperatures over the past two summers.

“The warmer the water becomes, the less oxygen it can absorb, and if during the summer there is a riot of aquatic vegetation growth, that vegetation will break down after the lake freezes over, further diminishing oxygen levels in the lakes,” he said. “Voila! Fish die-offs.”

Sandford said he suspects this could be happening in Vermilion Lakes.

“That will likely continue in the future as lake temperatures on average rise with each degree of global warming,” he said.

Documentation from Bow Valley Naturalists shows winterkill at Vermilion Lakes in 1956 and 1964, but Taylor said he’s skeptical that those years are the only times it has happened other than 2023 and this year.

“I’m sure it’s happened again where we just didn’t notice,” he said.

Considered one of Banff National Park’s environmentally sensitive sites, the Vermilion Lakes wetlands include a diversity of vegetation and many rare and significant plant species, as well as important habitat for a variety of birds, mammals, and aquatic species.

Taylor said white suckers and brook stickleback are a common species in the wider ecosystem of Canada and the United States; however, are not widely distributed throughout Banff National Park.

“Vermilion is unique in Banff and that’s why it’s the main place where you’re going to find white suckers and stickleback – the shallowness and it warms up really well in summer,” he said.

“You don’t find them commonly everywhere here because we have so much cold water that it’s more appropriate for trout,” added.

“But in the larger ecosystem of Canada and the U.S., those are very common species that are found quite wide-ranging and they’re definitely not a threatened species.”