Heat wave takes toll on fish

Richard Froese
South Peace News

A heat wave in late June has been determined as the cause of fish dying in streams, rivers and lakes in Alberta.

That’s the word from Mike Sullivan, the fisheries science specialist for Alberta Environment and Parks.

“We’ve seen it all over the province, with many reports from popular and visible waters like Lac Ste. Anne, Pigeon Lake and urban streams,” Sullivan says.

“The extent of the kill will be lake and river specific, but can easily be in the hundreds of fish.”

He says a summerkill is unusual in June.

“We typically see summerkills of fish in late July and early August,” Sullivan says.

Temperatures over a wide part of Alberta reached 35-40 C from June 25 to July 2. During that time, many fish were reported dead in waters, including the East Prairie River east of High Prairie.

“Summerkills of fish happen every year,” Sullivan says.

“This was earlier than normal, and the really high heat made it more severe than normal.”

Warm sunny days result in lots of plant and algae growth in the water, Sullivan says.

After the burst of summer growth, the plants and algae start to die off and use more oxygen than they produce.

“Low oxygen and warm water create situations where fish can’t get enough to breath,” he says.

“Sounds weird, but the fish drown.”

It’s part of natural weather conditions, he says.

“We expect most fish kills in Alberta’s most productive waters, places with good numbers of fish and lots of fish-food,” Sullivan says.

“That is a double-edged sword as more productive waters also can be on the verge of being too productive with algae blooms and consequently, occasional summerkills.”

Low water levels also cause fish to die.

“Really hot weather often means drought and low water levels and shallow water warms up fast,” Sullivan says.

Just a few types of fish are mostly affected by the heat.

“Mostly, we have been seeing dead white and longnose suckers in the agricultural areas because these fish are adapted to living in those slow, silty streams,” Sullivan says.

“On the lakes, we typically see lake whitefish and tullibee dying.”

Whitefish typically need cooler water and higher oxygen levels.

In faster rivers like the lower reaches of the McLeod and Pembina rivers, mountain whitefish were dying as that species needs cool water and high levels of oxygen.

He notes the temperatures were the highest he could recall in several decades.

“That heat wave was unprecedented in my 40 years as a fisheries biologist,” Sullivan says.

“I’ve seen some awful fish kills in years past, when excessive nutrient run-offs and warm [not hot] summers combined to create severe summerkills.”

He says 1990 was also a bad year.

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