The Bow River likely won't be able to support its "world-renowned" recreational fishery in future as the river's rainbow trout population is in drastic decline, according to a new study.

From 2003 to 2013, the population of rainbow trout in the river dropped between 43 and 50 per cent, biologists from the University of Calgary and the provincial government found in the study published in the Canadian Journal of Fisheries and Aquatic Sciences.

The reality is if you want a high-quality rainbow trout fishery in a place like Calgary, you might have to consider things like a restriction through a lottery. - Chris Cahill, study author

Rainbow trout were first stocked in the Bow River nearly 100 years ago.

Thousands of anglers flock to Calgary each year — some from around the globe — bringing in an estimated $24.5 million to the local economy, said PhD student Chris Cahill, the lead author on the study.

"There were some pretty large declines in the numbers of adult rainbow trout in the lower Bow River, and we believe if those trends continue, it's probably unlikely the river will be able to support the world-renowned rainbow trout fishery in the future," said Cahill.

"It's a blue-ribbon rainbow trout fishery and that was how the Bow River actually became famous originally. So it's certainly concerning."

Chris Cahill, a PhD student at the University of Calgary, is the lead author of a study that found the population of adult rainbow trout in the Bow River declined significantly in recent years. (Riley Brandt/University of Calgary)

Cahill said his first reaction, when he saw how large the population decline was over such a short period of time, was to double-check the numbers.

"I was certainly surprised. It's not exactly the happiest feeling in the world when you find those sorts of things, but I also work in fisheries conservation, so that's the work that I do."

Since the fish aren't native to the region, they don't qualify for protection as a threatened or endangered species — but if they were native to the region, the drop would be large enough to place them on the threatened list, Cahill said.

Multiple 'smoking guns' behind drop

The researchers weren't able to pin down a specific cause leading to the decline, but instead flagged multiple stressors that Cahill described as "smoking guns."

The first is flooding. Major floods in 2005 and 2013 likely displaced fish downstream and modified habitats enough to impact populations.

Another factor is whirling disease — a parasite that infects trout and makes them swim in circles. It's usually fatal.

One factor that might surprise people is the increased mortality rate from practising catch and release.

Catch and release still causing deaths

Anglers in the Bow catch and release trout, with a low mortality rate of just three to five per cent of released fish. However, repeated catches can take a toll. One angler in the province's latest fishing regulation's document describes catching and releasing 11 rainbow and brown trout in the river in a single day.

"Cumulative deaths can add up, even if the catch and release mortality is low, basically, because it appears the fish are being caught multiple times per year," Cahill said.

Not much can be done about floods or whirling disease — but there are things fishermen and the government can do to limit catch and release deaths.

Cahill recommended anglers try to minimize air exposure for fish they catch, especially during hot weather.

"If you're taking pictures, try to remember the fish was basically just running a marathon under water," he said.

Researchers for the province tag and give water to a rainbow trout from the Bow River in 2013. The population of rainbow trout declined significantly between 2003 and 2013, a study found. ((Kyle Bakx/CBC))

He also said some governments have implemented measures like warm-weather closures, or a lottery system that limits the number of anglers allowed to fish each year.

"The reality is if you want a high-quality rainbow trout fishery in a place like Calgary, you might have to consider things like a restriction through a lottery to try and maintain the fishery."

Many native trout populations in Alberta are considered at risk, and the province has considered stream and river closures in the past to help tackle the problems, but closures/lotteries on the Bow River were not under discussion as part of those regulations as the fish are non-native.

Whatever's done, Cahill said it needs to be done in a way that can help researchers figure out what's actually causing the population decline.

"The science won't be there unless we actually undertake these bold adaptive management programs … those programs often require short-term pain for long-term conservation gain."