Study shows precipitous decline in population of adult rainbow trout in Bow River

The population of adult rainbow trout in the Bow River has declined dramatically in recent years, a trend that puts at risk the world-famous recreational fishery, University of Calgary scientists have found.

In a study led by fisheries biologists in the Faculty of Science and which included provincial government biologists, a research team found the fish population in the lower Bow River fell by 43 to 50 per cent in the 10-year period between 2003 and 2013. 

Rainbow trout were introduced many decades ago into the Bow, considered to be a "blue ribbon" fishery and prime tourist attraction worth an estimated $24.5 million a year. Because the fish aren’t a native species in the river, they don’t qualify for assessment and protection under the federal Committee on the Status of Endangered Wildlife in Canada (COSEWIC). 

But if the rainbow trout were native to the Bow, “COSEWIC could recommend them as threatened under the Species At Risk Act based on our trend estimates,” says Chris Cahill, a PhD student in the Department of Biological Sciences and lead author on the study. 

The researchers were unable, given provincial fish-monitoring data available during the 10-year period, to link the decline to a specific cause or causes. 

However, they noted that the Bow River fishery is affected by multiple stressors, including one of the highest angling efforts for inland fisheries in North America, notable floods and whirling disease (caused by a parasite that impairs the nervous system, especially in juvenile trout). 

“We likely have multiple causes and monitoring data that’s not of the sort that you need to clearly disentangle these causes,” says Dr. John Post, PhD, professor of ecology and evolutionary biology in the Department of Biological Sciences, a study co-author along with one of his post-docs and four graduate students. 

The research team built three different quantitative models to clearly identify and account for uncertainties in the data, such as various fish-sampling sites. All three models consistently showed the steep population decline. 

Their study, “Multiple Challenges Confront a High-Effort Inland Recreational Fishery in Decline,” is published in the peer-reviewed Canadian Journal of Fisheries and Aquatic Sciences. 

Angling effort may need to be limited

Regulations require rainbow trout in the lower Bow to be caught and released. But so many anglers use the river, some fish are likely being caught and released a half-dozen times or more per year, he says. Even if the catch-and-release mortality rate is only three per cent each time a fish is caught, the cumulative deaths add up. 

“Resource management is all about tradeoffs,” Cahill says. “If you want a high-quality rainbow trout fishery in a place like Calgary where there are lots of anglers and additional stressors that managers can’t control, you might have to consider limiting angling effort, perhaps through a lottery system.” 

A lottery is now used in Alberta to provide a limited harvest in some high-effort walleye fisheries which had previously collapsed. In past years, the province also has temporarily closed fisheries during conditions of high temperature and low water flow. 

Earlier this year, provincial biologists proposed five-year closures in certain rivers in Alberta’s eastern slopes, to help recovery of threatened and endangered native trout species. But after some complaints in the angling community, Alberta Environment and Parks Minister Shannon Phillips backed away from the plan, saying the science wasn’t strong enough to warrant the closures.

“Our paper shows there’s a realistic threat in the Bow River trout fishery,” Cahill notes. “We have no reason to believe it’s any different for the eastern slopes fisheries.”

 ‘Adaptive management’ approach urged 

This fall, Cahill and another of Post’s graduate students joined provincial biologists in electro-fishing at sampling sites along the Bow River, to gather new data on the rainbow trout population. It will take six to eight months to analyze and model the data. 

Fish monitoring programs can be designed, and used with lotteries, temporary closures and other management techniques, to provide better information about a fishery — including testing potential causes of a population decline, Cahill says. 

His PhD focuses on using such an “adaptive management” approach in high-angling effort inland fisheries. “This learning-by-doing approach requires more time, more money, and it’s a heck of a lot harder to implement,” Cahill says. 

“Although the information isn’t perfect, some would argue that adaptive management is the only way forward that has any chance of improving things,” Post says. “The status quo is the worst option.” 

“Alberta Environment and Parks fisheries biologists are pleased to be working collaboratively with the university’s Faculty of Science staff and students to better understand Bow River fish populations and threats, and proactively identify management solutions to ensure this high-profile fishery is managed sustainably into the future,” says Paul Christensen, senior fisheries biologist for the Bow District and a co-author on the study. 

The UCalgary study was supported by the Natural Sciences and Engineering Research Council of Canada, and Cahill has a Vanier Canada Graduate Scholarship.