UNIVERSITY OF NORTHERN BRITISH COLUMBIA
Advancing understanding of trout population change to guide conservation and management efforts

As climate change reshapes rivers and ecosystems across northern British Columbia, new research led by Master of Science student Carly Walters is advancing understanding of how stream-dwelling trout populations are responding, offering critical insight to guide targeted conservation and management efforts in B.C. and beyond.

Drawing from an extensive dataset, research findings by Natural Resources and Environmental Studies graduate student Carly Walters have direct implications for conservation and fisheries management.

Working under the supervision of Rio Tinto Research Chair in Climate Change and Freshwater Fish Ecology Dr. Eduardo Martins, Walters examined long-term changes in rainbow trout population dynamics in the Stellako River, revealing that environmental effects on productivity and warming summer temperatures are reducing the survival of younger fish and driving population declines observed since the early 2000s.

In her first co-authored paper published in the journal Ecological Applications titled "Disentangling the contributions of density dependence and independence to population growth rates", Walters drew from a rare and extensive dataset 35 years of snorkel counts collected annually on rainbow trout by the B.C. government in the Stellako River running from François Lake to Fraser Lake between 1988 and 2022.

The snorkel counts were collected by three observers floating the river in a downstream direction. Each observer was responsible for counting rainbow trout in one of three lanes across the channel. The rainbow trout were counted by size classes 10-30 centimetre (cm), 30-50 cm and 50+ cm. 

"This is the longest-running, continuous snorkel count dataset in North America, that we know of," says the Natural Resources and Environmental Studies graduate student. "It was instrumental in identifying the main factors driving changes in the rainbow trout population over time."

By combining the decades of fish counts collected by provincial biologists with environmental data, and using newly developed modeling techniques, Walters was able to untangle how different factors influence trout survival, growth and reproduction at different life stages, and how that influenced periods of population growth and decline over the study period.

"Temperature, river flow and even the number of returning sockeye salmon all play a role but the biggest impact we saw was on the survival of the smallest fish during warmer summers, and unexplained environmental effects on reproduction" she says. "We found those have been the main contributors to the population decline since the mid-2000s."

The research shows that trout in the smallest size class are particularly vulnerable to rising temperatures, while larger fish are influenced by different ecological factors. In years when more sockeye salmon return to spawn, larger trout tend to survive at higher rates.

The ability to track these patterns through decades was critical to understanding when and why the population changed. The long-term dataset allowed Walters to pinpoint periods of growth and decline and connect those shifts to specific environmental conditions and life stages.

Supported by funding from the Natural Sciences and Engineering Research Council of Canada and the Nechako Environmental Enhancement Fund, the research has direct implications for conservation and fisheries management. 

"The findings point to the importance of focusing management efforts on smaller fish and underscores the impact that climate-driven warming is having on stream-dwelling trout populations" Walters notes. "It also highlights the importance of abundant sockeye salmon returns to support the recreational fishery of larger rainbow trout."

The study also sparked the next phase in the MSc student's ongoing research in UNBC's Freshwater Fish Ecology Laboratory (FFEL), alongside Master of Science student Allison Pugh.