Shrinking habitats, alien species threaten native trout

Climate change is often associated with the endangerment of species, so it’s natural that a recently discovered expansion of trout species in streams has been assumed to be positive. However, a new study from the University of Montana has discovered that this expansion is the result of climate change and is actually reducing stream habitat sizes of native trout.

“This study had three main questions: How have the distributions of native and invasive trout shifted in Montana over the last 30 years, how will they change in the future, and what factors are causing those changes?” outlined Donovan Bell, lead author of the study.

The research analyzed the impacts of climate change associated with a massive long-term dataset concerning the distributions of five trout species in the Rocky Mountains: native westslope cutthroat, bull, brown, rainbow, and invasive brook trout.

It was discovered that between 1993 and 2018, distributions of native bull trout and westslope cutthroat trout declined by 18 percent and six percent respectively, with predictive analysis suggesting further decreases of 39 percent and 16 percent by 2080. On the other hand, invasive brown and rainbow trout show potential to prosper due to the increased climate size.

The problematic situation for bull trout arises from the warmer water temperatures and lower summer water levels, as the species require cold streams and adequate flow to survive. Westslope cutthroat trout populations instead struggled due to the invasive competition from rainbow trout.

“Our two native trout species in Montana will decline in the future unless appropriate conservation action is taken,” Bell stated. “Our results suggest that tailoring conservation strategies to specific species and specific climate-change threats is important for native fish conservation.”

For example, suppression of invasive species will be more beneficial for the conservation of westslope cutthroat trout, whereas conserving bull trouts in streams will be more successful if their critical cold-water habitat is protected.

“Globally, climate-induced changes to aquatic habitats are predicted to threaten at least one-third of freshwater fishes, and some invasive species could take advantage of such changes,” explained co-author Clint Muhlfeld. “These scenarios seem to be playing out in our backyard with native and invasive trout.”

The study also highlights the importance of using and maintaining long-term datasets covering large regions to shed light on the relationships between changing climates and their species. 

“It’s exciting to have the opportunity to use data meticulously collected over decades in Montana to convincingly answer complex questions like these,” added David Schmetterling, fisheries research coordinator for Montana FWP.

However, study co-author Andrew Whiteley emphasized the consequences of this particular issue, explaining that if these trout species are not conserved, there will be an annual cost of around $650 million to the local economy. However, he remained positive, assuming that conservation efforts will continue to be put in place and researched further.

The study has been published in Science Advances.

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By Calum Vaughan, Earth.com Staff Writer