This Montana Trout Experiment Ran for Four Generations. Here's What It Found.

Fisheries agencies restoring a native trout population usually don't start from scratch. They pull fish from several existing populations and mix them into the new water, hoping the combination sticks. A new study out of Cherry Creek, Montana just tracked one of those mixed-source introductions across roughly four generations of Westslope Cutthroat Trout, and the source populations did not contribute equally.

The Setup

Researchers led by Donovan Bell, with Ryan Kovach and Andrew Whiteley among the coauthors, studied a large-scale, mixed-source introduction of Westslope Cutthroat Trout in Cherry Creek. As the study's authors put it, "introductions are an increasingly important conservation tool," but few long-term studies have looked at how the choice of source populations shapes what happens generations later. That's the gap this project set out to address.

Some Sources Won, Some Didn't

The team tracked how much each donor stock contributed to the final population relative to what you'd expect if every source contributed equally, a number the researchers treat as a proxy for fitness. The two donor stocks with the lowest genetic variation ended up with reduced contributions to the final population. The donor stocks with the highest genetic variation did the opposite, exceeding the expected neutral contribution by 152 to 198 percent. The relationship between a source's genetic variation and how much it contributed was strong, with an R² of 0.6. By the final study year, nearly every fish sampled had mixed ancestry, and average heterozygosity, a measure of genetic diversity within the population, hit its highest recorded level, higher than any single source population had on its own.

Worth Sitting With

That "proxy for fitness" language is doing real work here. The study measured genetic contribution relative to neutral expectations, and treats that as a stand-in for fitness, but the abstract doesn't report survival, growth, or reproductive success numbers alongside it. It's a reasonable proxy, but still a proxy. This is also one creek, one introduction project, tracked over roughly four generations. Whether the same pattern holds in a different stream, with a different mix of source stocks, isn't something this study can answer.

What This Means If You Fish Restored Trout Water

I'll be straight about this one: I've never targeted Westslope Cutthroat Trout, and Cherry Creek is nowhere near anywhere I've fished. My wild trout time is Sierra Nevada streams in California, a different species and a different mountain range entirely.

But the underlying idea travels. A lot of the water anglers fish for native or wild trout today exists because an agency reintroduced fish there at some point, sometimes decades ago, sometimes recently. This study is a reminder that those reintroductions aren't a simple matter of dumping fish in and walking away. At Cherry Creek, the source populations chosen and how genetically diverse those sources were tracked closely with which fish were still represented in the population generations later. Next time you read about a native trout restoration project on a stream you fish, or one you're hoping to fish someday, that's the kind of decision happening behind the scenes before a single fish gets released.

Citation

Bell, D. A., Kovach, R. P., Campbell, M. R., Delomas, T. A., Nelson, L., Clancey, P. T., Shepard, B. B., Williams, J., Kruse, C., & Whiteley, A. R. (2026). Long-term genetic outcomes of a mixed-source introduction of Westslope Cutthroat Trout. Canadian Journal of Fisheries and Aquatic Sciences. https://doi.org/10.1139/cjfas-2025-0417