Native fish species, particularly those with "pure" genetic lines, are crucial to maintaining the balance and health of aquatic ecosystems. For species like the Westslope Cutthroat Trout Oncorhnchus lewisi, preserving their pure genetic makeup is not just about maintaining a historical lineage but also ensuring that they continue to thrive in their native environments. I intend to discuss why pure genetics matter, why Westslope Cutthroat Trout are a priority, and the issues surrounding hybridization in Alberta.
Why Pure Westslope Cutthroat Matter: A COSEWIC Perspective
The Committee on the Status of Endangered Wildlife in Canada (COSEWIC) recognizes the Westslope Cutthroat Trout as a species of special concern (COSEWIC 2006). One reason for this is the importance of maintaining "pure" genetic lines. These native trout have evolved over thousands of years to adapt to specific ecological conditions, from water temperatures to food availability. When these fish interbreed with non-native species, such as Rainbow Trout Oncorhynchus mykiss, the genetic adaptations that made them successful in their native habitat can be diluted, making them less resilient to environmental changes and diseases.
COSEWIC's stance is clear: only populations of genetically pure Westslope Cutthroat Trout count toward conservation goals. Hybrids do not provide the same ecological and genetic value, which is why preserving pure populations is essential to long-term species survival.
The Difference Between Cutthroat Species
Cutthroat Trout are divided into four species, each with unique adaptations that make them suited to their respective environments. Although there are many more in the subspecies categories, the four species include:
Westslope Cutthroat Trout (Oncorhynchus lewisi): Native to the cold, clear waters of the Rocky Mountains, these trout are celebrated for their characteristic "cut" beneath the jaw and their beautiful coloration. They are known for thriving in smaller streams and high-altitude lakes. Native to the waters of southern Alberta, SE British Columbia, and some areas of the NW USA.
Rocky Mountain Cutthroat Trout (Oncorhynchus virginalis): Found in drainages further south than the Westslope Cutthroat in the Rocky Mountains of the USA, this species is known for being slightly larger and more aggressive than their Westslope cousins. They include the Finespotted, Yellowstone, Yellowfin, Colorado River, Greenback, Bonneville, and Rio Grande Cutthroat subspecies.
Coastal Cutthroat Trout (Oncorhynchus clarkii): Inhabiting rivers and streams along the Pacific coast, they exhibit more saltwater tolerance, often migrating between freshwater and marine environments. Found along the coasts of the Pacific Ocean from California to Alaska.
Lahontan Cutthroat Trout (Oncorhynchus henshawi): The largest of the Cutthroat, inhabiting large alkaline lakes in the great basin, primarily in Nevada. Subspecies include Alvord, Humboldt, and Paiute Cutthroat.
While these species share a common ancestor, their genetic differences are critical for their survival in distinct ecological niches. Hybridization threatens to blur these lines, reducing the overall fitness of each subspecies as evidenced by Rosenthal et.al 2022.
Issues Surrounding Hybridization
Hybridization occurs when Westslope Cutthroat Trout interbreed with introduced species like Rainbow Trout, or non-native Cutthroat Trout. While these hybrids can survive and reproduce, their genetic makeup is no longer pure. This poses several challenges:
Loss of Adaptive Traits: Westslope Cutthroat Trout are adapted to specific environmental conditions, like cold, fast-moving waters. When hybridized, these specialized traits can be lost, making the offspring less suited to the environment (Muhlfeld et.al 2014).
Reduced Conservation Value: Hybrids are not considered part of the pure population, which affects conservation efforts. Only genetically pure fish are counted toward recovery targets. Once Westslope Cutthroat Trout populations drop below 95% purity, they no longer qualify for protection under the Species at Risk Act, as only pure (99%) and near pure (95-98%) populations are protected (Government of Canada 2019).
Challenges to Identification: Hybrids can be difficult to distinguish from pure populations, complicating efforts to manage and restore native fish species. Genetic testing is required in areas where rainbows and non-native cutthroat are known to exist.
Historical Stocking and Its Role in Hybridization
The hybridization of Westslope Cutthroat Trout with other species, such as Rainbow Trout, didn’t happen naturally—it’s largely the result of human intervention. In Alberta, as in many other regions, historical stocking programs in the 20th century introduced non-native species, which unintentionally led to genetic mixing (Government of Canada, n.d.).
Stocking of Rainbow Trout and Other Cutthroat Species
Starting in the early 1900s, Rainbow Trout were widely stocked in Alberta’s rivers and lakes. At the time, the focus was on enhancing fishing opportunities, and little consideration was given to the potential ecological impacts. Rainbow Trout are fast-growing, aggressive, and popular with anglers, which made them a favored choice for stocking programs.
However, Rainbow Trout are capable of interbreeding with Westslope Cutthroat Trout, producing what are now called "cuttbows." This hybridization began diluting the genetic integrity of the native Westslope Cutthroat Trout populations. In addition to Rainbow Trout, other subspecies, such as Yellowstone Cutthroat Trout and Golden Trout Oncorhynchus mykiss aguabonita, were occasionally stocked in Alberta. This further complicated the genetic landscape and contributed to the risk of hybridization.
Although the stocking of non-native species on top of Westslope Cutthroat Trout populations has largely been stopped, the genetic impact continues to persist in many of Alberta's waterways. Today, some Westslope Cutthroat populations show varying degrees of hybridization, which affects their value from a conservation perspective.
Barriers: Protecting Pure Populations
In the face of this hybridization challenge, barriers have become an essential tool in conservation efforts. These physical structures help protect pure populations of Westslope Cutthroat Trout by preventing non-native species, such as Rainbow Trout, from accessing the same habitats.
Types of Barriers
Natural Barriers: In some cases, natural features like waterfalls or steep gradient changes serve as barriers that block the upstream movement of non-native species. These natural barriers have historically kept some populations of Westslope Cutthroat Trout isolated, allowing them to remain genetically pure.
Man-made Barriers: Where natural barriers do not exist, conservation efforts can involve constructing artificial barriers. These range from weirs to more advanced fish barriers that are strategically placed in streams and rivers. By blocking non-native fish from moving upstream into the habitats of pure Westslope Cutthroat Trout populations, these barriers play a critical role in maintaining genetic purity.
Fish Removal Projects: In some cases, whole stream reaches are targeted for the removal of non-native fish using methods like electrofishing, and/or the application of piscicides such as rotenone. Barriers are often installed pre-treatment to prevent re-invasion of non-native fishes.
The Importance of Isolated Habitats
Isolated populations of Westslope Cutthroat Trout often serve as the last refuges of pure genetic strains. These isolated habitats, often located in small headwater streams, are difficult for non-native fish to access, providing a natural line of defense. Protecting these areas from human disturbance and further stocking efforts is key to preserving the remaining pure populations.
The Role of Conservation in Protecting Pure Strains
Efforts to preserve pure populations of Westslope Cutthroat Trout often involve habitat restoration, removal of non-native species, and genetic monitoring. By focusing conservation and protection efforts on genetically pure populations, conservationists can maintain the ecological integrity of the watersheds where these fish live.
One method used to understand the remaining populations of Westslope Cutthroat Trout is genetic testing, which helps determine the purity of populations and identify areas where hybridization has occurred, and to what extent. Genetic analysis, paired with disease testing, can also reveal healthy source populations that may be used in obtaining recovery broodstock to support reintroduction efforts. Restoration projects, such as the removal of invasive species and the construction of fish barriers, help prevent further hybridization and protect remaining pure populations.
Moving Forward: Balancing Conservation and Recreation
As conservationists work to preserve these genetically pure populations, they must also balance the needs of recreational anglers and local communities. Many waters in Alberta still contain naturalized non-native fish which do support angling, but there is an increasing push to ensure that Westslope Cutthroat Trout habitats remain undisturbed. Ongoing efforts include genetic testing, removal of invasive species, and public education about the importance of preserving pure native fish. There are also a number of lakes within Alberta where Brook, Rainbow, Brown and Tiger trout are continually stocked, so anglers wishing to target these species still have a recreational opportunity to do so. Recent research suggests that Westslope Cutthroat Trout populations in Alberta could decline by as much as 70% over the next 10 years, with non-native species and climate change identified as the most significant threats (Sinnatamby et.al. 2019).
Why It All Matters
The push to conserve genetically pure populations of native fish species like the Westslope Cutthroat is about more than just preserving a name. It's about maintaining biodiversity, protecting ecosystems, and ensuring that these species can thrive in their native habitats for generations to come.
In the end, the purity of a species' genetics reflects its history, resilience, and future potential. Preserving pure Westslope Cutthroat Trout populations is a key part of safeguarding the ecological legacy of North America's freshwater ecosystems.
References
COSEWIC 2006. COSEWIC assessment and update status report on the westslope cutthroat trout Oncorhynchus clarkii lewisi (British Columbia population and Alberta population) in Canada. Committee on the Status of Endangered Wildlife in Canada. Ottawa. vii + 67 pp.
Government of Canada. (n.d.). History of fish stocking in the park - Banff National Park. Retrieved September 15, 2024, from: https://parks.canada.ca/pn-np/ab/banff/nature/conservation/aquatics/restoring/ensemencement-stocking
Government of Canada. (2019). Recovery Strategy and Action Plan for the Alberta Populations of Westslope Cutthroat Trout (Oncorhynchus clarkii lewisi) in Canada 2019. Retrieved September 24, 2024, from: https://www.canada.ca/en/environment-climate-change/services/species-risk-public-registry/recovery/westslope-cutthroat-trout-2019-proposed.html
Muhlfeld, C., Kovach, R., Jones, L. et al. Invasive hybridization in a threatened species is accelerated by climate change. Nature Clim Change 4, 620–624 (2014). https://doi.org/10.1038/nclimate2252
Rosenthal WC, Fennell JM, Mandeville EG, Burckhardt JC, Walters AW, Wagner CE. Hybridization decreases native cutthroat trout reproductive fitness. Mol Ecol. 2022 Aug;31(16):4224-4241. doi: 10.1111/mec.16578. Epub 2022 Jul 13. PMID: 35751487.
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