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Cutthroat restoration facing uncertainty; lineages baffle
Colorado fish biologists have been embroiled in a mystery surrounding Colorado’s native cutthroat trout.
For decades, biologists accepted that Colorado’s native cutthroat could be distinguished by their location: Greenbacks were east of the Continental Divide while Colorado River and Rio Grande cutthroat were in their namesake watersheds. This was important because the Colorado River and green back cutthroat are difficult to differentiate due to similar coloration and spotting.
Thought to be extinct by the 1930s, vestige greenback populations were discovered by biologists in the 1950s. Subsequent recovery efforts led to their down-listing from “endangered” to “threatened” in 1978. However, several years ago, researchers using innovative genetic technology, revealed half of these remnant greenback populations were actually Colorado River cutthroat trout.
This was a blow to recovery efforts since many of these populations were used to establish new populations. Spurred by the revelation, fish biologists tested cutthroat populations statewide and discovered that fish genetically-resembling greenbacks were numerous on the Western Slope, suggesting a possible deficiency in the genetic analyses.
At the time, genetic researchers were confident that their tests were reliable and thought the unexpected distributions of cutthroat could be reflecting the widespread sportfish stocking efforts in the late 19th and early 20th centuries. Colorado Parks & Wildlife, still wary of the findings, partnered with genetecists to develop a new genetic test to clarify the differences between our native cutthroats.
FISHING FOR INFORMATION
Fish taxonomists dug through historic federal and state records and accounts of fish stocking to develop a better understanding and a more detailed history of past events. Researchers also evaluated extensive museum collections of trout specimens assembled and preserved up to 150 years ago by explorers, before fish stocking was rampant. These historic specimens revealed that, prior to settlement, each major river basin had a distinct lineage of cutthroat trout.
It is now clear that Colorado historically had six – not three – distinct lineages of native cutthroat trout: Greenback cutthroat originated in the South Platte River; yellowfin cutthroat, thought to be indigenous only to Twin Lakes, actually inhabited cold waters throughout the Arkansas River basin; Rio Grande cutthroat continue to persist in their namesake watershed; a previously undescribed lineage existed in the San Juan River; and, two Colorado River cutthroat lineages were isolated in the Yamp/White and Upper Colorado watersheds. Historic fish stocking widely distributed fish, resulting in the inadvertent preservation of the greenback cutthroat outside of their native basin. Unfortunately, extensive searches for the descendants of yellowfin and San Juan cutthroat within and outside of their native drainages have failed.
Recovery efforts for our native cutthroat have always used what is considered to be the best science available. For a time, reintroduction efforts used fish that were not necessarily indigenous to the waters where they were introduced, but this increased the number of native cutthroat populations across Colorado, preserving the genetic diversity and resiliency of the species. As well, existing habitat was protected, rehabilitated and restored; and streams were secured from invasion by exotic fish species and disease. Now we are tasked with continuing these preservation efforts and expanding our unique remnat populations to ensure the legacy of Colorado’s cutthroat long into the future.