Last week, biologists released 1,200 greenback cutthroat trout into Zimmerman Lake, about an hour and a half west of Fort Collins. The four-inch-long, one-year-old fish were raised in hatcheries and are the descendants of the last known wild greenback population, which lives in Bear Creek outside Colorado Springs. The release was the latest effort in a decades-long journey to save the fish from extinction -- a journey that has included several rounds of setbacks, successes and scientific breakthroughs.
We wrote about the greenbacks' saga in this week's cover story, "One Fish, Two Fish." Thought extinct in the 1930s, the colorful trout were rediscovered in the 1950s -- or so biologists thought. In the 1970s, the greenbacks were included on the first-ever Endangered Species List, and state and federal biologists began efforts to conserve the species and increase its ranks through breeding and stocking.
But a 2007 University of Colorado study cast doubt on whether the greenbacks they were saving were greenbacks at all. A follow-up study published in 2012 confirmed that the fish they'd spent so much time on was actually a different subspecies of cutthroat trout.
Researchers concluded that the last known greenbacks live only in a four-mile stretch of shallow Bear Creek, and counts showed that there were only about 700 of them left. So the biologists started over again, transporting some of the Bear Creek fish to hatcheries and breeding them with the idea of releasing their offspring into the wild.The first release happened on August 8 in Zimmerman Lake. The lake was rendered fishless to prepare for the greenbacks' arrival. If non-native species such as brook or rainbow trout encounter greenbacks, they'll replace them or breed with them, creating hybrid species. Therefore, it's important that the greenbacks not have any competition.
Continue for more on the release of the greenbacks into Zimmerman Lake. Doug Krieger, a senior aquatic biologist with Colorado Parks and Wildlife, says the greenbacks were driven to Zimmerman Lake from the Leadville National Fish Hatchery in a pickup truck with a water tank in back. Once they arrived, biologists selected two hundred of the fish to be measured, weighed and photographed. They also snipped off a bit of those fishes' tail fins in order to conduct a genetic analysis on each one.The reason, Krieger says, has to do with deformities that are common among the hatchery-raised greenbacks. Unlike the fish who live in Bear Creek, many of the greenbacks raised in hatcheries have only one eye or a double chin. Biologists believe the defects are the result of decades of inbreeding among the last greenback population in Bear Creek -- though the deformed fish tend not to survive long in the wild.
Researchers expect the same survival-of-the-fittest phenomenon will happen in Zimmerman Lake, with the deformed fish dying off and the hardiest fish living to adulthood. The plan, Krieger says, is to return to Zimmerman Lake after some time has passed, take genetic samples from the fish that have survived and then compare the samples to see if geneticists can pinpoint the genes responsible for the deformities.
By 2016, biologists hope the fish in Zimmerman Lake will be ready to spawn, creating a new generation of less-deformed greenback cutthroat trout, some of which can eventually be moved to hatcheries for another round of breeding and stocking. "We're hoping to pass on the strongest traits," Krieger explains.
Usually, fish are stocked by driving the pickup truck that transported them to the edge of the lake and using metal tubes to shoot them out of the tank and into the water, Krieger says. But the lake's banks were too muddy to do that, he says, so the crew resorted to a bucket brigade method instead. "But that worked out really well," Krieger says, "because everybody wanted to put some fish in the lake."The honor of putting the first bucket of fish into the lake went to Jessica Metcalf and Andrew Martin, the CU researchers whose genetic studies identified that the last known population of greenbacks live in Bear Creek.
Krieger says the hatchery-raised fish took a few minutes to get acclimated but that the crowd of thirty to forty people who'd come to witness the historic release soon saw signs that the fish were swimming around and searching for food, as fish should.
Because the greenbacks are currently listed as a "threatened" species (the U.S. Fish and Wildlife Service is evaluating whether to upgrade them to "endangered"), anglers are allowed to catch them with the caveat that they release them immediately.