by Robert Schelly, Utah Division of Wildlife Resources
Growing to lengths of three feet and living for up to 40 years, Razorback Suckers (Xyrauchen texanus), with their brilliant yellow undersides and prominant nuchal humps, are one of the iconic fishes endemic to the Colorado River Basin. As dams have wrought changes to the natural flow cycle, Razorback Suckers have steadily declined, due to plummeting rates of larval survival, and the species is currently listed as endangered. Factors contributing to poor Razorback Sucker recruitment include loss of off-channel wetlands important as nursery habitats for young suckers and an expanding presence of nonnative fishes in the basin acting as predators and competitors. With the middle Green River population maintained in recent years only by intensive stocking of large hatchery-raised fish, recovery efforts have focused on improving sucker recruitment by more closely mimicking the timing and intensity of historical peak-flows associated with spring runoff, and on restoring connectivity with wetland nursery habitats.
Stewart Lake, a gated wetland on the Green River near Jensen, Utah, managed by the Utah Division of Wildlife Resources, is playing a vital role in these efforts. It is a final link in a complex chain of events involving the cooperation of numerous State and Federal agencies under an experimental scheme called the Larval Trigger Study Plan. In the weeks following Razorback Sucker spawning along a Green River gravel-bar in Dinosaur National Monument, the U.S. Fish and Wildlife Service deploys light-traps at various points downstream. As Razorbacks begin to hatch and drift in the current, they are detected in the light-traps, with rapid identification made possible by the work of the Colorado State University Larval Fish Lab. The presence of drifting Razorbacks signals the Bureau of Reclamation to increase releases from Flaming Gorge Reservoir. This restores the historical coupling of peak Green River flows with the period of Razorback Sucker larval drift, allowing larval suckers to enter warm, productive nursery habitats in off channel wetlands that only connect to the river at high flows. Stewart Lake is one such wetland.
After a dry year in 2013 in which Stewart Lake was partially filled for two months but nevertheless produced a successful cohort of juvenile Razorbacks, 2014 provided the wettest year yet to test the Larval Trigger Study Plan. Using picket weirs with openings of ¼ inch to exclude large-bodied nonnative fishes, Stewart Lake’s gates were opened for two weeks during the period of larval drift in early June, when peak Green River flows (supplemented by Flaming Gorge Reservoir releases) reached nearly 20,000 cubic feet per second. Lighttrapping inside Stewart Lake confirmed the entrainment of Razorback Sucker larvae in the wetland, which was filled to capacity before closing the gates to the river.
With the wetland completely full, a longer period of entrainment was possible, allowing the 2014 year -class of Razorbacks three months to grow in the Stewart Lake nursery. This additional month of growth produced great results. During the two weeks of drawdown in early September, over 700 wild-spawned juvenile Razorback Suckers were netted in a fish trap at the outlet gate and measured before being released to the Green River. (That more than 110,000 small nonnative fishes, mostly carp, were also trapped and removed underscores the severity of the nonnative threat in the system.)The largest Razorback Suckers were nearly double the size of the largest fish from the previous year,with one sucker having grown to 168mm in just three months. These large suckers, in excellent condition, are less at risk of predation and have sufficient reserves to overwinter, greatly increasing their chances of survival to adulthood. Such promising results at Stewart Lake demonstrate that the Larval Trigger Study Plan is on track toward restoring the necessary conditions for successful Razorback Sucker recruitment—excellent news for the prospect of Razorback Sucker recovery.