Flow Conditioning of Hatchery‐Reared Razorback Sucker Increases Apparent Survival in the Wild

Abstract

AbstractHatchery augmentation of dwindling wild populations has become a common strategy to avoid extinction of endangered fishes. While benign hatchery conditions can lead to low persistence of stocked individuals, experiments have demonstrated that flow conditioning of Razorback Sucker Xyrauchen texanus can increase swimming performance and reduce downstream movements from desired locations after stocking in the Colorado River basin, USA. However, managers have been slow to adopt its use, likely because linking experimental flow conditioning to scales that are relevant to conservation has been difficult. We reared age‐2 Razorback Sucker in two raceways (control: n = 530; flow conditioning: n = 533) for 10 weeks to assess whether flow conditioning could increase apparent survival after stocking into the San Juan River, New Mexico. We used riverwide active sampling and remote PIT tag antennas to resample individuals for 3 years after stocking. Using Cormack–Jolly–Seber mark–recapture models, we assessed the effects of flow conditioning, size at stocking, relative condition (Fulton’s K), and their interactions on apparent survival and downstream movement after stocking. Flow conditioning of fish doubled the mean apparent survival (at a mean size of 325 mm TL) during the first overwinter period (control fish: 0.14, 95% CI = 0.09–0.21; flow‐conditioned fish: 0.28, 95% CI = 0.19–0.38), with both groups having similar mean overwinter apparent survival estimates thereafter (both > 0.75). Size at stocking and K were positively associated with first‐overwinter apparent survival regardless of treatment (mean survival was <0.20 for fish <275 mm TL but was >0.65 for fish >450 mm TL). Downstream movement after stocking was negatively associated with fish size, indicating that emigration likely contributed to higher apparent mortality of smaller fish. Our results suggest that transitioning hatchery practices to include flow conditioning of Razorback Sucker (and potentially other endangered Colorado River fishes) prior to stocking could substantially increase the conservation efficiency of these augmentation programs.