Effects of hatchery flow training on stocked age-1 Colorado Pikeminnow

Abstract Objective Stocking of Colorado Pikeminnow Ptychocheilus lucius in the San Juan River, USA, has been necessary due to population declines. Stocking of age-1 fish began in 2021 followed by a hatchery-flow-training protocol implemented in 2022 and 2023 to evaluate if hatchery enrichment altered behavior or fate of stocked fish. Methods Radio telemetry and passive integrated transponder (PIT) tag detections were used to assess differences in movement rates, habitat use, and fate of stocked Colorado Pikeminnow following rearing in different hatchery conditions: control raceway (control), flow-trained raceway (flow-trained), and pond reared with larger size at stocking (larger pond-reared). Movement rates were based on detections of radio- or PIT-tagged fish within the first 60 d after stocking. Fate and habitat use was based on detections of radio-tagged fish during the same 60-d period. Passive integrated transponder tag detections were used to evaluate fate of fish beyond the 60-d battery life of radio transmitters. Results Colorado Pikeminnow moved downstream following stocking with no significant differences between flow-trained and control treatments for either radio- or PIT-tagged fish. Movement rates of radio-tagged fish averaged 3.7 km/d and PIT-tagged fish movements were bimodal, ranging from 0.35 to 45.0 km/d. Radio-tagged Colorado Pikeminnow used low velocity (<0.4 m/s) and moderately deep (∼0.5 m) habitat dominated by sand and cobble, with no differences among treatment groups. Additionally, after 60 d poststocking, only a fraction (0–35%) of each radio-tagged Colorado Pikeminnow treatment was accounted for in our study reach during the final tracking trip. Presumed avian predation contributed to the loss of Colorado Pikeminnow, with radio transmitters and PIT tags found on shore or in a live bird following stocking. Only 130 PIT-tagged individuals from all treatments were detected several years following stocking, less than 2% of all stocked individuals. Conclusions Results suggest that stocked Colorado Pikeminnow dispersed quickly downstream, current hatchery-flow-training methods did not alter movement rates or habitat use, and avian predators may be a relatively large source of poststocking mortality.