Colorado pikeminnow re-introduction and population augmentation in rivers of the Western USA

Summary

The Colorado pikeminnow (Ptychocheilus lucius), a federally endangered river fish that can exceed 1.5 meters and 35 kg, was targeted for population augmentation in two reaches of its native Colorado River basin where dams and diversions had cut off remnant populations from completing their life cycle. In the upper Colorado River, where reproduction was already documented, two years of stocking 2,000-3,000 hatchery juveniles annually failed outright: nearly all dispersed downstream out of target reaches, some ending up in diversion canals later drained, and the effort was suspended in favor of building fish passage facilities and screening diversions instead. In the San Juan River, where fewer than 50 fish remained with essentially no reproduction in the 1990s, an aggressive eight-year program targeting 300,000 annual stockings across all life stages initially showed similar rapid dispersal, but a revised release strategy acclimating fish in river water and moving them by raft to sheltered off-channel habitats improved outcomes. After years of sustained stocking, multi-year survival, a steadily growing population, and eventually some natural reproduction and larvae were documented, indicating stocked fish were beginning to complete their life cycle in the river. Major difficulties included river regulation altering flow and temperature, dominant non-native fish competing with and preying on young pikeminnow, and diversion structures fragmenting migration routes. The project rates itself Partially Successful, since augmentation succeeded in the San Juan River but failed in the upper Colorado River, with insufficient time to confirm a fully self-sustaining population in either.