A Physical Habitat Suitability-Based Framework for Artificial Spawning Ground Restoration in Hydropower-Regulated Rivers: A Case Study of the Critically Endangered Sichuan Taimen (Hucho bleekeri) in the Upper Yangtze River

Sichuan taimen (Hucho bleekeri Kimura 1934) has experienced severe loss of spawning habitat due to hydropower development, as reservoir impoundment has inundated and altered the hydraulic conditions of its original riverine habitats, substantially reducing suitable habitats for its survival and reproduction. Identifying alternative spawning sites and creating suitable hydraulic conditions are therefore critical for this endangered species. Field investigations and literature synthesis identified key spawning requirements, including water velocity of 0.4–0.8 m s−1, water depth of 0.2–0.8 m, sand–gravel substrates with ≥60% sand and fine gravel, and water temperature of 4–10 °C. An integrated ecological engineering scheme combining gravel-bar construction, substrate reconstruction, hydraulic optimization, and nursery habitat enhancement was developed. A depth-averaged, 2-D, hydrodynamic–habitat model showed that restoration increased WUA during the spawning period from 5.2–6.5% to 39.5–47.6%, corresponding to a 70–287% increase, with WUA increasing to up to ~3.9 times the pre-restoration value. The results demonstrate that ecological engineering can effectively enhance spawning habitat under hydropower-regulated flows and provide a transferable framework for endemic rheophilic fishes in regulated alpine rivers.