Engineered spawning grounds in alpine rivers: Substrate stability is more important than substrate quality

Management of salmonid spawning grounds in non-natural mountain rivers is challenging due to anthropogenically altered discharge regimes and disturbed sediment transport. This study explored the potential of a cost-effective and easily implementable approach to salmonid spawning ground restoration using autochthonous gravel obtained from sediment trap excavation as spawning substrate. Spawning grounds were constructed in three alpine tributaries of the River Inn and assessed through a combination of abiotic habitat characterization and active bioindication with brown trout and Danube salmon eggs. Ecologically relevant parameters of interstitial water and substrate composition revealed heterogeneity across sites, but remained in suitable conditions throughout the developmental period of eggs. Egg-to-fry survival was often higher than 90 % at all sites, even exceeding the open water and hatchery references. Temperature-corrected larval growth was site-dependent and influenced by incubation environment, with interstitial conditions supporting more efficient energy allocation than turbulent open water settings. As scouring is a known problem in streams with high gradient catchments, we measured substrate stability over the egg and larval development. While substrates for fall-spawning brown trout remained stable, spring trials with Danube salmon revealed substantial loss of incubated eggs due to increased discharge extremes and substrate relocation, underscoring the importance of substrate stability. Our findings highlight the viability of combining spawning ground management and periodic sediment trap excavation, and they stress the importance of timing, flexibility and site-specific measures in high-energy mountain streams. • We tested the functioning of ecologically engineered spawning grounds in alpine rivers. • Salmonid spawning grounds were constructed from excavated sediment trap material. • Re-used gravel provided an ideal substrate resulting in high egg-to-fry survival. • Spawning ground stability was the most crucial factor, with seasonal variability. • Sediment trap management offers a cost-effective restoration measure in alpine rivers.