Nature-Like Fishways in Spain: A National Overview of Design, Construction Costs, and Effectiveness

Nature-like fishways (NLFs) are a key restoration measure for fragmented rivers at low-head barriers, yet their economic and functional performance is poorly documented. This study provides a comprehensive analysis of 134 NLF projects in Spain (2003–2025), classifying them by typology, energy dissipation elements, and construction method. We quantified construction costs using standardized indicators and assessed available hydraulic and biological efficiency data. Results show a predominance of public funding schemes and a strong geographical concentration of NLFs in the northern half of the country, with ramps (76.1%) being more frequent than bypass channels (23.9%). Construction costs varied markedly among designs, with concrete boulder ramps consistently representing the most cost-intensive NLF configurations, while also being strongly influenced by local site conditions and construction constraints. Only a small fraction of projects (13.4%) underwent post-construction efficiency assessments, but those evaluated generally showed favorable performances for multiple fish species. Our findings provide a state-of-the-art overview of NLFs in Spain, together with a practical classification framework and standardized cost indicators to support the planning and prioritization of river connectivity restoration projects.