T‐Shaped Remote Underwater Video (<scp>RUV</scp>): A Cost Friendly and Easy Use <scp>RUV</scp> Technology to Estimate Fish Length in Small Alpine Streams

Alpine streams represent some of the most challenging yet ecologically valuable freshwater environments to study, due to their remoteness, fast flows and extreme climatic conditions. Traditional fish survey methods are often impractical or invasive in these habitats. This study presents a lightweight, low-cost, T-shaped remote underwater video (RUV) system optimized for fish monitoring in small, high-altitude streams of the European Alps. Field tests were conducted across three alpine streams in northwestern Italy in June 2025. The RUV system proved effective in recording salmonid presence, estimating fish length using an integrated reference scale, and capturing natural behaviours such as feeding. The compact steel frame (~1 kg) ensured stability in fast-flowing waters while remaining portable for deployment in remote areas. Despite challenges such as bubble interference and site accessibility, the RUV allowed reliable, permanently recorded observations with minimal disturbance. This method offers a promising non-invasive approach for ecological research and conservation monitoring in alpine freshwater systems, with potential applications in species distribution studies and behavioral ecology.