Abstract
Introduction: The upper Arlanza River (Duero Basin, Burgos, Spain) hosts a genetically distinct local lineage of brown trout (Salmo trutta fario), the “Arlanza strain”, largely free from hatchery-derived introgression, alongside other native cyprinids of conservation concern, including the Iberian chub (Achondrostoma arcasii, Vulnerable—IUCN). The river also supports the Iberian desman (Galemys pyrenaicus, Endangered—IUCN) and Eurasian otter (Lutra lutra). Despite these values, the study reach presents multiple transverse obstacles limiting longitudinal connectivity and degraded riparian cover in critical sections due to livestock erosion, compromising habitat quality for all species. Objective: This study aimed to design engineering interventions to improve fluvial and riparian habitat in a 4 km reach of the upper Arlanza River, restoring longitudinal connectivity and thermal refuge availability while strictly preserving the genetic integrity of the native Arlanza trout strain. Methodology: The reach was characterised through electrofishing surveys, riparian quality assessment (modified RQI index), hydraulic refuge evaluation (IR index), and hydrological analysis based on a 30-year flow record. Brown trout population dynamics were modelled using dimP 1.0 software, with a comparative analysis between upstream (Quintanar de la Sierra village) and downstream (Vilviestre del Pinar village) sampling points to identify connectivity bottlenecks. Engineering works were scheduled to avoid reproductive periods of all target species. Results: The upstream population showed a rejuvenated age structure (density: ~1.40 ind/m; mean length: 12.0 cm), consistent with good spawning conditions but limited growth capacity due to cold temperatures and low summer flows. The downstream point exhibited a severely reduced population (~0.10 ind/m), indicating marked loss of connectivity and habitat degradation. Priority intervention zones were identified in the Camping and lower Prado Mayor sub-reaches. Proposed measures included weir notching to restore fish passage, livestock watering points to reduce bank erosion, and riparian restoration by planting native species (Populus tremula, Betula alba, Salix spp.) protected with fences. Conclusions: Restoring longitudinal connectivity and riparian cover in the upper Arlanza River are essential to protect the genetically valuable Arlanza trout strain, the endangered G. pyrenaicus, and other native fish species, providing a transferable framework for headwater fluvial restoration that jointly addresses biodiversity conservation and genetic resource protection.
Author Contributions
Conceptualization, J.d.M.-A., F.J.S.-R. and F.J.B.-C.; methodology, J.d.M.-A., F.J.S.-R. and F.J.B.-C.; validation, J.d.M.-A., F.J.S.-R., A.G.-V. and J.F.F.-P.; formal analysis, J.d.M.-A. and F.J.B.-C.; investigation, F.J.B.-C. and J.d.M.-A.; data curation, F.J.S.-R., A.G.-V. and J.F.F.-P.; writing—original draft preparation, F.J.B.-C.; writing—review and editing, all authorsproject administration, J.d.M.-A. and F.J.S.-R.; funding acquisition, J.d.M.-A. and F.J.S.-R. All authors have read and agreed to the published version of the manuscript.
Funding
The presenting author acknowledges the financial support provided by the Gasterosteus Fund of the Iberian Society of Ichthyology (SIBIC), through the Gasterosteus Grant covering the SIBIC 2026 congress registration fee.
Institutional Review Board Statement
The animal study protocol was approved by the Ethics Committee of the University of Valladolid (protocol code 12107335 and date of approval 1 March 2024).
Informed Consent Statement
Not applicable.
Data Availability Statement
The data presented in this study will be made available by the authors on request.
Conflicts of Interest
The authors declare no conflicts of interest.
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