Weak Reproductive Response of European River Lamprey to a Newly Built Fishway—A Case Study From Latvia

The European River Lamprey is an anadromous migratory fish species of high ecological, cultural, and commercial importance. It is experiencing population decline across its range due to overfishing, habitat degradation, and migratory barriers. However, it remains questionable whether fishways mitigate the impact of barriers and contribute meaningfully to population improvement. In this study, we provide insights into the effectiveness of the first natural-type fishway constructed in Latvia, located on the Rīva River to facilitate European River Lamprey migration. Results showed a partial improvement in larval abundance after the completion of the fishway, with statistically significant increases detected only in the progeny of generations that migrated two or more years post-construction. However, larval abundance remained inconsistent. Prior to the fishway's installation, some years even exhibited relatively higher abundance. These findings suggest that while the fishway offers limited benefit, overall, it has not made a significant contribution to improving the natural reproduction success of European River Lamprey, most likely due to suboptimal fishway design for lampreys. This case study highlights the challenges in designing fish passage systems suitable for non-salmonid species like lampreys. It emphasizes the need for species-specific solutions and long-term monitoring to ensure comprehensive evaluation of the spawning success and effective conservation outcomes.