Selective Migration: Unravelling Phenotype‐Dependent Fishway Passage Success in Anadromous Three‐Spined Sticklebacks

ABSTRACT Fishways mitigate barriers blocking fish migration, but their effectiveness varies among species. Whether passage success also varies among individuals of the same species based on their morphology, physiology, or behaviour is largely unknown, particularly for small species. Yet, this knowledge is crucial for conservation practices due to potential artificial selection of pheno‐/genotypes with consequences for populations and ecosystems. This study tested whether a fishway induced a selective bias in exploration, length, and timing of migration in anadromous three‐spined sticklebacks ( Gasterosteus aculeatus ) in the Netherlands. Sticklebacks were PIT tagged, measured, tested for exploration, and released back near the fishway entrance. The fishway was monitored for 2 months. Passage success was 12.7% for all fish and 23% for those detected at the entrance, with fish passing 5.4 days after release on average. Passage success and time were unrelated to exploration or length. However, the timing of migration strongly predicted both. Individuals that were tagged later in the season passed the fishway faster and more successfully, likely due to increased reproductive demands and the need to access breeding grounds. Capture‐mark‐recapture analysis showed that increased passage was not explained by higher survival of late migrants, but by increased temperatures likely driving higher activity levels later in the season. High predation risk near the fishway may further explain the low overall passage success. Long‐term monitoring over multiple years, combined with prey–predator interaction studies, is needed to better evaluate the population‐level consequences of selective passage and predation pressure. This will help improve fishway design aimed at enhancing effectiveness.