Is it possible to contribute to the recovery of European grayling (Thymallus thymallus, Salmonidae) populations by stocking cultured brood fish in the pre-spawning period?

European grayling populations have declined significantly in their central range, prompting numerous stocking programs with reared fish that did not bring desirable population prosperity. This study evaluated the effectiveness of stocking long-reared grayling brood fish before their spawning period. It focused on monitoring the presence of juveniles coming from their natural spawn in the stream and on recapture rates and growth parameters of stocked fish to estimate their adaptability in a natural river environment. The results revealed that the recapture rate of the stocked brood fish was notably low, with significant increases only in length growth but a decreases in condition factor, suggesting poor adaptability in the wild. The limited number of young-of-year grayling from natural spawning further indicated low reproductive success. These results highlight the inadequacies of this stocking approach to strengthen the wild grayling population. We suggest alternative strategies, such as using younger stock and implementing protective measures like catch-and-release, which may improve conservation efforts and enhance the success of grayling stocking programs.