Can stocking with advanced European grayling fry strengthen its populations in the wild?

European grayling, Thymallus thymallus (Linnaeus, 1758) (Actinopterygii: Salmoniformes: Salmonidae), represents one of the highly attractive riverine fishing species in Europe. Its populations have declined in the Czech Republic due to various adverse factors. Current approaches for strengthening these populations based on restocking with artificially reared 1+ or 2+ old individuals have proven to be ineffective. This study focuses on the possibilities of supporting or restoring these populations by reintroducing two-month-old, fast-growing fry reared in ponds. In June 2021, 5400 advanced fry marked with Alizarin Red (ARS) were introduced into three free-flowing South Bohemian streams. The recapture rates and biometric data (length and weight) of stocked grayling were assessed at the release sites as well as further downstream. The first electrofishing monitoring was conducted at the end of the growing season (October) and after their first winter (March). Water temperature and flow rate at the stocking sites were monitored throughout the growing season. The substantial number of recaptured individuals across all monitored sites suggests that the fry successfully adapted to their new habitat and significantly contributed to the composition of local fish communities. Downstream movement of stocked fry correlated with lower water temperatures and higher flows. Our findings demonstrate that some introduced individuals successfully overwintered at all three reintroduction sites. Marking with ARS has proven to be a very effective non-invasive method of group marking juvenile fish and is suitable for monitoring stocking programs. The production and stocking of fast-growing advanced fry of European grayling is thus a promising strategy for revitalizing and strengthening the populations of this threatened fish in running waters.