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.

Small larvae in large rivers: observations on downstream movement of European grayling Thymallus thymallus during early life stages

Behaviour of early life stages of the salmonid European grayling Thymallus thymallus was investigated by assessing the timing of larval downstream movement from spawning areas, the depth at which larvae moved and the distribution of juvenile fish during summer in two large connected river systems in Norway. Trapping of larvae moving downstream and electrofishing surveys revealed that T. thymallus larvae emerging from the spawning gravel moved downstream predominantly during the night, despite light levels sufficient for orientation in the high-latitude study area. Larvae moved in the water mostly at the bottom layer close to the substratum, while drifting debris was caught in all layers of the water column. Few young-of-the-year still resided close to the spawning areas in autumn, suggesting large-scale movement (several km). Together, these observations show that there may be a deliberate, active component to downstream movement of T. thymallus during early life stages. This research signifies the importance of longitudinal connectivity for T. thymallus in Nordic large river systems. Human alterations of flow regimes and the construction of reservoirs for hydropower may not only affect the movement of adult fish, but may already interfere with active movement behaviour of fish during early life stages.