The European grayling (Thymallus thymallus) is a species native to Slovenia, whose populations have declined significantly in European waters in recent decades. One of the most important strategies for conserving this species is to reintroduce it into rivers using fish that have been reared in hatcheries. In order to improve the rearing of grayling in hatcheries, we investigated the impact of different factors on the quality of grayling gametes, focusing on the type of feed. Three different commercial feeds were used: the high-carbohydrate feed INICIO 702, the low-carbohydrate feed INICIO Plus, and the carotenoid-enriched feed EFICIO Enviro 978. The average grayling fertilization rate was 70%, the survival to the eyed embryo stage was 25%, and the larval hatching rate was 5%. Our results suggest that several factors influence the success of grayling rearing in hatcheries, with the age of the broodstock proving to be one of the most critical. First-time spawners had almost 50% lower absolute fecundity compared to older fish, and their eggs were smaller and lighter. Differences were also observed in the biochemical composition of the eggs, particularly in the content of polyunsaturated fatty acids (PUFAs), and in progressive motility, velocity, and concentration of sperm in seminal fluid. The influence of feed on grayling rearing was less than expected, as no significant differences in embryo survival were observed between the different feed types. However, some differences in the fatty acid composition of eggs and sperm concentration in seminal fluid were observed. The most significant differences in the biochemical composition of the eggs were observed when comparing wild- and hatchery eggs. The fatty acid ratios of EPA/ARA and DHA/EPA in hatchery eggs were suboptimal, possibly contributing to survival failure and developmental deformities in hatchery-reared larvae.
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