Broodstock age and diet as determinants of gamete quality and early development in hatchery-reared grayling (Thymallus thymallus)

Grayling (Thymallus thymallus) populations have declined significantly in European waters over past few decades. One of key conservation strategies involves stocking rivers with hatchery-reared fish. To improve the grayling rearing processes in hatcheries, this two-year study investigates the effects of broodstock age (3+ and 4+) and different commercial feeds on gamete quality, measured through egg biochemical composition, fertile egg proportion, sperm concentration and mobility, and early embryonic development in grayling. Broodstock age emerged as the dominant factor influencing reproductive success, with older females demonstrating higher fecundity and larger eggs. In addition, eggs of first-time spawners exhibited higher DHA/EPA ratios, which may be linked to early mortality syndromes; however, no differences in embryo survival were observed between age groups. Age also affected sperm quality, with higher progressive motility and velocity, but lower concentration in older males. Feed had a moderate effect on gamete quality, influencing the biochemical composition of eggs, including fatty acid profiles, and sperm concentration. Combined with reproductive performance, it was concluded that none of the tested feeds fully met the species' nutritional requirements for optimal reproduction. The average fertile egg proportion was 70 %, survival to the eyed egg stage was 25 %, and the hatching rate of larvae was below 10 %. Growth differences were observed, with males responding positively to carbohydrate-rich diets. Eggs from females fed high-carbohydrate diets exhibited altered n-3/n-6 PUFA ratios and reduced essential fatty acid levels. The findings underscore the necessity for tailored broodstock feed for grayling, which supports both growth and reproduction.