The grayling (Thymallus thymallus) is a salmonid species that is threatened in its natural environment by a number of factors. One obstacle to breeding is the very high mortality rate during embryonic development. One of the most important factors affecting egg quality is the composition of fatty acids (FA). In this work, we investigated the influence of the FA composition of the egg on egg quality, which was evaluated by the fertilisation rate, hatching rate and the incidence of malformations. Eggs from female grayling from three wild stocks and two farms were included in the experiment. Compared to eggs and larvae from farmed females, eggs and larvae from wild females were found to have higher hatchability, higher survival rates, and, in terms of the timing of embryonic development, the embryonic heartbeat started later and the first larvae took longer to hatch. Of the FAs affecting embryonic development, eggs from farmed females contain more LA and DHA, while eggs from wild females contain more ARA and EPA. The DHA/EPA ratio is higher in the eggs of farmed females. We confirmed the influence of the DHA/EPA ratio on embryonic development; DHA/EPA ratio in eggs is negatively correlated with the hatching rate. Eggs from farmed females contain more FA and also more polyunsaturated fatty acids (PUFA) than eggs from wild females. Our results indicate that the mass fraction of PUFA in eggs is negatively correlated with hatching rate. Deformed larvae contain more linoleic acid and adrenic acid compared to well-developed larvae, which may influence spine development. We found that: (1) FA composition of eggs varies according to female origin (wild, farmed) and depends on female diet; (2) embryos from eggs with higher DHA/EPA ratio have higher mortality rate.
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