Influence of Nutritional Spectra on the Fatty Acid Composition of Muscle Tissue and the Omega-3 PUFA Content in the Brain, Muscle, and Adipose Tissues of Fish of the Genus Thymallus

For humans, fish is a primary source of physiologically valuable polyunsaturated fatty acids (PUFAs) of the omega-3 family: eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). The content of these PUFAs in the biomass of different fish species varies considerably. Differences in the content of EPA and DHA at the higher taxonomic levels (order and family) are associated with phylogenetic factors, while intraspecific variations are linked to the spectrum of diet. However, the impact of these factors on PUFA content in lower taxonomic ranks (genus) remains understudied. This study focused on three species of grayling from nine water bodies. Thymallus baicalensis was collected from the Yenisei, Angara, Mana, Bazaikha, Gladkaya Kacha, Krutaya Kacha, and Tamasul rivers; T. thymallus from the Sylva River; and T. arcticus from Lake Sobachye. Fish diets were determined based on the fatty acid (FA) composition of their adipose tissue and stomach contents. The research revealed that the diet has a more significant influence on the FA composition of grayling muscle tissue than the taxonomic position of the fish. The absolute content of physiologically valuable EPA and DHA in muscle tissue depended on the diet, ranging from 2.3 mg/g wet weight in fish with a high proportion of allochthonous organic matter in their diet to 6.5 mg/g wet weight in fish whose diet consisted exclusively of autochthonous organic sources. The impact of fish diet on the EPA and DHA content in the brain tissue was negligible. Among the studied tissues, adipose tissue was the leader in EPA + DHA content (35.8–120.3 mg/g wet weight), surpassing the content of these PUFAs in muscle tissue by 15–31 times. Thus, the nutritional value of grayling for human consumption, assessed based on the EPA and DHA content, varied threefold.