## 4 Discussion
Amongst other fatty acids, EPA and DHA are very important for human consumption because they can significantly affect human health early development and the prevention of some diseases; therefore, consuming foods containing these fatty acids is highly recommended [49]. In human nutrition, fish is the main source of polyunsaturated fatty acids, which unfortunately are very susceptible to degradation processes such as oxidation. Polyunsaturated fatty acids often serve as targets for nascent free radicals that trigger lipid peroxidation [50], threatening the quality of foods, especially those containing highly unsaturated fats [51]. The values for MDA recorded in our study were overall lower than the values determined in the similar study reported by [52] for fish fed with fish oil with a peroxide value of 11.5 meqkg\({}^{-1}\) (corresponding to 0.037 umolg\({}^{-1}\)). Although the experimental diet group (A) demonstrated the highest value of MDA (0.031 umolg\({}^{-1}\)), all three groups had values that can be described as low peroxidation levels in muscle tissue. No health problems or differences in survival were observed. Differences in MDA values between groups may be the result of different amounts of fat. Ref. [53] found that different areas of muscle tissue in fish can have different MDA values and that it is correlated with the amount of fat. Since the samples of fish muscle tissue in the study were collected immediately after harvest and stored properly, external influence in the form of oxidation is excluded, and the dispersion of the data within the feeding groups can be attributed to the suboptimal winter periods of sampling (December; T = 14.38 \(\pm\) 0.38 \({}^{\circ}\)C) since the best temperature for growth of meagre is between 17-21 \({}^{\circ}\)C, with an acceptable range of 14-23 \({}^{\circ}\)C [4]. This would be in accordance to [54], who found that the lowest stress levels (MDA and catalase as indicators) occurred on _Dicentrackus labrax_ at the optimal temperature (24 \({}^{\circ}\)C) and increased outside the upper and lower optimal thermal limits of this species. Otherwise, no relative importance of the temperature seasonality was observed over growth, lipid composition and overall condition of meagre. In fish, the biosynthesis of lipids involves key mechanisms such as the elongation and desaturation of fatty acids, which are critical for maintaining cellular membrane integrity and function [55]. Oxidative stress, characterized by an imbalance between the production of reactive oxygen species (ROS) and antioxidant defenses, can be influenced by the lipid content in their diet. Diets high in certain lipids can exacerbate oxidative stress, impacting fish welfare by impairing growth, immune function, and overall health [56]. Studies have shown that a balanced lipid intake is essential for optimizing fish health and welfare, minimizing oxidative damage, and ensuring efficient metabolic function [56; 57]. The PCA indicated a clear distinction between meat quality of the fish fed three different diets. The fish were grouped according to their fatty acid content and clearly separatedaccording to the total lipid content and HUFA proportions and ratios. The overall effect of the protein enrichment in the commercial feed for sole (B) was positive on C18:3 (n-3) and EPA, resulting in high content of n-3 fatty acids, and at the same time the lowest indication of the oxidative stress. The meagre fed experimental feed A enriched in both the fat and the protein contents demonstrated expectedly high proportions of HUFA as well as the highest n-3/n-6 ratio, qualifying for the highest meat quality in the presented experiment. Since marine fish have restricted enzymatic capacity for elongation and desaturation, EPA and DHA are considered essential FA in marine fish species, meaning their presence in the muscle tissue depends mostly on the dietary intake [8].