Recent research is increasingly shedding light on the important role that microbial metabolites such as γ-aminobutyric acid (GABA) play in the context of nutrition, cognition, immune function, and the modulation of the gut microbiome. Yet, very few trials were conducted to assess the effects of its supplementation on biomarkers of fish health. Therefore, an eight-week feeding trial was devised to evaluate GABA supplementation in juvenile olive flounder, (Paralichthys olivaceus). A total of 630 fish with an average weight of 4.90 ± 0.10 g (±SD) were randomly assigned to one of seven triplicate groups and fed a non-GABA supplemented diet (CON, with 92 mg/kg GABA content), a positive control with 4 g/kg oxytetracycline (OTC), and five other diets supplemented with 50, 100, 150, 200 and 250 mg/kg GABA (corresponding to a total GABA content of 154, 229, 282, 327 and 352 mg/kg, respectively). Growth, blood chemistry, nonspecific immunity, digestive enzyme activity and disease resistance were assessed. The results showed that 100 and 150 mg/kg GABA supplementation consistently yielded significant improvements (p < 0.05) in growth, intestinal amylase, serum lysozyme, and survival against infection with Streptococcus iniae. Based on polynomial analysis, the optimal supplementation level was determined to be 237 mg/kg. These results support GABA as an important functional feed additive in juvenile olive flounder.

Tilapia (Oreochromis niloticus) previously reared on a commercial feed were shifted to 3 experimental diets with added 5 % of soybean, linseed oil or fish oils, for 42 days as a finishing diet, according to literature recommendations. Fillet, liver and mesenteric fat total lipid fatty acid composition was determined and evaluated taking health and dietary recommendations into consideration. It was found that dietary vegetable oil fatty acids are effectively incorporated into tilapia hepatic and muscular total lipids, but have no pronounced effect on further fatty acid metabolism, in particular on the n-3 fatty acids. Liver was found to sensitively indicate elevated dietary lipid intake, as proven by its higher, most probably endogenous palmitate synthesis. Based on our results the application of vegetable oils to partially substitute fish oil for tilapia can be recommended in relation to the most important dietary lipid quality indicators.

A six-week experiment was carried out to test the effects of total (100%) and partial (50%) replacement of fish meal in the diet of African catfish growers with black soldier fly (B) meal, yellow mealworm (M) meal, and a 1:1 combination of both (BM) on the production and health of fish. A total of 420 fish with an average initial body weight of 200 ± 0.5 g were randomly distributed in triplicate to seven diet groups (C, B50, B100, M50, M100, BM50, and BM100, respectively). The growth performance and feed utilization of fish fed with partial or total replacement levels of FM with B were not significantly affected (p > 0.05) during the 6 weeks of feeding. In contrast, significant differences were observed between the groups fed with a diet where FM was totally replaced with M meal and the control in terms of final body weight, specific growth rate, feed conversion ratio, protein efficiency ratio, and protein productive value. Among the blood plasma biochemistry parameters, total cholesterol exhibited a significant difference (p = 0.007) between the M treatments and the control diet. The fatty acid profile of the liver was changed with respect to the long-chain polyunsaturated fatty acid content in all experimental groups. Parallel with this, the upregulation of elovl5 and fas genes in liver was found in all experimental groups compared to the control. Overall, this study shows that fish meal cannot be substituted with yellow mealworm meal in the practical diet of African catfish without compromising the growth, health and feed utilization parameters.

Study of enrichment of different vitamins (C, B 1 , B 6 and E) in common carp larvae through 4 weeks feeding in recirculation system was carried out. For this purpose, procedure for enrichment of Artemia nauplii with different vitamins was developed and investigated the effectiveness in larvae nutrition. The habituation of larvae to dry feeds was done using supplemented dry diets with the same vitamins. To study the effect of vitamins in young fish, the most common stress situation was planned for carp nursed fries, which occurs during transportation. Levels of vitamins and parameters of immune response in mucus were monitored before and after stress situation. At the end of the feeding trials using enriched Artemia nauplii, level of vitamin B 1 , B 6 and C were increased in the body of fish, but remained on similar level or decreased after dry feeding period. Therefore, we concluded that Artemia is a good delivery vector of these vitamins. While handling stress, vitamin C and E concentration was decreased and sparing effect of antioxidant vitamins was confirmed in groups supplemented with both vitamins. Vitamin B 1 was consumed intensively during the stress, vigorously in groups without supplementation. Contrary to this, vitamin B 6 level increased in stress condition in group without supplementations. The levels of immunoglobulins in mucus were increased in all treated groups independently of treatment. Our results confirm that carp larvae reared in tank condition needs sufficient amount of vitamins C, B 1 and B 6 supplements to protect them from the mild handling stress.