Effects of Dietary Fish Oil Supplementation on the Growth, Proximate Composition, and Liver Health of Chinese Stripe-Necked Turtle (<i>Mauremys sinensis</i>)

Dietary lipids provide energy for animals and can also be converted into other nutrients (such as non-essential amino acids), which play a role in saving protein. The Chinese stripe-necked turtle is a protected and endangered species that has been bred in captivity; however, basic data on lipid requirements remain unavailable. In this study, 360 <i>Mauremys sinensis</i> (body weight of 65.32 ± 0.15 g) were randomly divided into six groups with three replicates per group; the turtles were fed experimental diets supplemented with various levels of fish oil (i.e., 1% (control group, CG), 3.5% (HF-1), 6% (HF-2), 8.5% (HF-3), 11% (HF-4), and 13.5% (HF-5)) for 10 weeks. The results showed that compared with CG, increasing the fish oil level promoted the growth performance of turtles, and the HF-3 group achieved the best effect. The HF-4 group showed the highest increases in the hepatosomatic index and viscerosomatic index. In addition, increased lipid levels also increased the crude lipid content and reduced the crude protein content in muscle tissue. Oil red O staining showed that the liver lipid content increased with the level of supplemented fish oil, which is consistent with the results of the hepatosomatic index. Compared with CG, triglyceride, total cholesterol, and low-density lipoprotein cholesterol increased significantly in both the liver and serum when fish oil levels exceeded 8.5% (<i>p</i> < 0.05), while high-density lipoprotein cholesterol decreased significantly. Aspartate transaminase and cerealthirdtransaminase levels in serum increased significantly when fish oil levels exceeded 8.5% (<i>p</i> < 0.05). Moreover, the activities of antioxidant enzymes (GSH-Px, SOD, T-AOC, and CAT) and MDA showed similar results, indicating that high fish oil levels (8.5–13.5%) caused liver tissue damage in <i>M. sinensis</i>. Increased fish oil levels significantly upregulated the expression levels of cytokines (<i>IFN-γ</i>, <i>TNF-α</i>, <i>TGF-β1</i>, <i>IL-10</i>, and <i>IL-12</i>) (<i>p</i> < 0.05), downregulated the expression levels of antioxidant enzyme-related genes (<i>cat</i>, <i>mn-sod</i>, and <i>gsh-px</i>), and increased apoptosis of liver cells. Supplementation of the diet with 3.5–6% fish oil improved the growth performance of <i>M. sinensis</i>, and the turtles maintained a beneficial immune status. The results provide a scientific basis for optimizing the commercial feed formula of <i>M. sinensis</i>.