Physiological Response Of Juvenile Nile Tilapia, Oreochromis Niloticus, To Dietary Lipid Nutrition At Different Salinities

Nile tilapia, Oreochromis niloticus, is one of the most important economic fish in the world, because it grows fast, and has many merits for aquaculture, such as high disease resistance and wide environment adaptation. The global annual production was as high as about 3.44 million tons in 2013, and the rapid development of tilapia aquaculture industry means that more freshwater resources will be needed, but lack of affordable freshwater will become a main constraint facing its rapid development. Because china is rich in sea and brackish water, it would be a better choice to culture tilapia with wide salinity torelance in brackish or sea water for the sustainable development. However, the growth of tilapia would be inhibited by salinity, as osmoregulation led by salinity change would cost much energy, resulting in growth inhibition and physiological changes of tilapia. As one of the major energy nutrients in fish, lipid can not only provide energy for life activities, it also can meet the requirements of necessary fatty acid and promote the absorption of lipid-soluble vitamins which will help tilapia alleviate the salinity stress. But the knowledge of whether and how lipid will relieve salinity stress for tilapia was still limited. This study explored the function of lipid on stress resistance of salinity in tilapia, in order to provide a theoretical reference in the aquaculture and feed formulation of O. niloticus in the future.1. Growth, body composition, muscle fatty acid profile and antioxidant status of juvenile tilapia, at different salinitiesJuvenile Nile tilapia, O.niloticus, were reared in freshwater (control), and waters with salinity of 8,16 or 24practical salinity unit (psu) for eight weeks, respectively, with triplicate tanks for each group and 25 fish in each tank. The weight gain rate was higher in freshwater group than in 16 or 24psu groups (P< 0.05). Feed conversion ratio was lower in freshwater group than in 16 or 24psu groups (P< 0.05). Whole body crude lipid contents were higher in freshwater and 8psu groups than in 16 and 24psu groups (P< 0.05). No significant differences were detected in moisture, crude protein and ash among all groups (P> 0.05). The muscular polyunsaturated fatty acids (PUFAs) (Linolenic acid, Arachidonate and Docosahexaenoic acid) increased with the increase of salinity until it reached 16psu(P< 0.05), then PUFAs decreased with the increase of salinity (P< 0.05). With the increase of salinity, the activities of superoxide dismutase (SOD) and glutathione peroxidase (GPX) and the contents of malondialdehyde (MDA) tended to increase (P< 0.05), but no significant differences were detected between freshwater and 8psu groups (P> 0.05). There were no significant differences in the activity of CAT among all the groups (P> 0.05). The results indicate that salinity (16psu) will decrease the growth, lead to activate the antioxidant systems of Nile tilapia, elevate the profiles of PUFAs and decrease the crude lipid content of whole body composition. However, no significant differences were detected between freshwater and 8psu groups.2. Effect of dietary lipid level on growth, antioxidant status and non-specific immunity of juvenile GIFT tilapia at different salinitiesThree purified diets containing 6(control),9 or 12% lipid were prepared by using corn oil as the only lipid source, and were fed to juvenile GIFT tilapia (1.09±0.03g) in freshwater or 16psu respectively for 8 weeks. The results show that the growth performance of tilapia was not affected by salinity or lipid level (P> 0.05). The antioxidant status related parameters of tilapia were not affected by salinity (P> 0.05). In freshwater, the lipid level increase induced the reduction of SOD activity. However, in 16psu, the SOD activity increased at first, then decreased (P< 0.05), with the increase of lipid level. The CAT activity in freshwater and 16psu both decreased with the increase of lipid level (P< 0.05), and the GPX activity was not affected by lipid level (P> 0.05). The contents of MDA were not affected by salinity (P> 0.05), but increased with the increase of lipid level (P< 0.05), with no significant differences between 6% and 9% groups (P> 0.05). The contents of serum lysozyme were not affected by lipid level (P> 0.05), but the contents of serum lysozyme were lower in 16psu groups than in freshwater groups (P< 0.05). The contents of serum protein were higher in 16psu groups than in freshwater groups (P< 0.05). And the content of serum protein increased with the increase of lipid level (P< 0.05), but no significant differences were detected between 9% and 12% groups (P> 0.05). These results indicate that the growth performance of O.niloticus is not significantly affected by salinity (< 16psu) and lipid level (6-12%), which may be related to its high salinity tolerance. But the diet which contains too much lipid (12%) will induce lipid peroxidation in fish. However, the proper lipid level (9%) in diet may help fish adapt the brackish water environment by increasing the content of serum protein and enhancing the antioxidant properties.3. Effect of dietary lipid source on the growth, antioxidant status and non-specific immunity of juvenile GIFT tilapia at different salinitiesFour purified diets were prepared using corn oil, linseed oil, fish oil and mixed oil (corn oil/linseed oil/fish oil=1/1/1) respectively as lipid source and were fed to juvenile tilapia (0.96±0.04g) in freshwater or 16psu water respectively for 8 weeks. The results showed that the growth performance of tilapia was not affected by salinity or lipid source (P> 0.05). The hepatosomatic index was higher in corn oil group than in linseed oil or fish oil groups (P< 0.05), but no difference was found between corn oil and mix oil groups (P> 0.05). The MDA contents were higher in 16psu groups than in freshwater groups (P< 0.05). At 16psu, the activities of SOD and CAT were lower in fish oil group than in other three lipid source groups (P< 0.05). In freshwater, the CAT activity was higher in linseed oil group than in the other three groups (P< 0.05). The GPX activity of corn oil group was the lowest (P< 0.05), but no significant differences were detected among the other three groups (P> 0.05). When compared to other groups, the highest content of MDA was in fish oil group (P< 0.05), and the lowest in corn oil group (P< 0.05). Both serum lysozyme and protein content were higher in 16psu groups than in freshwater groups (P< 0.05), but were not affected by lipid source (P> 0.05). These results indicate that the growth performance of tilapia could not be affected by lipid source and salinity (<16psu), but the antioxidant status of fish can be alerted. Besides, adding fish oil into the diet could not promote the growth of tilapia, even induce the occurrence of lipid peroxidation in tilapia. So fish oil could be totally replaced by plant oil in the diet without significantly affecting growth performance and with reducing the degree of lipid peroxidation.4. Cloning and expression response to dietary lipid source and salinity of fatty acid elongase gene from juvenile GIFT tilapiaThe hepatopancreas of GIFT tilapia was used in this experiment. A partial sequence of elov!5 was cloned from juvenile GIFT tilapia reared in 16psu water. The obtained sequence was 383bp, including a partial sequence of 5’non-coding region (57bp), and a partial open reading frame (326bp), which encodes 109 amino acids. Amino acid sequences comparison showed that the homology ranged 69%-85% with those from other fish species. In freshwater, the relative expressions of elovl5 were higher in plant oil groups (corn and linseed oil) than in fish oil and mix oil groups (P< 0.05). The elovl5 expression offish fed linseed oil in 16psu was lower than that of linseed oil group in freshwater (P< 0.05). While, the elovl5 gene expression of fish oil group was significantly higher than that of fish oil group in 16psu (P< 0.05). These results indicates that, regardless of salinity, tilapia prefers to utilize corn oil. In brackish water, although fish oil can provide highly unsaturated fatty acids for fish, the arachidonic acis in fish oil is scarce, so the elovl5 gene expression of fish oil group is higher in 16psu than that in freshwater. These results indicate that fish oil also can be replaced by corn oil in the feed formulation for the brackish water aquaculture of tilapia, but not linseed oil.