| As one of the important environmental factors in aquaculture, stocking density has a direct impact on a series of physiological activities such as growth, survival and immunity of aquatic animals, and is also related to aquaculture yield and economic benefits. A series of studies had shown that suitable stocking density could improve the growth performance, digestion and antioxidant capacity of cultivation object. However, when the stocking density was too high, it would increase the competition of aquatic animals for living space and biological bait, cause density stress, lead to physiological dysfunction, immune defense dysfunction and other negative effects, and finally lead to low feed utilization rate, slow growth rate, serious diseases, and even death, affecting the breeding yield. In previous reports, there were no relevant studies on the stocking density Japanese seabass (Lateolabrax japonicus). In addition, most of the previous studies were mainly conducted under the conditions of small laboratory water bodies, industrial cement ponds and soil ponds, and there were few relevant studies on seawater cages.
Therefore, A 70-day feeding experiment was conducted to evaluate the effects of different stocking density on growth performance, digestive enzyme activities, non-specific immunity and anti-stress ability of Lateolabrax japonicus [mean initial weight (220.52±2.50) g]. Four culture density gradients were set, 20 tail/box (4.43 kg/m3), 30 tail/box (6.61 kg/m3), 40 tail/box (8.82 kg/m3) and 50 tail/box (10.98 kg/m3), named M20 group, M30 group, M40 group and M50 group, respectively, three replicates in each group, and randomly placed in 12 seawater cages (1m×1m×1.2m). The results showed as following: 1) The weight growth rate (WGR) and specific growth rate (SGR) reached the maximum values in M40 group (90.33% and 0.92), and there was no significant difference with M30 group (P>0.05), but significantly higher than M20 and M50 groups (P<0.05). The condition factor (CF) of M50 group was the lowest (1.72), significantly lower than the other three groups (P<0.05), and the CF of M40 group was the highest (1.84). 2) The highest ether extrac content in whole fish and muscle in M30 group was significantly higher than that in M50 group (P<0.05), and the lowest ether extrac content in muscle in M50 group was 87.53%, which was significantly lower than that in other groups (P<0.05). 3) The activities of trypsin in intestinal tract reached the highest value at M40 group, and was significantly higher than other groups (P<0.05). At the same time, activities of lipase reached the lowest value at M50 group, and was significantly lower than other groups (P<0.05). 4) The activities of superoxide dismutase (SOD) of M40 group were significantly higher than M20 and M50 groups (P<0.05), but there was no significant difference with that in M30 group (P>0.05). But the malondialdehyde (MDA) content in M50 group was the highest, and significantly higher than other three groups (P<0.05). Meanwhile, the activities of alkaline phosphatase (AKP), lactate dehydrogenase (LDH) and lysozyme (LZM) were similar tendency with SOD. 5) With the increase of stocking density, the activity of corticosterone (COR) in serum decreased at first, and then increased, the activity of M40 group was the lowest and significantly lower than that other groups (P<0.05), while the activity of M50 group was the highest and significantly higher than that of the other groups (P<0.05). The activity of creatine kinase (CK) and adrenocorticotropic hormone (ACTH) showed a similar tendency with COR. In conclusion, the stocking density had significant effects on growth performance, digestive capacity, antioxidant and anti-stress ability of Lateolabrax japonicus.
Under the experimental conditions, the growth performance and non-specific immune ability of Lateolabrax japonicus were better in M30 and M40 groups, the stocking density was 6.61~8.82kg/m3, and higher or lower stocking density was not conducive to the growth and healthy of Lateolabrax japonicus. Moreover, the results of this study enrich the relevant data of the stocking density of Lateolabrax japonicus, and also lay a theoretical foundation for the healthy development of its industry. |
