High stocking density affected the growth performance, digestive enzyme activity, intestinal histology, and immune response of Nile tilapia (Oreochromis niloticus) fry grown in brackish water

It is crucial to ensure the suitable stocking density for Nile tilapia fry since Nile tilapia is among the most consumed fish species globally. In this study, fry were distributed at three stocking densities, 1000 fry/m³ (low density, LD), 2000 fry/m³ (middle density, MD), and 4000 fry/m³ (high density, HD). Then each stocking density was subdivided into two groups where three aquaria were supplied with fresh dechlorinated water (FW, 0.35 g saline /L), and the other three aquaria were fortified with underground brackish water (BW, ≈ 8 g saline /L). Subsequently, fry were kept under these experimental conditions and offered 40% crude protein thrice daily at 4–5% of the body mass for 15 days, then switched to 30% thrice daily at 3–4% of the body mass until the end of the trial (60 days). Except for the dissolved oxygen, salinity, and total ammonia nitrogen, the measured water characteristics (temperature and pH) were not markedly (P˃0.05) affected by the stocking density or water salinity. Stocking density was a significant factor in the case of final body weight, weight gain, specific growth rate, and survival rate (P<0.05). The water salinity and stocking density markedly affected the digestive enzyme activity (protease, lipase, and amylase) (P<0.05). The stocking density, water salinity, and their interaction were significant factors in the lysozyme activity. The stocking density significantly affected the superoxide dismutase (SOD), catalase activity (CAT), and glutathione peroxidase (GPx), and fish fry in LD showed higher SOD, CAT, and GPx than fish in MD or HD in FW or BW (P<0.05). Malondialdehyde (MDA) was affected by the stocking density, and fish fry in LD showed lower MDA than in MD or HD under FW or BW (P<0.05). Increased density to HD in the FW or BW induced slight degeneration of the intestinal mucosal lining. In conclusion, Nile tilapia fry can grow in brackish water (≈ 8 g saline /L) with 1000 to 2000 fry/m³ without affecting growth performance, feed utilization, digestive enzymes, intestinal histological features, and immune and antioxidative responses.