Combined effects of surface area of periphyton substrates and stocking density on growth performance, health status, and immune response of Nile tilapia (Oreochromis niloticus) produced in cages

A 4-month feeding trial was conducted to investigate the effect of surface area of periphyton substrates (PS) and stocking density (SD) of juvenile Nile tilapia, Oreochromis niloticus , on growth, major blood constituents, and some immunity biomarkers. Six treatments (T1–T6) were established in 18 floating cages (1 m 3 water each) fixed in an earthen pond for growing tilapia (1.12 ± 0.10 g), in a 2 × 3 factorial design experiment (2 SD × 3 PS). For T1, T2, and T3, fish were stocked at a rate of 70 m −3 with 1, 2, or 3 PS units (area: 0.7, 1.4, and 2.0 m 2 , respectively). For T4–T6, fish were stocked at 90 m −3 , also with 1, 2, or 3 PS units. All fish were fed a 20% crude protein supplemental diet. The best growth rates, feed efficiency, body composition, immune response, and overall health status were attained for T2 group with 70 fish m −3 in the presence of 2 PS units (1.4 m 2 ) followed by T5 with 90 fish m −3 plus 2 PS. Stocking density affected albumin while periphyton substrates impacted cholesterol, triglycerides, and alanine aminotransferase levels in fish serum. Meanwhile, total proteins and glucose-6-phosphate dehydrogenase were not significantly affected. Serum immunoglobulin (IgM and IgG) values were relatively higher in fish reared at 70 m −3 than at 90 fish m −3 indicating better immunity response. Periphyton biomass (dry matter, DM and ash-free DM) was lower in T2, than in other treatments, whereas periphyton populations belonged mainly to Chlorophyta (52–75%), Cyanobacteria (17–23%), and Charophyta (4–21%). Accordingly, the present study suggests that 70 fish m −3 and 2 PS units (surface area of 1.4 m 2 ) would be an optimal combination for the best growth, health status, and immunity response of juvenile Nile tilapia reared in periphyton-based cage system.