The influence of carbon source on growth, feed efficiency, and growth-related genes in Nile tilapia (Oreochromis niloticus) reared under biofloc conditions and high stocking density

Abstract

Evaluation of the growth rate of Nile tilapia in bioflocs with different carbon sources under low and high stocking density provides insight into the suitability of each system for tilapia culture. The experiment was conducted for 98 days with two different fish densities (low stocking density (LSD), 140 fish /m3 and high stocking density, (HSD), 280/m3). Initial fish weight was 5.15 ± 1.12 g. Three carbon sources were used in biofloc units–glycerol, molasses, and starch. The highest concentrations of total ammonia, nitrite, and nitrate and the lowest levels of dissolved oxygen (DO) were observed in control fish reared under LSD or HSD, Moisture, total lipids, and crude proteins in bioflocs formed with glycerol as a carbon source under LSD showed the highest values, while total ash and fiber showed the lowest levels for other combinations of conditions. Growth parameters of Nile tilapia, including final body weight, total body weight gain (BWG), average daily gain, and specific growth rate, were substantially affected by stocking density. Generally, whole-body protein and lipid % showed positive trends with the addition of a carbon source, especially glycerol-based BF reared at LSD that showed the highest (p < 0.05) protein and lipid %. Additionally, the lipid content of control fish at LSD displayed the lowest values among all experimental groups. The addition of glycerol at LSD downregulated GH gene expression 0.6-fold, without affecting MSTN expression. Further, significant upregulation of both IGF-1 and GHR1 genes 5 and 2.1-fold, respectively, were recorded under this condition. At HSD, the addition of glycerol upregulated expression of GHR1, GH, and IGF-1 genes 1.7, 2.5, and 1.9-fold, respectively, but MSTN gene expression was maintained at the control level. Growth rate and feed utilization were improved and growth-related genes upregulated in fish under LSD with glycerol and molasses as carbon sources.