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Abstract
This study determined the appropriate stocking density for juvenile hybrid grouper (Epinephelus lanceolatus ♂ × E. coioides ♀) reared in marine cages, for which information remains scarce. A 60-day trial in Nha Trang Bay, Khanh Hoa province, followed a completely randomised design with four stocking densities (25, 30, 35 and 45 fish/m³) and four replicates each (16 net hapas of 8 m³). Fish (initial weight 12.23 ± 0.72 g) were fed a commercial pellet diet (46% crude protein) to apparent satiation three times daily and were not graded throughout the trial. Individual growth declined linearly with density (P < 0.001): final weight at 25–35 fish/m³ (74.91–83.18 g/fish) did not differ among densities but was significantly higher than at 45 fish/m³ (56.22 g/fish). In contrast, survival, coefficient of variation in body weight (CVW), feed conversion ratio (FCR) and harvested biomass responded non-linearly to density (significant quadratic component, P ≤ 0.006), with optima converging within 29–35 fish/m³. The 30 fish/m³ treatment achieved the highest survival (85.63%), the most uniform population (CVW = 18.02%) and the lowest FCR (2.18), whereas 35 fish/m³ yielded the highest biomass (2.16 kg/m³). Notably, the lowest density (25 fish/m³) was not optimal, showing lower survival and greater size heterogeneity than 30 fish/m³ (P < 0.05). CVW was strongly negatively correlated with survival, suggesting that size divergence mediates mortality. The highest density (45 fish/m³) performed worst for all traits (67.50%; 28.47%; 2.81), requiring 64.8% more feed and 2.15 times more juveniles per kg of fish produced without any gain in biomass per unit volume. Stocking at 30–35 fish/m³ is therefore recommended: ≈30 fish/m³ when survival, size uniformity and feed efficiency are prioritised, and ≈35 fish/m³ when cage volume is limiting.
Keywords: Hybrid grouper, Epinephelus lanceolatus × E. coioides, stocking density, marine cage culture, size uniformity, feed utilisation.