Investigating stocking density, culture duration, and the combination ratio of artificial feed and fresh feed for mangrove crab Scylla serrata megalopae reared in pond-based net cages

To address the crablet supply shortage for grow-out ponds, two experiments were conducted. The first aimed to identify the optimal combination of stocking density (SD) and culture duration (CD), while the second utilized this combination to determine the most effective ratio of artificial feed (F) to mussel meat (M) for nursery rearing of mangrove crab (Scylla serrata) megalopae into crablets. Each pond-based net cage (1 m² area) was provided with 1 unit each of shelter and feeding tray. The first experiment assessed four SDs (50, 100, 200, and 300 ind.m⁻²) at two CDs (3 and 4 weeks). Results showed that crablets at 50 ind.m⁻² had carapace width (CW) of 17.3 mm and final body weight (FBW) of 0.65 g after 4 weeks while the highest specific growth rate (SGR) of 19%/day was observed at 3 weeks. Although survival rates were higher (80–84%) at SD 50, 100, and 200 ind.m⁻², a higher final harvested number of crablets was achieved at higher SD 200–300 ind.m⁻² of 162 and 215 ind.m⁻², respectively. Using the SD 200 ind. m⁻², the second experiment evaluated 50F:50 M, 70F:30 M, 80F:20 M, and 100F alone on megalopa reared for 3 weeks. Results indicated that the 70F:30 M treatment obtained the highest SR of 69.9%, FBW of 0.28 g, CW of 12 mm, and SGR of 21.33%/day (p > 0.05). Our results indicate that a SD of 200–300 ind.m² and a feed ratio of 70F:30 M for 3 weeks are recommended for optimal growth and maximizing crablet production of Scylla serrata megalopae in nursery rearing.