Effects of densities and alkalinities on the rearing performance of all-male giant freshwater prawns ( <i>Macrobrachium rosenbergii</i>  De Man, 1879) postlarvae

Farming all-male giant freshwater prawns ( Macrobrachium rosenbergii ) is becoming increasingly common in aquaculture due to the faster growth and larger maximum weights in males than females. However, the key factors influencing the development of all-male giant freshwater prawns (post-larval AMGFPs) under captive conditions remain insufficiently explored. This study evaluated the growth and survival of post-larval AMGFPs under varying stocking densities (200–600 ind m −3 ) and alkalinities (80–160 mg CaCO 3 L −1 ) across two completely randomized experiments in three replicates in 1 m 3 tank systems with a biofloc environment, where a carbon source (rice flour with a C:N ratio of 15:1) was added for biofloc formation. Post-larval AMGFPs (initial mean weight and length of 0.012-0.016 g and 1.13-1.53 cm, respectively) were maintained at a salinity 5‰ and fed commercial feed (40% crude protein) for 60 days. The stocking density in Experiment 2 was 500 m −3 (the best stocking density obtained in Experiment 1). The results showed that after 45 rearing days under the experimental conditions, all growth parameters, such as mean weight (MW), daily weight gain (DWG), specific growth rate in weight (SGRw), mean length (ML), daily length gain (DLG), and specific growth rate in length (SGRL), increased as stocking density rose within 200-500 ind m −3 but declined at 600 ind m −3 . The highest survival rate (SR) was observed at a density of 500 ind m −3 , which was significantly higher than at other densities. Additionally, the highest final biomass (FB) was in 500 and 600 ind m −3 and differed from other densities. Besides, from rearing day 30 onward, within the alkalinity range of 80–160 mg CaCO 3 L −1 , all growth parameters (weight and length), SR, and FB increased in proportion to both alkalinity level and rearing time, with the highest values at 160 mg CaCO 3 L −1 and significantly differing from those at lower levels at the experimental end. A stocking density of 500 ind m −3 and an alkalinity of 160 mg CaCO 3 L −1 were recommended as optimal for rearing post-larval AMGFPs under the tested conditions.