An experimental study on the budget of organic carbon in polyculture systems of swimming crab with white shrimp and short-necked clam

The budget of organic carbon (OC) in polyculture systems of swimming crab Portunus trituberculatus, white shrimp Litopenaeus vannamei and short-necked clam Ruditapes philippinarum was studied with land-based enclosures from July 2 to October 3, 2012. Six treatments were designed, including monoculture of crab (C), polyculture of crab with shrimp (CS) and polyculture of crab with shrimp and clam (CSB1, CSB2, CSB3 and CSB4). Crab were stocked at 6 ind/m2 in all treatments, and shrimp were stocked at 45 ind/m2. Short-necked clam were stocked with crab and shrimp in four different densities, i.e., 7.5 (CSB1), 15 (CSB2), 30 (CSB3) and 60 ind/m2 (CSB4). Results showed that the yield of crab was the highest in CSB4 system (P < 0.05), while the survival rate and yield of white shrimp were highest in CSB3 system (P < 0.05). The short-necked clam in CSB4 system exhibited the best yield (P < 0.05). Primary production of phytoplankton and periphyton was the main input of OC, accounting for 46.08%-74.34%, and the feed provided 21.50%-51.40% of the total input. The major output of OC was biological and sediment respiration followed by accumulation in sediment, which accounted for 59.39%-68.61% and 21.86%-28.49%, respectively, of the total output. Accumulation of OC in the sediment of CSB1, CSB3 and CSB4 was significantly lower than the other systems (P < 0.05). The CSB3 showed the highest efficiency of OC utilization and followed by the CSB4 (P < 0.05). In conclusion, the optimum stocking density for the tri-species polyculture systems in this study was crab at 6 ind/m2, shrimp at 45 ind/m2, and short-necked clam at 30-60 ind/m2, which exhibited better ecological efficiency and economic benefit.