Optimization of culturing the freshwater pearl mussels, Hyriopsis cumingii with filter feeding Chinese carps (bighead carp and silver carp) by orthogonal array design

Traditional pearl culture needs frequent fertilization with organic materials to cultivate phytoplankton for the crops, consequently resulting in high loads of nutrients and the frequent outbreak of harmful algal blooms (esp., cyanobacterial bloom) in the mussel ponds, increasing the risk of pollution to the surrounding environment by aquaculture effluents, which is becoming an obstacle to the sustainable development of this industry. The orthogonal array design of OA9 (34) was used to evaluate the effects of 4 factors, (namely, A, the two-carp ratio between silver carp Hypophthalmichthys molitrix and bighead carp Aristichthys nobilis co-cultured in the mussel pond; B, stocking densities of silver and bighead carps; C, density of pearl mussels; and D, fertilization intensity) on survival rate, growth of mussels and water quality from July 22 to December 8, 2007. The objectives of this study were: 1) to determine what factors might have more significant effects on the survival rate, growth of mussels and water quality; 2) to obtain an optimum level of each tested factor; and 3) to determine a best combination of the 4 tested factors to be used as an improved technology to upgrade the traditional freshwater pearl culture. The results showed: 1) Factors A and B at the tested levels had even more influences on the growth of the mussels than Factors C and D; 2) Factor A had significant effects on the growth of shell length of the mussels (P < 0.05), with a significant difference observed between level 1 and level 3 (P < 0.05); while other factors had no such significant effects (P > 0.05); 3) Factor D had significant effects on Chemical Oxygen Demand (COD) (P < 0.05), with a significant difference occurred between level 1 or 2 and level 3; while it had no significant effects on other water quality parameters, and the other 3 factors had no significant effects on all parameters of water quality (P > 0.05); 4) the four factors at the tested levels had no significant effects on both the survival rate and the growth of shell width of the mussels (P > 0.05); 5) The best combination for the four factors with the optimal levels was A1B2C1D1, which could not only improve the growth of mussels, but also the water quality, i.e., the optimized technology for pearl mussel culture was to culture the pearl mussels with the two-carp ratio of 3/7, fish density of 0.075 ind./m3, mussel density at 0.75 ind./m3 and fertilization intensity of 1.75 g/ m3.