Polyculture culture of black tiger shrimpPenaeus monodonand red seaweedGracilaria tenuistipitataunder different densities: effects on water quality, post-larvae performance and their resistance againstVibrio parahaemolyticus

The integration of seaweeds (extractive, low-footprint species) into aquaculture systems has been considered an alternative approach for the sustainable development of the shrimp industry, which aims to expand in an environmentally friendly way. Two consecutive experiments were conducted to evaluate the effects of integrating various densities of black tiger shrimpPenaeus monodonpost-larvae (PL) and red seaweedGracilaria tenuistipitataon water quality and shrimp performance in the nursery phase. The first experiment involved a 3 x 4 factorial design with three levels of shrimp density (1000; 2000 and 3000 PL m(-3)) and four levels of red seaweed density (0, 1.0, 1.5 and 2.0 kg m(-3)) randomly allocated in triplicate tanks for 30 days. Shrimp PL (with a mean weight of 0.012 +/- 0.002 g and length of 1.12 +/- 0.09 cm) and red seaweed were reared in 150 L tanks at a salinity of 15 g L-1. Significant synergistic effects between shrimp and seaweed densities were observed for the NO(3)(-)total nitrogen (TN) and PO(4)(3-)contents, as well as for the survival (p< 0.05) and production of shrimp (p< 0.01). The integration of shrimp and red seaweed significantly decreased the concentrations of nitrogen and phosphorus in the rearing tanks and greatly enhanced the survival and growth rate of shrimp. In particular, increased stocking density resulted in poorer growth performance but enhanced production. Applying stocking densities of 1000 and 2000 PL m(-3)obtained larger shrimp size, while a density of 3000 PL m(-3)achieved the highest production output in the integrated system. In the second experiment, which followed the 30-day growth trial, shrimp quality was assessed via an immersion challenge test using pathogenicVibrio parahaemolyticus(a concentration of 2 x 10(8)CFU mL(-1)) on the shrimp groups previously stocked at a density of 3000 PL m(-3)and integrated with different quantities of seaweed. After 14 days of challenge, the cumulative mortality in the monoculture system averaged 75.6%, which was significantly higher (p< 0.05) than those in the integrated groups (17.8-31.1%). It is concluded that the integration ofP. monodonshrimp andG. tenuistipitataimproved tank water quality as well as shrimp survival and growth while also enhancing the antibacterial activity of shrimp againstV. parahaemolyticusinfection in the nursery phase.