Polyculture culture of black tiger shrimp Penaeus monodon and red seaweed Gracilaria tenuistipitata under different densities: effects on water quality, post-larvae performance and their resistance against Vibrio parahaemolyticus Nguyen Thi Ngoc Anh 1 & Nguyen Hoang Vinh 1,2 & Bui Nguyen Thu An 1 & Lam My Lan 1 & Tran Ngoc Hai 1 Received: 2 July 2020 /Revised and accepted: 9 September 2020 # Springer Nature B.V. 2020 Abstract 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 shrimp Penaeus monodon post-larvae (PL) and red seaweed Gracilaria tenuistipitata on water quality and shrimp performance in the nursery phase. The first experiment involved a 3 × 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 pathogenic Vibrio parahaemolyticus (a concentration of 2 × 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 of P. monodon shrimp and G. tenuistipitata improved tank water quality as well as shrimp survival and growth while also enhancing the antibacterial activity of shrimp against V. parahaemolyticus infection in the nursery phase. Keywords Polyculture . Stocking density . Penaeus monodon . Gracilaria tenuistipitata . Rhodophyta . Water quality . Shrimp performance . Vibrio parahaemolyticus Introduction Farmed shrimp is one of the fastest-growing aquaculture prod- ucts and represents an important globally traded food commodity. Most shrimp production originates from Asian countries and is intensifying through all major producing countries (Portley 2016). Global farmed shrimp production reached approximately 4 million tonnes in 2018, increasing by 3 to 5% compared with 2017. The black tiger shrimp (Penaeus monodon) had a 12 to 15% share in global farmed shrimp production in 2018, motivated by relatively stable and high prices when compared with white leg shrimp (Litopenaeus vannamei). Hence, numerous shrimp farmers throughout Southeast Asia (Thailand, Indonesia, India and Vietnam, among others) are shifting from L. vannamei to P. monodon, which provides improved financial efficiency (FAO 2019). Vietnam is the fifth-largest global shrimp * Nguyen Thi Ngoc Anh ntnanh@ctu.edu.vn 1 College of Aquaculture and Fisheries, Can Tho University, Campus II, 3/2 street, Can Tho City, Ninh Kieu District, Vietnam 2 Bac Lieu Agricultural Extension Center, Bac Lieu City, Bac Lieu Province, Vietnam Journal of Applied Phycology https://doi.org/10.1007/s10811-020-02253-7