Acute Toxicity of Ammonia and Nitrite to Pacific White Shrimp, Litopenaeus vannamei, at Low Salinities

This study was conducted to determine the individual and combined effects of ammonia and nitrite on Litopenaeus vannamei postlarvae (25- to 45-d old PL) at 10 ppt salinity, 28 C, and a pH of 7.8. The independent variables were salinity, total ammonia nitrogen (TAN), and nitrite nitrogen (NO2-N). The TAN experiments were conducted at 18 and 10 ppt salinity, and the NO2-N test was conducted at 10 ppt salinity. Combined TAN and NO2 tests were also conducted at 10 ppt salinity. The 48-h lethal concentration (LC50) values for TAN at 18 ppt salinity, TAN at 10 ppt salinity, and NO2-N at 10 ppt were observed to be 42.92, 39.72 (2.26 and 2.09 mg/L unionized ammonia-N), and 153.75 mg/L, respectively. When NO2-N was adjusted to the lowest observed effect concentration (LOEC) level and TAN concentrations were varied, synergistic effects were observed, with a 48-h LC50 calculated to be 28.2 mg/L TAN (1.49 mg/L unionized ammonia-N). However, when the ammonia level was adjusted to the LOEC and nitrite was varied, antagonistic effects were observed with a 48-h LC50 calculated to be 163.3 mg/L NO2-N. This study demonstrated that the combined toxic effects of ammonia and nitrite need to be considered when developing standard operating procedures for shrimp culture.