|Nitrite and Ammonia LC50’s on Small Juvenile Spotted Gar (Lepisosteus oculatus)
Ammonia and nitrite toxicities vary among fish species. Ammonia is the main nitrogenous waste of fish and exists in two forms: ionized NH4+ and un-ionized NH3, and the un-ionized form is primarily responsible for toxicity by diffusion through the gills. Ammonia can be oxidized to nitrite, and nitrite can oxidize hemoglobin to methemoglobin, which cannot bind and deliver oxygen in the organism’s circulatory system. Monitoring of such toxicants is essential in aquaculture systems because overcrowding of fish can lead to toxic ammonia and nitrite levels. The purpose of this study was to determine the median lethal concentration (LC50) at 24, 48, 72, and 96 hours for nitrite-N, total ammonia-N (TAN), and un-ionized ammonia-N (UIAN) for small juvenile spotted gar (Lepisosteus oculatus).
Five replicates were designed for each nitrite-N concentration (0, 5, 10, 20, 40, and 50 mg/L) and for each total ammonia-N [un-ionized ammonia-N] concentration (0 , 50 [0.974], 100 [1.285], 200 [1.930], and 400 [2.845] mg/L). Three fish were placed in each replicate and given a 24-hour acclimation period. The time of exposure lasted 96 hours with no feeding or water exchange. Mortalities were recorded and removed daily, and water quality parameters (dissolved oxygen, pH, temperature, and conductivity) were taken daily. LC50 values were determined using the Probit Analysis of the SAS Program.
The TAN LC50 values for 24, 48, 72, and 96 hours were 40.48 mg/L + 13.62, 30.72 mg/L + 11.45, 25.60 mg/L + 3.37E-7, and 25.60 mg/L + 3.37E-7, respectively. The UIAN LC50 values for 24, 48, 72, and 96 hours were 1.025 mg/L + 0.04, 0.996 mg/L + 0.04, 0.978 mg/L + 1.49E-8, and 0.978 mg/L + 1.49E-8, respectively. The nitrite-N LC50 values were indeterminable due to high control mortalities, possibly caused by low metabolic reserves or poor water quality conditions in the holding tank prior to the LC50.