This study evaluated the effects of cumulative nitrate nitrogen concentrations on blackhead seabream (Acanthopagrus schlegelii) in recirculating aquaculture systems.
Firstly, two acute toxicity tests were conducted with two different sizes of fish (Trial A: 7.1±0.6 g, Trial B: 71.3±3.5 g). Experimental nitrate concentrations for Trial A were 0(control), 500, 1500, 2500, 3500 and 4500 mg NO3-N/L, while those for Trial B were 0(control), 600, 1200, 1800, 2400 and 3000 mg NO3-N/L. Each experiment was implemented for seven days, evaluating survival rates and the median lethal concentrations (LC50) for 96 and 168 hours. The result showed that final survival rates of fish exposed to nitrate nitrogen concentrations of 0(control), 500, 1500, 2500, 3500 and 4500 mg/L in Trial A were 99.0, 95.2, 74.8, 13.3, 0 and 0%, respectively. In Trial B, final survival rates to nitrate nitrogen concentrations of 0(control), 600, 1200, 1800, 2400 and 3000 mg/L were 100.0, 100.0, 100.0, 96.3, 51.8 and 0%, respectively. In Trial A and B, survival rate decreased with increasing nitrate nitrogen concentration. The 96 and 168 hours lethal concentrations (LC50) for 7.1 g fish were 2505 and 1806 mg NO3-N/L, while those for 71.3 g fish were 2663 and 2377 mg NO3-N/L. This result indicated a higher susceptibility of nitrate nitrogen in smaller fish.
Secondly, an chronic toxicity test conducted to evaluate long term effects of nitrate nitrogen on production of blackhead seabream in recirculating aquaculture systems. Blackhead seabream with an individual body weight of 18.8±2.2 g were exposed for 120 days to different nitrate nitrogen concentrations of 0(control), 62.5, 125, 250, 500 and 1000 mg/L. The experimental concentrations for chronic studies was based on the results of acute toxicity testing. After 120 days, final total biomass and total length were significantly lower in 500 and 1000 mg NO3-N/L treatments compared to control (P<0.05).Growth and survival indicators did not show significant differences among treatments except for 1000 mg NO3-N/L (P>0.05). Overall results related to growth and survival factors indicated that blackhead seabream showed comparable performance to a nitrate nitrogen concentration of 500 mg NO3-N/L. Hemoglobin and met- hemoglobin did not show significant differences between all treatment, while the cortisol level was significantly higher at 1000 mg NO3-N/L with a value of 8.60±5.64 ng/mL. Insulin like growth factor(IGF)-1 and IGF-1 receptor were significantly lower at 250, 500 and 1000 mg NO3-N/L compared to control, 62.5, 125 mg NO3-N/L. In the chronic toxicity test, growth performance, survival rate and cortisol were significantly affected by nitrate nitrogen over 500 mg NO3-N/L, while IGF-1 and IGF-1 receptor decreased over 125 mg NO3-N/L. The results found in the present study contribute to understanding the nitrate toxicity of blakhead seabream in recirculating aquaculture system.