Previous onsite research determined that post-smolt Atlantic salmon Salmo salar growth, health, and welfare were unaffected by exposure to 99 mg/L nitrate-nitrogen (NO3-N) in freshwater recirculating aquaculture systems (RAS). A subsequent study was conducted to determine if higher concentrations negatively impact Atlantic salmon within a similar size range. Salmon production metrics were compared in triplicate RAS with NO3-N levels increasing from 100 to 250 mg/L ("high") vs. 50-100 mg/L NO3-N ("low"). An average feed loading rate of 2.53 kg feed/m(3) makeup water/day was maintained to facilitate natural NO3-N accumulation up to 75-100 mg/L, and sodium nitrate was continuously dosed to achieve higher concentrations. All-female diploid and all-female triploid Atlantic salmon were comingled in six replicated RAS, resulting in 227 fish/tank with an initial mean weight of 0.35 kg. Six diploid salmon with surgically implanted heart rate bio-loggers were included in each RAS. Continuously logged heart rates were matched with frequently measured NO3-N levels to determine the onset of a physiological response. At the end of the 6-month study, salmon exposed to the high and low NO3-N conditions weighed 1.84 +/- 0.05 and 1.91 +/- 0.02 kg, respectively (p > 0.05). Diploid and triploid salmon growth rates, feed conversion ratios, maturation prevalence, survival, fin condition, and cataract scores were unaffected (p > 0.05). However, salmon heart rates were generally higher in the high NO3-N treatment after concentrations exceeded 150 mg/L NO3-N. Higher plasma chloride, hematocrit, and hemoglobin levels (p < 0.05) consistent with an adaptive response to a stressor were also measured in salmon from the high NO3-N RAS during this period. These findings suggest that NO3-N concentrations < 150 mg/L do not affect post-smolt Atlantic salmon under similar operating conditions, including freshwater RAS without denitrification technologies and water hardness >300 mg/L as CaCO3. Additional research is required to determine if higher NO3-N concentrations are suitable for Atlantic salmon production over longer exposure periods and with naturally produced NO3-N levels imparted by reduced water exchange. Feed- and nitrogen-loading rates conducive to maintaining the tested NO3-N levels are reported.
