Evaluating the suitability of nitrate-nitrogen levels for post-smolt Atlantic salmon Salmo salar production in RAS with assistance from heart rate bio-loggers

Authors: DAVIDSON, III, JOHN - Freshwater Institute; CROUSE, CURTIS - Freshwater Institute; LEPINE, CHRISTINE - Freshwater Institute; GOOD, CHRISTOPHER - Freshwater Institute

Submitted to: Aquacultural Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/18/2024
Publication Date: 8/22/2024
Citation: Davidson, Iii, J.W., Crouse, C., Lepine, C., Good, C. 2024. Evaluating the suitability of nitrate-nitrogen levels for post-smolt Atlantic salmon Salmo salar production in RAS with assistance from heart rate bio-loggers. Aquacultural Engineering. 107. Artcile 102461. https://doi.org/10.1016/j.aquaeng.2024.102461.
DOI: https://doi.org/10.1016/j.aquaeng.2024.102461

Interpretive Summary: Nitrate accumulates as an end product of nitrification in recirculating aquaculture systems. Understanding the threshold at which nitrate negatively affects fish is critical to maintaining optimal growth, health, and welfare. Therefore, a study was conducted to compare the effects of nitrate-nitrogen levels increasing from 100 to 250 mg/L versus safe levels of 50 to 100 mg/L on Atlantic salmon. Surgically implanted heart rate bio-loggers were used to determine the onset of a physiological response. Most Atlantic salmon growth and health metrics were unaffected by the highest concentrations. However, increasing heart rate and blood chemistry indicative of a stress response were evident when nitrate-nitrogen exceeded 150 mg/L, suggesting lower levels should be maintained when growing Atlantic salmon in freshwater recirculating aquaculture systems. These results guide daily water use, feed amounts, and engineering criteria required to maintain an optimal culture environment for Atlantic salmon.

Technical Abstract: 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 to 100 mg/L NO3-N (“low”). An average feed loading rate of 2.53 kg feed/m3 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 are safe for post-smolt Atlantic salmon under similar operating conditions, including freshwater RAS without denitrification technologies and relatively high water hardness >300 mg/L as CaCO3. Feed- and nitrogen-loading rates conducive to maintaining safe NO3-N levels are reported.