COMPARING THE EFFECTS OF "HIGH" (100 mg/L) VERSUS "LOW" (10 mg/L) NITRATE-NITROGEN ON POST-SMOLT ATLANTIC SALMON Salmo salar PERFORMANCE AND PHYSIOLOGY IN RECIRCULATION AQUACULTURE SYSTEMS
Recently, there has been increased interest in culturing Atlantic salmon smolts and, in some cases, post-smolts in recirculation aquaculture systems (RAS). As this is a relatively new frontier in salmon aquaculture, establishment of optimal water quality thresholds, (particularly for nitrate) is ongoing. In RAS that are not equipped with denitrification processes, nitrate accumulates as an end-product of nitrification and is only controlled by dilution and feeding rates. In addition, previous research indicates that accumulating nitrate can be chronically toxic to certain fish species produced in RAS, including other salmonids such as rainbow trout. Nitrate has also been identified as an endocrine disrupting compound and, as such, has been found to hasten maturation onset in some fish, which can be undesirable for production.
Thus, an 8-month trial was conducted to evaluate the effects of "high" (100 mg/L) versus "low" (10 mg/L) nitrate-nitrogen (NO3-N) on post-smolt Atlantic salmon cultured in RAS. Atlantic salmon (102 ± 1 g) were stocked in equal numbers into six replicated RAS (9.5 m3 total volume). Sodium nitrate was continuously dosed to three RAS to maintain the high NO3-N target concentration, while low NO3-N conditions were maintained in three RAS via typical nitrification. Each experimental RAS was continuously flushed with 3.7 L/min of water, which, in combination with other minor flushing, resulted in a system hydraulic retention time of 1.7 days and enough dilution to maintain other water quality constituents at low concentrations.
The NO3-N levels tested did not negatively impact Atlantic salmon performance. At the conclusion of the study, mean salmon weight was 1.15 ± 0.02 and 1.17 ± 0.01 kg; survival, excluding culls for sampling and several fish that jumped out of tanks, was 99.4 ± 0.5 and 99.2 ± 0.2%; and cumulative FCR was 1.00 ± 0.03 and 0.99 ± 0.01, for the high and low NO3-N treatments, respectively. A relatively high percentage of fish (mostly males) matured for each treatment. Plasma 11-KT levels increased in males during the study, corresponding to increases in gonadosomatic index and phenotypic maturation traits. However, no significant differences were detected between treatments for any of these maturation indicators, suggesting that early maturation was independent of NO3-N concentration and possibly related to other factors. Overall, these results indicate that post-smolt Atlantic salmon can be safely cultured in RAS at < 100 mg/L NO3-N under conditions similar to those of the present study.