Comparing the effects of high vs. low nitrate on the health, performance, and welfare of juvenile rainbow trout Oncorhynchus mykiss within water recirculating aquaculture systems

Previous research indicates that rainbow trout Oncorhynchus mykiss begin to exhibit health and welfare problems when cultured within water recirculating aquaculture systems (WRAS) operated at low exchange (6.7 days hydraulic retention time) and a mean feed loading rate of 4.1 kg feed/m3 daily makeup flow. These studies could not conclusively determine the causative agent of the health and welfare issues, but accumulation of mean nitrate nitrogen (NO3-N) to approximately 100 mg/L was determined to be a potential cause of abnormal swimming behaviors such as “side swimming” and rapid swimming velocity. A subsequent controlled, 3-month study was conducted to determine if NO3-N concentrations of 80–100 mg/L resulted in chronic health issues for rainbow trout. Equal numbers of rainbow trout (16.4 ± 0.3 g) were stocked within six replicated 9.5 m3 WRAS. Three WRAS were maintained with a mean NO3-N concentration of 30 mg/L (“low”) resulting from nitrification, and three WRAS were maintained with a mean concentration of 91 mg/L (“high”) via continuous dosing of a sodium nitrate stock solution in addition to nitrification. All six WRAS were operated with equal water exchange (1.3 days mean hydraulic retention time) and mean feed loading rates (0.72 kg feed/m3 daily makeup flow), which provided enough flushing to limit the accumulation of other water quality concentrations. Rainbow trout growth was not significantly impacted by the high NO3-N treatment. Cumulative survival for fish cultured within the high NO3-N WRAS was lower and bordered statistical significance, which resulted in total rainbow trout biomass that was significantly lower for this group at study's end. In addition, a significantly greater prevalence of side swimming rainbow trout occurred in the high NO3-N treatment, as was observed during previous research. Swimming speeds were generally greater for rainbow trout cultured in the high NO3-N treatment, but were not always significantly different. Although most water quality variables were controlled, significant differences between treatments for the concentrations of other water quality parameters inhibited definitive conclusions regarding the effect of NO3-N. However, due to the unlikely toxicity of confounding water quality parameters, study results provided strong evidence that relatively low NO3-N levels, 80–100 mg/L, were related to chronic health and welfare impacts to juvenile rainbow trout under the described conditions.