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Title: 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

Author
item DAVIDSON, JOHN - Freshwater Institute
item GOOD, CHRISTOPHER - Freshwater Institute
item WELSH, CARLA - Freshwater Institute
item SUMMERFELT, STEVEN - Freshwater Institute

Submitted to: Aquacultural Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/16/2014
Publication Date: 3/1/2014
Citation: Davidson, J.W., Good, C.M., Welsh, C., Summerfelt, S.T. 2014. 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. Aquacultural Engineering. 59: 30-40. dx.doi.org/10.1016/j.aquaeng.2014.01.003.

Interpretive Summary: Recirculating aquaculture systems (RAS) must be operated with minimal water exchange due to the limited availability of clean water resources. As a result, the concentrations of various water quality parameters, including nitrate nitrogen (NO3-N), can build up within RAS and can become harmful to fish. Definition of a chronic NO3-N toxicity threshold for rainbow trout is needed because it would: 1) provide a guideline for culture conditions that are conducive with optimal rainbow trout health, performance, and welfare; and 2) aid in the establishment of an engineering parameter that impacts many aspects of RAS design. A controlled study was conducted using six replicated RAS to evaluate the potential effects of relatively low levels of NO3-N on rainbow trout performance, health, and welfare. Three RAS were maintained with 80-100 mg/L NO3-N and three RAS were maintained at control concentrations of 20-40 mg/L NO3-N. An important finding was replication of a previously observed fish behavior described as "side swimming" within the high NO3-N treatment systems. The side swimming behavior was abnormal and not consistent with optimal rainbow trout health and welfare. Cumulative rainbow trout survival was also lower for the high NO3-N treatment which resulted in total rainbow trout biomass that was significantly lower at the end of the study. This research provided strong evidence that relatively low NO3-N levels, 80-100 mg/L, were at least partly related to the chronic performance, health, and welfare impacts measured under the described conditions. Diminished health, decreased survival, and reduced fish biomass resulting from the high NO3-N treatment represent negative consequences for a private venture trout farmer, which lead to decreased profitability. Based on these findings, the authors currently recommend 75 mg/L NO3-N as the upper limit for optimal rainbow trout culture and for RAS engineering design.

Technical Abstract: 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 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 30 ± 0 mg/L NO3-N and three WRAS were maintained with 91 ± 0 mg/L NO3-N by continuous addition of a sodium nitrate stock solution. 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 contaminants. Rainbow trout growth was not negatively 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 nitrate nitrogen. 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.