Nitrate has a low impact on performance parameters and health status of on-growing European sea bass (Dicentrarchus labrax) reared in RAS

Coming along with the intensification of fish production in modern recirculating aquaculture systems (RAS) the water re-use in RAS is increasing. Accompanied by the water re-use, metabolic end products of fish and bacteria continuously accumulate in RAS process water. Nitrate, which is an end-product of biological nitrification processes, accumulates to high concentrations potentially influencing various physiological parameters of fish. The present study focusses on the effects of different nitrate concentrations on performance and health status of on-growing European sea bass (Dicentrarchus labrax). On-growing sea bass were exposed to four different nitrate-N (NO3−-N) concentrations (control (C): 0 mg L−1; low nitrate-N (LN): 125 mg L−1; medium nitrate-N (MN): 250 mg L−1; high nitrate-N (HN): 500 mg L−1) in a triplicate experimental design in twelve small-scaled RAS for 10 weeks. The nitrate concentrations were adjusted with a sodium nitrate and potassium chloride solution (Na+/K+ weight-ratio of 1:27). Rising nitrate levels during the exposure period, caused by the accumulation of metabolic nitrate, were avoided by batch water exchange twice a week. Final biomass, final individual weight, condition factor (CF), feed conversion ratio (FCR) and spleen-somatic index (SSI) were not significantly affected by nitrate exposure within the concentration range tested. Specific growth rate (SGR), and total mortalities did not significantly differ between the treatment groups either, although a trend towards decreasing SGR and increasing mortalities at high nitrate levels was obvious. This study revealed, that the hepato-somatic index (HSI) and daily feed intake (DFI) were significantly negative correlated to with increasing nitrate levels. However, the sensitivity of on-growing sea bass towards nitrate toxicity seems to be low compared to other aquaculture fish species recently tested.