Elevated nitrate levels affect the energy metabolism of pikeperch (Sander lucioperca) in RAS

The increasing demand for pikeperch (Sander lucioperca) from recirculating aquaculture systems (RAS) has raised the need for detailed knowledge on water quality parameters. Nitrate thresholds are of interest as nitrate accumulates in RAS and influences fish physiology. The trial was conducted in a recirculating aquaculture respirometer with pikeperch (average body weight (BW) 367 ± 1.1 g) successively reared at three different nitrate-nitrogen (NO3-N) concentrations (N30: 30 mg L−1, N120: 120 mg L−1 and N240: 240 mg L−1) and compared to a control group reared at the lowest possible nitrate-nitrogen concentration (N0: 5 mg L−1). Pikeperch were fed once per day with a commercial diet at three different feeding levels corresponding to 0.3% BW, 0.6% BW and 0.9% BW in triplicates for 8 days at either nitrate-nitrogen concentration before fasting for additional 3 days. Oxygen consumption and ammonia excretion were measured for 22-h in fed and fasting pikeperch to examine the influence of nitrate on energy metabolism. Metabolisable energy, retained energy and digestible energy requirements for maintenance (DEm) as well as the efficiency for energy utilisation (kg) and relative protein utilisation for energy metabolism (as ammonia quotient (AQ)) were determined. Specific dynamic action (SDA) was calculated to estimate the amount of energy spend on ingestion, digestion, absorption and assimilation of feed at different nitrate levels. SDA was significantly increased in the N240 treatment. Results of DEm showed a significant difference between N30 and N120 with DEm of N120 being about 73% higher than DEm of N30. The efficiency kg was significantly decreased between N240 and both N120 and N30 by 10% and 11% respectively. Increased values in AQ in the feed depleted fish in the N240 treatment indicates that fasting pikeperch in the high nitrate treatment had to use body protein to fuel an average of 46 ± 7% of their energy metabolism. The results of this trial show that pikeperch tolerate NO3-N concentrations of up to 240 mg L−1 but energy requirements are most favourable at concentrations of 30 mg L−1 NO3-N.