Water exchange rate in RAS and dietary inclusion of micro-minerals influence growth, body composition and mineral metabolism in common carp

Abstract

Recirculation aquaculture systems (RASs) operated at low water exchange rates are known to accumulate min-erals in the water. This study examined the dietary mineral requirement and metabolism in common carp rearedin RAS of contrasting water exchange rates. Two independent RAS (water exchange rates, 70 vs. 2000 L/kg feed)andfive experimental diets with graded levels of micro-mineral premix inclusion (0.0, 0.3, 0.6, 1.0 and 1.5%)were tested in a 2 × 5 factorial arrangement. Common carpfingerlings (8.5 g) were reared in either of the RASand fed the experimental diets in triplicates for 8 weeks at 24 °C. Water quality,fish growth, body composition,tissue mineral concentrations, blood haemoglobin and haematocrit levels, biochemical and molecular markers ofoxidative stress, mineral uptake and metabolism were studied. RAS operated at low water exchange rate showedsignificantly high conductivity, nitrate, nitrite and dissolved mineral concentrations in water. A tendency forhigher growth, significantly higher whole body mineral levels except Cu and Zn were observed infish rearedin RAS with high accumulation of minerals (H-RAS). Of the micro-minerals studied, effect of RAS on the minimaldietary inclusion level was significant only for Se; lower infish reared in the H-RAS (0.28 mg/kg) compared to L-RAS (0.32 mg/kg). Increasing premix inclusion decreased growth and feed efficiency, increased the whole bodyconcentration of Cu, Se and Zn, while Fe and Mn were unaffected. Plasma P, Ca, K and Mn were higher andhaematocrit was lower in H-RAS rearedfish; plasma mineral levels were also influenced by premix inclusion. En-zymes involved in micro-mineral uptake and metabolism (ferric reductase, cupric reductase and alkaline phos-phatase) and oxidative stress markers (glutathione peroxidase, catalase, glutathione reductase and glutathioneS-transferase) were analysed in gill, intestine and liver. Infish reared in H-RAS, reduced glutathione peroxidase(GPx) and increased glutathione reductase (GR) activities were observed in liver and intestine, respectively. Ac-tivity of GPx in all the analysed tissues increased with premix supplementation. Differential regulation in mRNAexpression of molecular markers related to micro-mineral uptake, metabolism and oxidative stress were ob-served in the tissues in response to RAS and premix inclusion. To conclude,fish reared in high accumulationRAS had higher mineral levels in whole body and vertebrae, but did not result in a lower estimate of micro-min-erals, except for Se. Difference in rearing system had multiple effects on the physiology and metabolism offish onthe whole, apart from mineral balance alone.