Effect of addition of mineral salts to recirculating aquaculture system: evaluation of water quality, growth performance and some biochemical and immune indices of African cichlid (Labidochromis lividus) fish

In the present study, the effects of startup and addition of ammonium chloride (NH4Cl) and sodium nitrite (NaNO2) salts for 4 weeks before fish stocking on efficiency of biofilter for removal of total ammonia nitrogen (TAN) from water, survival rate, growth performance, ammonia metabolizing and urea cycle enzymes activities as well as mucosal immunity of African cichlid (Labidochromis lividus) fingerling. The experimental unit was a recirculating aquaculture system (RAS) with water holding capacity of 200 L in which 20 fingerling fish with average weight of 3.06±0.07 g were stocked and cultured for 30 days. The experimental groups included the RAS without startup and adding salts (control, group 1), startup of RAS before fish stocking (group 2) and startup RAS with adding salts (group 3). Water from each experimental unit was sampled after 0, 10, 20 and 30 days of fish stocking. The results showed that the TAN concentration in the groups 2 and 3 was significantly lower than the control group (P<0.05). Nitrate concentration in the groups 2 and 3 was higher than the control group (P<0.05). The highest nitrate concentration was observed in the group 3 (P<0.05). Growth performance including final weight, weight gain, specific growth rate (SGR%), daily growth index (DGI) and body weight increase (BWI%) were higher in the groups 3, 2 and control, respectively (P<0.05). The food conversion ratio (FCR) in the groups 2 and 3 was lower than the control (P<0.05). The activities of glutamate dehydrogenase, glutamine synthetase and ornithine carbamyl transferase enzymes in liver and muscle tissues increased in the control and group 2 compared to the group 3 (P<0.05). The total immunoglobulin, lysozyme and alternative complement pathway activity (ACH50) of skin mucus increased in the groups 2 and 3 compared to the control; the highest levels were observed in the group 3 (P<0.05). The findings indicate that the startup of RAS with the addition of salts before fish stocking can be an effective way to improve the efficiency of biofilter in converting water ammonia to nitrate, survival rate, growth performance and health of the fish.