Effects of water reuse and stocking density on water quality, blood physiology and growth rate of juvenile cod (Gadus morhua)

The effects of increasing degree of water reuse on water quality and subsequent growth and physiological response in juvenile Atlantic cod, Gadus morhua, were examined. Fish with a mean (SD) initial weight of 11.7 (1.6) g, were reared for 63 days in tanks connected in series, where the water (10.1 ± 1.5 °C) was reused from tank to tank (n = 4 with replicates) with reoxygenation as the only water treatment being used. Groups were designated “Control” (no reuse), “Low”, “Medium” and “High” degree of reuse. Mean initial and final stocking density was 7.9 and 19.3 kg m− 3, respectively, and did not vary significantly between groups. Mean individual growth trajectories and growth-at-age trajectories varied (P < 0.001) between the four treatment groups during the study period. The overall growth rate was highest in the Control group (1.62% d− 1) and lowest in the Low group (1.45% d− 1), while the Medium and High groups displayed overall growth rates of: 1.55 and 1.53% d− 1, respectively. Mean weights did not vary significantly between groups at the end of the experiment.Overall, the concentration of CO2, TAN (total ammonia nitrogen) and TOC (total organic carbon) increased, while pH decreased in the tanks as a function of water reuse. Water CO2 concentrations were highly correlated with increased blood pH and HCO3− concentrations, indicating that a compensatory alkalosis occur in response to an increasingly acidic environment. Overall, the results demonstrate that growth rates in juvenile cod may be maintained high at relatively high densities, and with efficient utilization of water resources. The blood physiology of the fish reflected the increased degradation of the ambient rearing water, and such measurements could be used for routine investigations of the physiological condition of the fish.