Water quality and microbial community structure in juvenile Atlantic cod (Gadus morhua L.) cultures

The effect of water treatment and flow rate on young Atlantic cod juveniles was investigated in a 36-days experiment. Four different flow rates (10, 20, 40, and 70 times the effective tank volume per day) were set up in triplicate tanks within each of three rigs with recirculated, UV-radiated, and untreated water, respectively. Each of the 36 tanks was stocked with 200 weaned cod juveniles at a mean weight of 0.048 g. Fish mortality was recorded daily in all tanks, and growth (wet weight) was determined at the end of the experiment. The microflora in the rearing water was investigated by means of PCR-DGGE and flow cytometry. Observed mortality was significantly higher at low flow rates while otherwise unexplained mortality (presumed to be due to cannibalism) was lowest in the recirculation system. No correlation was found between survival and growth. Growth was significantly affected by both water exchange rate and treatment, as the juveniles from high flow rates and the UV-treatment showed elevated growth rates. Both growth and survival scaled in accordance with metabolic factors like oxygen saturation and unionized ammonia. Bacterial concentrations increased in all tanks and treatment from the beginning of the experiment to the end. The UV-treated and untreated water started at typical seawater concentrations (0.5–1 × 106 mL−1) and increased five to tenfold during the experiment. The recycled water tanks started with bacterial concentrations 2–5 times higher than the UV- and untreated experiments at the time of fish transfer, and ended up with 10 times higher concentrations in the end. Cluster analysis of the DGGE profiles separated the recirculation tanks, including the respective inlet water, from the flow-through systems, with one exception (the highest flow rate). Eighty-five% of the sequences clustered within the Gammaproteobacteria, further divided into four distinct clusters. One of the clusters was only detected in the recirculation system, and showed highest affiliation to bacteria belonging to the Alteromonas/Pseudoalteromonas genera. In contrast, bacteria belonging to the family Vibrionaceae were detected in the flow-through systems.