Effects of water quality and stocking density on growth performance of juvenile cod (Gadus morhua L.)

Abstract

Abstract In a 5-month experiment where groups of juvenile cod were reared in a flow-through system at low density (Group 1) and in a recirculating system at low (Group 2) and high densities (Group 3), the recirculated water had negative effects on growth rate, nutritional condition, and mortality. After the first month, mean weight was significantly larger in Group 1 than in the other two groups. The effect of stocking density on mean weight was not significant until the end of the experiment, when densities were 9.0 and 48.3 kg m−3 in Groups 2 and 3, respectively. Initial mean weight of fish was 37.3 g and at the termination of the experiment mean weights were 225.2, 181.8, and 167.9 g in Groups 1, 2, and 3, respectively. After 5 months, mean condition factors were 1.074, 0.965, and 0.946, mean liver indices 9.5, 9.0, and 7.6, and mean mortalities over the course of the experiment were 1.0%, 5.1%, and 2.4% in Groups 1, 2, and 3, respectively. Water temperature (10.4–10.6°C), pH (7.2–7.4) and oxygen concentration (8–10 mg l−1) were similar among groups. Concentration of total ammonia nitrogen (TAN) was similar among groups for the first half of the experiment (0.3–0.4), but during the last month it was 0.6, 1.3, and 1.5 mg l−1 in Groups 1, 2, and 3, respectively. There was a negative correlation (r2 = 0.48, n = 36) between relative growth rate and TAN, suggesting that ammonia may have been a limiting factor in the recirculating system. The apparent threshold limit of TAN for reduced growth was approximately 1 mg l−1.