Water flow requirements in the intensive production of Atlantic salmon (Salmo salar L.) parr-smolt at two salinity levels

Atlantic salmon (Salmo salar L.) smolts (out-of-season), average weight: 42 - 77 g were exposed during 40 days to three water flow rates: 0.55-0.71 l kg(-1) min(-1) (high), 0.25-0.31 l kg(-1) min(-1) (medium) and 0.15-0.20 l kg(-1) min(-1) (low) at two different salinity levels, 4.5 ppt (High) and 1.6 ppt (Low). There were two replicate tanks in each group. Oxygen was added to the inlet water to maintain discharge water oxygen concentration higher than 7.5 mg l(-1) at 8-9 degreesC. pH was 6.1-6.4 and 6.3-6.7 in the tanks supplied low and high salinity water, respectively. Concentrations of carbon dioxide were 14 mg l(-1) at low flow, 10 mg 1(-1) at medium flow and 7 mg l(-1) at high flow. After 6 weeks, all groups were transferred from the brackish water to normal seawater. Gill Al accumulation was low in all groups at the end of the brackish water period (highest mean value was 11 mug Al g(-1) gill tissue). However, gill Al accumulation was significantly higher in the low salinity groups compared to the high salinity groups (p < 0.05). No significant differences due to different water flow rates were detected. The mean plasma chloride concentration was slightly and significantly reduced by lower flow rate (p < 0.05) but was not significantly affected by salinity. At the end of the brackish water period and at the end of the seawater period, the mean condition factor was significantly influenced by both water flow exchange and by salinity (p < 0.05). The highest condition factor was found in the high salinity-high water flow group (p < 0.05). The growth rate was fairly good for all groups during the brackish water and the seawater period and no mortality was observed. The results of the pH-measurements do indicate that a specific water flow exchange rate of 0.3 l kg fish(-1) min(-1) is too low when the buffer capacity is low. pH is easily reduced to a level which is unfavourable for smolts. Further studies are needed to quantify water flow requirements of Atlantic salmon parr and smolts. (C) 2004 Elsevier B.V. All rights reserved.