Samenvatting

Water velocity in rearing units for Atlantic salmon post-smolt can influence fish robustness. Studies have shown that training at velocities between 1-2 body lengths/second (bl/s) increases disease resistance in salmonid species. Monitoring and timely adjustments to the water velocity during post-smolt production could therefore prevent losses after transfer to sea cages. The objective of this study is to explore which factors significantly influence the water velocity in the rearing units. Four 3,2 m3 and 0,5 m3 tanks have been calibrated by adjusting the inlet pipe and the flow in the tank. Two of the 3,2 m3 tanks were set to a flow of 140 L/min and contained 90 kg/m3 of post-smolts while the other two tanks were set to a flow of 40 L/min and contained 25 kg/m3. These densities correspond to the same mass specific water use 1,5 (L/min/kg). The 0,5 m3 tanks were set to a flow of 30 L/s and contained 10 fish. Velocities in the tanks were measured while being empty and when containing fish, both while feeding was off as well as during continuous feeding. The Vector 3D current meter by Nortek was used in the 3,2 m3 tanks to measure the velocity in 36 locations evenly divided over the entire tank volume in order to create complete tank profiles. Results showed that (1) Feeding regimes have no influence on acoustic water velocity measurements; (2) fish presence can decrease tank velocities by up to between 20 and 30%, both at a density of 25 kg/m3 as 90 kg/m3. As this decrease was uniform for both flow settings a decrease of 20-30% occurs at a mass specific water use of 1,5 L/min/kg. It can be concluded that the presence of post-smolt in rearing units significantly decreases the water velocity in the tank. After fish transfer the water velocity in the rearing units should be adjusted accordingly to 1-2 bl/s to apply the optimal training regime increasing fish robustness. The Vector instrument using acoustics to measure water velocities proved to be a reliable alternative to traditional instruments and capable of efficient and accurate measurements.