2.4. The superchilling system
Two insulated polyethylene fish chilling tubs (900 l) were used. At the bottom of each tank a tap was inserted for occasional drainage of water. A fish net was firmly attached to a wooden frame (1.04 0.85 m) which was inserted into each tub at a height of 0.3 m. After filling the entire tub with slurry, the gutted fish were placed on top of this net in a volume (above the frame) of 0.44 m3. The fish (n = 42) were divided equally between tubs. The resulting gutted fish density in the slurry was 204 kg m3. To be able to store the fish in a relatively uniform slurry consistency (ice:water mixture) throughout the experiment, freshly made slurry was occasionally added in a batchwise manner. Under the stagnant conditions in the tub, the ice and water phases would gradually separate. Before the fish were added to the tub, we had therefore established a routine for maintaining a fairly consistent- looking slurry in the tub. After filling the tub on the first day, we produced fresh slurry when needed, typically once every day or every second day. The slurry was moved to another tub (reservoir) and from there the needed amounts could be pumped to the experimental tubs. The slurry level in the latter tubs was always constant as the slurry input volume was balanced by draining off the water phase from the bottom of the tank. Except from when fish were withdrawn for weighing and measurements of subcutaneous and core temperatures, the system was completely closed (a lid was used).