Effects of reduced waterflow (increased loading) in soft water on Atlantic salmon smolts ( Salmo salar L.) while maintaining oxygen at constant level by oxygenation of the inlet water

Abstract

Atlantic salmon ( Salmo salar L.) smolts were exposed to three different waterflows: 0.49–0.59 (control), 0.27–0.33 (medium) and 0.16–0.21 liter kg −1min −1 (low) for about two months in soft water. Oxygen was added to the inlet water prior to the medium and low waterflow tanks in order to ensure adequate dissolved oxygen levels. However, as a result of the reduced waterflow to the medium and low waterflow groups, carbon dioxide and total ammonia concentrations were increased and pH was reduced. The concentration of un-ionized ammonia was about the same for the three groups and far lower than established criteria for freshwater fish. The amount of food spillage seemed to be highest in the low waterflow group, probably because of less appetite and a reduced self-cleaning efficiency of the tanks. The respiration frequency of the medium group was slightly increased (although not significantly) during most of the exposure period. After 65 days, the smolts in this group showed reduction in weight and condition factor when compared to the control group. However, the length was not significantly different from the control group. The smolts in the low waterflow group showed increased respiration frequency during most of the exposure period. After 62–65 days of exposure, the hematocrit and hemoglobin levels were increased and plasma chloride, fish weight, length and condition factor were significantly reduced. Histological examination revealed chronic gill lesions with lamellar hypertrophy and hyperplasia. However, a high mortality rate was not observed. When the three groups later were exposed to equalconditions in seawater for 77 days, their mean weight and length were not significantly different. Compensation mechanisms had been stimulated to regain the weight reduction observed during the freshwater exposure. After 102 days in seawater the gill epithelium of the low waterflow group had obviously recovered.