Effects of Increases in Freshwater Temperature on Loss of Smolt Characteristics in Atlantic Salmon (Salmo salar)

Two-year-old (2+) Atlantic salmon (Salmo salar) raised under ambient temperature and simulated natural photoperiod (45-degrees-N) completed smolting between February and May, exhibiting significant increases in gill Na+, K+-ATPase activity and salinity tolerance (96 h, 37.5 ppt) but no significant changes in plasma osmolality. On May 2 the smolts were divided into four groups (each n = 80) and subjected to a rapid (2-3 h) increase in temperature from ambient (5-degrees-C) to 10, 13, or 16-degrees-C or remained ambient (control) which rose to 12-degrees-C by the end of the experiment on June 5. In the 10, 13, and 16-degrees-C groups, mean gill Na+, K+-ATPase activity (micromoles inorganic phosphorus per milligram protein per hour) declined significantly from 6.6 units on May 2 to 4.3, 3.6, and 2.3 units, respectively, on May 23. In the control group, Na+, K+-ATPase activity showed no significant changes during the study. Plasma osmolality was maintained in all groups between 290 and 304 mosmol.kg-1. Salinity tolerance tests revealed a significant decline in survival in the 16-degrees-C group from May 2 and in the 10-degrees-C, 13-degrees-C, and control groups from May 13, but no significant differences were observed among the three groups. The results support the hypothesis that increases in freshwater temperature accelerate the loss of hypoosmoregulatory capacity.