The objective of this study was to compare the parr-smolt transformation of hatchery reared offspring of one cultured strain (Aqua Gen) and two wild Norwegian populations (Namsen and Alta) of Atlantic salmon. The fish were held in freshwater at two different temperatures (3 and 10 °C) under a simulated natural photoperiod (69°N). Changes in condition factor and gill Na+/K+-ATPase activity of freshwater adapted fish, and plasma osmolality and Cl-concentration following seawater challenge tests at 6 °C, were measured on nine occasions during spring. The fish from each of the three experimental groups went through a successful parr-smolt transformation, characterised by reduced condition factor, increased Na+/K+-ATPase activity in the gills, and decreased plasma osmolality and plasma chloride concentration after challenge to seawater. The development of smoltification indices was advanced by 2-3 months in fish held at 10 °C, as compared to that displayed by the fish held at 3 °C. At 3 °C, there were no apparent differences in the development of smolting characters between the strains. However, at 10 °C the Aqua Gen strain developed full seawater tolerance approximately one month earlier (mid-March) than the Namsen strain (late April) and also showed a shaper de-smoltification delineation later in the experiment. The results from the present study suggest that there are genetic components affecting the time-pattern of smoltification and de-smoltification between strains of Atlantic salmon. © 2007 Elsevier B.V. All rights reserved.
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