The influence of three unchanging photoperiods on growth and parr‐smolt transformation in Atlantic salmon, Salmo salar L.

Abstract

Groups of pre‐smolt Atlantic salmon were reared under three experimental photoperiods. Growth rate was significantly enhanced under 24 h of light: 0 h of darkness per diem (24 L: 0 D) compared with 16 L: 8 D and 8 L: 16 D from early January to early May. From the beginning of May until the termination of the experiment on 27 May, growth rate was highest under 8 L: 16 D.All groups developed bimodal length‐frequency distributions during the experiment. The proportion of the population in each of the two growth modes was significantly affected by photoperiod treatment.The level of plasma cortisol increased significantly from February to May. There were no differences in levels of plasma cortisol among photoperiod treatments.Judged by development in plasma cortisol, changes in condition factor and external appearance, the parr‐smolt transformation was not completed under any of the experimental photoperiods.