IMPORTANCE OF THRESHOLD LENGTH AND PHOTOPERIOD FOR THE DEVELOPMENT OF BIMODAL LENGTH FREQUENCY-DISTRIBUTION IN ATLANTIC SALMON (SALMO-SALAR)

Juveniles of 0 + Atlantic salmon (Salmo salar) were graded in narrow length intervals to test the hypothesis that a threshold length determines the segregation into bimodal length-frequency distributions. In the experiment, 1350 juveniles of an initial length of 70-80 mm were held under different light and temperature conditions. Distinct decreases in temperature for short or long periods did not influence the degree of bimodal segregation and temperature did not, thus, seem to be a winter stimulus for segregation in fish of this size. Transfer of fish from continuous light to natural photoperiod was followed by a segregation in growth rates into lower and upper mode fish. Upper mode fish developed smolt appearance the following spring. Under continuous light the bimodality was weak or absent and these fish did not develop morphological characteristics typical of smolts. It is concluded that the individual decision to enter the upper mode and undergo silvering the following spring is strongly dependent on the length of the fish at the time of the winter light stimulus. Fish smaller than a threshold length of about 75 mm reduce their growth rate in response to the natural photoperiod, while fish above this size enter the upper length-frequency mode.