Aspects of parr-smolt transformation in anadromous and resident forms of brown trout ( Salmo trutta) in comparison with Atlantic salmon ( Salmo salar)

This study established the existence of a parr-smolt transformation in the anadromous form of brown trout but no such change in the resident form when reared under ambient photoperiod and temperature. Smolting occurred in March–April in the larger trout of the anadromous form and in April–May in salmon. Silvering, a peak of gill Na +,K +-ATPase activity and a simultaneous decrease in plasma prolactin were observed in both salmon and the anadromous form of trout. Bimodality in length frequency and a surge in growth hormone were observed only in Atlantic salmon. No physiological or morphological changes were observed in smaller (