ASPECTS OF PARR SMOLT TRANSFORMATION IN ANADROMOUS AND RESIDENT FORMSOF 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 activ ity and a simultaneous decrease in plasma prolactin were observed in b oth salmon and the anadromous form of trout. Bimodality in length freq uency and a surge in growth hormone were observed only in Atlantic sal mon. No physiological or morphological changes were observed in smalle r (< 13 cm) trout of the anadromous form or in the resident form. Hypo -osmoregulatory ability was greatest at the time of peak gill Na+,K+-A TPase activity and increased in spring in all trout, irrespective of s molt status. It is concluded that smolting is not as well developed in brown trout as in Atlantic salmon, and is not necessary for seawater adaptation of brown trout. It is also dependent on both stock origin a nd fish size.