EFFECTS OF A DECREASING PHOTOPERIOD ON SM OLTING IN UNDERYEARLING ATLANTIC SALMON (SALMO-SALAR) REARED UNDER ARTIFICIAL PHOTOPERIOD

Artificial photoperiod and temperature regimes lead to an early smolti fication on atlantic salmon juveniles and allow an out of phase seawat er transfer. In this paper, we are studying the effect of different ph otoperiod and spectral composition changes. After a 80-day period of d ay-length increase (that is the 18th of August) and under an. artifici al daylight exposure (with incandescent light), fish were subjected, d uring the autumn, to natural conditions (decreasing photoperiod). Thes e changes were realized again 11 and 35 days later. In order to dissoc iate the photoperiod from the spectral composition effects, fish were also subjected to artificial day-length increase under fluorescent ''t rue lite'' tube and to incandescent light and natural photoperiod. The se experiments show that the spectral composition of the light has onl y an effect on the smolting and not on the growth. A day length decrea se following a short photoperiod increase as it was realized does not lead to a change in the (Na+-K+)-ATPase activity unlike with a constan t photoperiod.