EFFECT OF FEEDING TIME ON POSTPRANDIAL NITROGEN-EXCRETION AND ENERGY-EXPENDITURE IN RAINBOW-TROUT

The effect of feeding time (dawn or midnight) on nitrogen excretion an d energy expenditure was studied in immature rainbow trout using measu rements of respiratory gas exchange. Fish (mean individual weight 70 g ) were maintained indoors under natural photoperiod and fed by hand (c ommercial food pellets) at a rate of 1% weight/day(-1). Rates of ammon ia and CO2 excretion and O-2 uptake were measured every hour. Ammonia excretion increased immediately after feeding in fish fed at midnight, and 2 h after feeding in fish fed at dawn. Ammonia excretion and ener gy supply from protein catabolism, were higher in trout fed at midnigh t than in those fed at dawn, while total energy expenditure was the sa me in both groups. The results suggested that trout fed in phase with their natural feeding rhythm use dietary protein more efficiently for growth than do trout fed out of phase with the natural rhythm. (C) 199 8 The Fisheries Society of the British Isles.