Aquaculture International 2, 165-179 (1994) Effect of the daily meal time on the growth of rainbow trout fed different ration levels P.K. Reddy’, J.F. Leatherland ‘*, M.N. Khan’ and T. Boujard* ‘Institute of Ichthyology~ Department of Zoology, University of Guelph, Guelph, Ontario NlG 2W1, Canada 2/ N R.A., 64310 Saint Pbe-sur-Nivelle, France . , This study examined the effect of ration level and time of delivery of a single daily meal on the growth performance of rainbow trout held under constant water temperature and a natural photoperiod. Two similar trials were conducted, the first from March to July, 1992, using ration levels of 0.5, 1.5 and 2.5% BW d-’ and the second from August 1992 to January 1993,using ration levels of 1.5, 2.0 and 2.5% BW d-‘. In five of the six treatment groups (three ration levels x two seasons), when the daily meal was presented in the post-dawn period the growth rate was lower than in trout fed at either mid-light or pre-dusk. No time-of-feeding effect was found when trout were fed excess rations. Specific growth rate (SGR) was not affected by feeding time, but was correlated with ration level; furthermore, there was a significant increase in SGR during the first section of the first trial and a significant decrease in the SGR throughout the duration of the second trial. Ration level was significantly correlated with condition factor, carcass visceral adipose tissue content and skeletal muscle lipid content, but there was no effect of the timing of the daily meal on these variables. KEYWORDS:Rainbow trout (Oncorhynchus mykiss), Growth rates, Feeding regimes, Time of feeding, Ration levels, Visceral adipose tissue, Skeletal muscle lipid INTRODUCTION For many species of fish, growth is indeterminate and continues throughout the life of the animal, provided there is an adequate source of food available. For most species studied to date, this indeterminate growth pattern is remarkably plastic (Weatherley and Gill, 1987; Weatherley, 1990). The quality and quantity of food is a primary factor in determining the growth performance of most fish, but the efficiency of assimilation of available energy is markedly influenced by other factors, such as ambient temperature, stocking density, ontogeny and season (e.g. Wurtsbaugh and Davis, 1977; Brett, 1979; Ricker, 1979; Elliott, 1982; Jobling, 1983, 1987; Leatherland and Cho, 1985; Vijayan and Leatherland, 1988; Jorgensen and Jobling, 1992; Leatherland et al., 1992; Leatherland, 1993). Some studies have also provided evidence to show that the growth performance of * To whom all correspondence should be addressed. 0967-6120 0 1994 Chapman & Hall