Effect of feeding time on hepatic nucleic acid, plasma T3, T4, and GH concentrations in rainbow trout.

The effect of feeding time (dawn, midnight) on the growth performance and daily pattern of liver nucleic acid concentrations, plasma thyroid hormone, and growth hormone concentrations was studied in immature rainbow trout (Oncorhynchus mykiss, mean individual weight: 55 g). Fish were held in groups of 20 individuals (12 groups per treatment) and maintained in flow-through tanks supplied with river water under natural photoperiod. Food pellets (Aqualim, 49% crude protein) were delivered daily over a 30-min period by means of belt feeders. Growth performance and protein retention efficiency were higher for trout fed at dawn than for those fed at midnight, whereas both groups of fish ingested the same total amount of feed. All parameters studied showed significant daily variations. The daily patterns of liver RNA concentrations, RNA/DNA, and protein/DNA ratios were significantly different between fish fed at dawn and at midnight, indicative of a higher liver protein synthesis in the trout fed at dawn. On average, plasma thyroid hormone levels were higher and plasma GH concentrations were lower in trout fed at dawn compared with those fed at midnight. The hormonal patterns were only affected by feeding time when the fish were fed at dawn. These results suggested that the observed differences in growth and protein retention efficiency were linked to the observed differences in plasma hormone levels.