Effects of dietary lipid and carbohydrate levels and chronic 3,5,3′-triiodo-L-thyronine treatment on growth, appetite, food and protein utilization and body composition of immature rainbow trout, Oncorhynchus mykiss, at low temperature

Abstract

Twelve dietary treatments were each given to duplicate groups of immature rainbow trout held at 6–7 °C for 12 weeks. Three isonitrogenous (protein = 39%) and isocaloric (3905 kcal digestible energy/kg) diets which varied in their proportions of lipid to carbohydrate (17.6%/1.48%;13.1%/14.9%;7.4%/28.3%) were each supplemented with 0, 4, 8 or 12 ppm T 3 (3,5,3′-triiodo-L-thyronine). All fish were fed their prescribed diet by hand to satiation four times daily. As the ratio of lipid to carbohydrate decreased, growth rate (weight and length), condition factor, food intake, gross food conversion efficiency (GFCE) and protein efficiency ratio (PER; dry basis) decreased. By contrast hepatosomatic index (HSI), plasma glucose concentration, carcass water content and percent lipid deposited increased. Growth was not depressed significantly by inclusion of 13.4% cerelose (D-glucose monohydrate) in the diet but was when 26.8% cerelose was added. All supplemental T 3 doses significantly enhanced growth in weight and length. However, while 8 or 12 ppm T 3 completely offset the reduced growth in length of trout ingesting the high carbohydrate diet, these doses only partially counteracted diminished growth in body weight. Consequently, the condition factor declined. T 3 treatment increased food intake (g/fish) in proportion to body weight which resulted in no change in appetite on a body weight basis. Likewise, percentages of protein and lipid deposited and GFCE were not significantly influenced by T 3 administration. By contrast, feed efficiency and PER values (wet weight basis) were improved significantly by 8 or 12 ppm T 3 in the diet. In conclusion, T 3 administration was efficacious in stimulating skeletal growth of trout at low temperature, and it is especially noteworthy that the hormone completely offset the reduced linear growth of trout receiving a high proportion of carbohydrate in their diet under out test conditions. The mechanism for this effect may have involved synergism of T 3 with growth hormone and/or improvement of carbohydrate utilization, since in the latter case plasma glucose level and HSI were respectively increased and decreased by T 3 treatment.