Cholesterol supplementation in plant-based diets for rainbow trout (Oncorhynchus mykiss)

Abstract

Total replacement of marine oils and proteins with plant proteins and oils in rainbow trout diets reduces growth performance and feed intake. It has been suggested that this is due to the presence of growth factors present in marine products but absent in plant ingredients. One putative factor is cholesterol. Thus, the effect of supplementing cholesterol with and without dietary polyunsaturated fatty acids in plant-based diets fed to rainbow trout was investigated in two experiments. In both experiments, the experimental period was 12 weeks and fish were weighed on day 0 and 84. Fish were fed to satiation twice daily. All diets were formulated to contain 43.0% crude protein (as is basis) and 92.0% digestible energy (as is basis). In Experiment 1, there were 8 experimental diets arranged in a 2 x 4 factorial design, with 2 levels of PUFA (olive oil or olive oil + linseed oil) and 4 levels of cholesterol (0.00, 0.05, 0.10 and 0.15%) added using synthetic cholesterol. Rainbow trout (n=13/tank; initial body weight 13.4 g and 3 tanks/treatment) were randomly assigned to one of the 8 diets. Between days 28 – 35, there was an outbreak of bacterial cold water disease (BCWD) (Flavobacterium psychrophilum). Fish fed the olive oil-based diets had significantly higher mortalities than fish fed the olive/linseed oil diets. The interaction between PUFA level and cholesterol was significant for average daily feed intake (ADFI) and near significant for average daily gain (ADG) (P = 0.085) and specific growth rate (SGR) (P = 0.082). The experiment was therefore reanalyzed using regression analysis. Fish fed the olive oil-based diets had significantly increased ADG, SGR and feed intake when cholesterol levels were increased, while there was no significant effect of cholesterol level on growth parameters of fish fed the olive oil + linseed oil-based diets. In Experiment 2, there were 6 experimental diets arranged in a 2 x 3 factorial design, with 2 levels of PUFA (olive oil or olive oil + linseed oil) and 3 levels of cholesterol (0.00, 0.075 and 0.15%) added using synthetic cholesterol. Rainbow trout (n=20/tank; initial body weight 27.7 g and 4 tanks/treatment) were randomly assigned to one of 6 diets and fed to satiety 2x daily for 84 days. Growth and feed intake were measured over the entire experimental period and at the end of the trial, blood and liver samples were taken from 4 fish per tank for measurement of cholesterol. There was a trend towards increased ADG (P = 0.089) and SGR (P = 0.082) with increasing cholesterol levels. There were no clinical signs of disease and both main effects and interactions were not significant for mortality. There were no significant effects of cholesterol, effects of PUFA level or an effect of the interaction between the two factors on serum or hepatic cholesterol levels in the fish. However, values for cholesterol content in both blood serum and hepatic tissue samples were found to be lower in the high PUFA diets, than those in the low PUFA diets (1.80 mg/g vs. 2.00 mg/g in hepatic tissue and 2.39 mg/ml vs. 2.66 mg/ml in serum). The results of the two experiments suggest that cholesterol is a conditionally limiting nutrient in rainbow trout and one constituent of marine products that improves growth when supplemented in a completely plant-based diet.