Impact of different dietary lipid sources on growth, lipid digestibility, tissue fatty acid composition and histology of rainbow trout, Oncorhynchus mykiss

The aim of the present study was to determine the impact of replacing at least 50% of the fish oil by alternative lipid sources in diets for rainbow trout on growth, lipid digestibility, fatty acid profiles of liver and muscle and tissue histology. Four experimental diets were formulated combining two fish oils (capelin and anchovy) with different vegetable oils (soybean, rapeseed, palm and olive) and one type of animal fat (lard), in order to obtain 60% to 80% fish oil replacement. A fifth diet prepared with pure fish oil (capelin oil) served as control. The diets were fed to apparent satiation twice a day to triplicate groups of 30 rainbow trout with an initial weight of 250 g for 64 days at 12 degreesC. Growth was good (TGC(3) 3.7 - 3.9) and independent of diet. Final body weight was on average 760 g. Feed conversion ratios (FCR) ranged from 0.72 to 0.79. The only significant difference in FCR was found in the group fed an olive oil-lard combination replacing 80% of the fish oil. This diet also had a lower apparent lipid digestibility (79% vs. 91 - 94% for the rest of the diets). The liver and muscle fatty acid compositions were similar within groups and reflected that of the diets. But, tissue levels of 20:5n - 3 were lower and levels of 22:6n - 3 were higher than their respective dietary percentages. Histologically, a supranuclear accumulation of lipid droplets was observed in the intestinal cells of some of the groups fed diets supplemented with vegetable oils. Similarly, livers from these groups showed large amounts of lipid droplets within the hepatocytes. The present results suggest that most of the fish oil can be replaced by the alternative sources tested without compromising growth and feed utilisation. However, the histological changes observed suggest an impact of dietary lipid source on the transport and/or metabolism of fat in the fish. This requires further investigation. (C) 2002 Elsevier Science B.V. All rights reserved.