REPLACEMENT OF FISH OIL WITH CONVENTIONAL AND HIGH OLEIC ACID SOYBEAN OIL IN THE STARTER DIETS OF RAINBOW TROUT Oncorhynchus mykiss

The replacement of fish meal and fish oil with plant-based ingredients in fish feeds is a promising solution to sustainability and profitability concerns associated with Salmonid aquaculture. While conventional soybean oil has been extensively studied as a lipid source in in Salmonid feeds, no studies to date have evaluated the high oleic acid soybean oil (HOSBO) that is rising in popularity with the U.S. soy industry. This investigation examined the culture performance, gastrointestinal histology, and lipid composition of rainbow trout fry fed experimental  starter  diets with  conventional and high oleic acid soybean oil sources.

Rainbow trout fry (165 mg)  were randomly distributed to eighteen 20  L flow-through tanks with initial stocking of 60 fish/tank. Fish were f ed six experimental diets for  eight weeks  on a restricted feeding regime (5-6% biomass/day), and tanks were weighed and sampled every two weeks .  Six experimental diets were formulated to have 41% protein and 20% lipids . Two diets were formulated with fish meal protein and cod liver oil or conventional soybean oil (FM+ CLO and FM+SBO, respectively). The other four diets had 1:1 fish meal and soybean meal protein and either cod liver oil (SBM+CLO), conventional soybean oil (SBM+SBO),  HOSBO  (SBM+HOSBO), or  HOSBO  supplemented with n-3 polyunsaturated fatty acids (SBO+HOSBO+PUFA).  Soybean oils were provided by Archer Daniels Midland^™ and SmithBucklin^™ .

The mean weight and feed conversion ratio (FCR) of fish were significantly different (α=0.05) between diet groups throughout the first four weeks of feeding (p<0.0001; Figure), but the FCR was not significant at six and eight weeks (p>0.1). The fishmeal diet groups had significantly faster growth and lower FCR than all soybean meal diet groups throughout the trial. While the two  HOSBO diet groups had a higher FCR at four weeks of feedin g, this trend was not apparent as fish gre w larger.  These results suggest that rainbow trout have reduced growth efficiency on high oleic acid soybean oil in starter diets, but that this may be an appropriate lipid source for grow-out stages.