PARTIAL AND TOTAL REPLACEMENT OF FISH OIL WITH FATTY ACID ETHYL ESTERS IN THE STARTER DIETS OF RAINBOW TROUT Oncorhynchus mykiss

Fish oil is a necessary dietary ingredient for meeting the omega-3 polyunsaturated fatty acid (PUFA) requirements in most fish. The rising price of fish oil, as well as the limited harvest of wild fish to meet aquafeed demands, have led to an interest in finding suitable replacements for this PUFA source. Fatty acid ethyl esters (FAEE) are formed as a byproduct of PUFA refinement, and are a potential dietary alternative to fish oil. This investigation examined the culture performance and lipid composition of rainbow trout fry fed experimental starter diets with graded levels of FAEE and fish oil.

Rainbow trout fry (144±20 mg) were randomly distributed to eighteen 20 L flow-through tanks with an initial stocking density of 60 fish/tank. Fish were fed six experimental diets for 62 days on a restricted feeding regime (4-5.5% biomass/day). Fish were weighed and sampled at 31 and 62 days of feeding, after which density was reduced to 40 fish/tank, then 15 fish/tank, respectively. Six experimental diets were formulated to be isonitrogenous with 20% lipids. One control diet was formulated using full-fat menhaden meal and menhaden oil (M. Control), while the other diets used defatted menhaden meal and Alaskan pollock oil and/or FAEE (Organic Technologies, Coshocton, OH). These five diets had 0% (P. Control), 25%, 50%, 75%, and 100% replacement of Alaskan pollock oil with FAEE, and were formulated with analogous fatty acid profiles.

The total growth and feed conversion ratio of fish were significantly different (α=0.05) between diet groups in the first 31 days of feeding (phase I; p<0.0001), but were not significant from 32 to 62 days of feeding (phase II; Figure). Trends in the data suggest that rainbow trout fry have less efficient growth on diets containing high FAEE concentrations during first-feeding, but that these trends are diminished as fish grew larger (phase II). Additionally, this investigation suggests that dietary FAEE do not affect fish health and survival. Further studies are needed to confirm that any initial growth limitations can be overcome as juveniles/adults.