EVALUATING EFFECTS OF FISH OIL REPLACEMENT WITH ALGAL OIL (Schizochytrium sp.) IN THE DIET OF Oncorhynchus mykiss      

With the recent and predicted surge in human population it is vital that we find a way to safely feed all the people in the world. Fish are a viable source of important nutrients, yet we need to find a way to produce fish for us to eat without further damaging natural stocks of fish. Heterotrophic algae may provide an alternative source of omega-3 HUFA's (highly unsaturated fatty acids), as they are a renewable source of EPA and DHA. The objective of this study was to evaluate the effects of fish oil replacement with 12% algal oil in combination with 8% soybean oil in an Oncorhynchus mykiss diet (50% protein/ 17% fat/ 3% fiber) versus a conventional diet (45% crude protein/ 16% crude fat/ 3% crude fiber) by evaluating growth, feed utilization and body composition of O. mykiss grown in conditions similar to an industrial setting.

A 70-day feeding trial was conducted with O. mykiss. Randomly selected fish (average age of 96 days post "start hatching", hatched and raised at the HSU Fish Hatchery) were stocked into 8          (4 experimental and 4 control) tanks filled to 145 L each (raised to 400 L on day 23) at a density of 10 kg/m3. Water was recirculated and kept at or below 18°C. Weight samples were taken at the beginning of the experiment and weekly throughout the experiment. Fish were fed at a rate of 1.29- 4.5% body weight one time a day. Upon completion of the experiment fish from each tank were euthanized, weighed and measured to determine the condition factor and the livers were removed and weighed to determine the hepatosomatic index. Furthermore, intestinal tissue was sampled to evaluate the morphology and health of the epithelium. Results were analyzed using a t-test to determine differences between the diets, which were significant when p<0.05.

Fish from the algal diet grew bigger, had a lower feed conversion ratio and had the same protein conversion ratio, condition factor, hepatosomatic index, and survival (Table 1), indicating that a mix of algal and soybean oil can be used to replace at least 20% of fish oil. Algal oils in combination with soybean oil are a more sustainable ingredient and an excellent replacement for fish oil in the diet of O. mykiss, however a longer-term study raising O. mykiss to market-size is necessary for further analysis.