Emulisfiers in soybean oil-based diets for california yellowtail

California yellowtail (YT), Seriola lalandi, is a top candidate species for ocean farming.  It is a high quality food fish in both sushi and conventional markets.  Hubbs-SeaWorld Research Institute has been developing low fish meal soy-based diets for marine finfish through research funded by the United Soybean Board since 2008 and soy-based diets with soybean-oil as a replacement for fish oil since 2010 with co-researchers at Southern Illinois University.  Previous research showed that it was possible to successfully replace a portion of fish oil with various types of soybean oils in diets for YT, but that digestibility might be a limiting factor for hydrogenated soybean oils.  As a continuation of this research, a trial was conducted to assess whether dietary supplementation with emulsifying agents could improve the digestibility and utilization of hydrogenated soybean oil-based feeds for YT.  

Two diets previously verified in our work with YT were formulated; a positive control containing menhaden fish oil and a negative control diet containing hydrogenated soybean oil and LC-PUFA supplements.  Fully hydrogenated soybean oil was chosen as it represents a 'worst case scenario' in terms of fat digestibility.  Four experimental diets were derived from the basal soybean oil-based formulation by the addition of 1% mixed bile acids, monoglycerides, whey protein, or casein as emulsifiers.  Juvenile fish (10.2 ± 0.0 g) were stocked in 300L tanks in a partial reuse system at 15 fish/tank.  Diets were randomly assigned to triplicate tanks and fish were fed at approximately 6-8% body weight per day for 8 weeks.

Except for fish fed the diet supplemented with 1% mixed bile acids, there were no significant differences observed with regard to weight gain, survival, or FCR.  Although the growth performance observed was not considered exceptionally poor, the 1% mixed bile acid diet was associated with inferior weight gain (459% vs. 655-682%) and FCR (1.64 vs. 1.25-1.28), as well as poor survival (49% vs. 96-100%).  Mortalities occurring in this treatment were associated with an emaciated appearance, though anorexia was not apparent in all fish fed the 1% mixed bile acid diet, some fish appeared to accept this diet quite well.  Unlike previous trials, reduced growth performance was not observed among fish fed the negative control diet.  Thus, we could not determine whether the addition of emulsifying agents might improve utilization of soybean oil-based diets when lipid digestibility is a limiting factor.  Although no benefit of supplementation was observed in the present case, the addition of monoglycerides, whey protein, or casein does not appear to negatively affect yellowtail.  Conversely, the addition of mixed bile acids at 1% of the diet is not recommended.  

Whole body proximate and fatty acid composition analysis is pending and results will be discussed.