Many farmers of marine fish are hesitant to use high levels of soybean products in feed due to a fear of reduced feed palatability and digestibility, slower growth, and/or poorer feed conversion compared to traditional feeds containing high levels of fishmeal and fish oil. Some consumers also fear that replacement of the fish meal in the feed could change the nutritional value of edible fish products.
A study was conducted to determine the optimal inclusion levels of soy products in a fish meal-based practical diet for gilthead sea bream (Sparus aurata) one of the main species of marine fish produced in the Mediterranean region. Three isonitrogenous and isoenergetic diets with 45 percent crude protein and 20 percent crude fat were formulated for testing with gilthead seabream. The experimental diet was formulated with fish meal making up 65, 50, or 35% of the total dietary protein. The fish meal was replaced with vegetable sources of protein, mainly soy protein products due to their cost efficiency compared to other plant ingredients. The replacement of fish protein/fat was balanced by the supplementation of the essential amino acids lysine and methionine, phosphorus and a blend of palatability/digestibility enhancers.
The results of the 10-week experiment are presented in the following table. As can be see replacing 30% of the fish meal protein with soy protein resulted in similar growth rates and feed-conversion ratios.