Rainbow Trout Rearing Performance, Intestinal Morphology, and Immune Response after Long-term Feeding of High Levels of Fermented Soybean Meal

The incorporation of PepSoyGen (PSG), a commercially produced fermented soybean meal product, was evaluated in a 205-d feeding trial. Three isonitrogenous and isocaloric experimental diets containing 0, 35, or 50% PSG and 40, 15, and 0% fish meal, respectively, were fed to Rainbow Trout Oncorhynchus mykiss. The diets containing either 0% or 35% PSG produced similar total tank weight gains, percent gains, and feed conversion ratios and were significantly different from the diets containing 50% PSG at the end of the trial. Mean individual fish lengths and weights were significantly greater in the fish receiving 35% PSG than either of the other two diets. Specific growth rate (SGR) was significantly lower in the 50% PSG treatment, but there was no significant difference in SGR between the other two diets. Viscerosomatic index was significantly lower in the fish fed 35% PSG. Hepatosomatic index was not significantly different among the diets, but fish receiving 50% PSG had a significantly poorer liver condition scores. No other significant differences were observed in health assessments of fat, fin condition, gills, pseudobranchs, opercles, eyes, guts, kidneys, or spleens. Significant differences were observed in distal intestine morphology among the diets at day 94, but no differences were observed at the end of the trial on day 205. Splenosomatic index, macrophage activity, and respiratory burst activity were not significantly different among the dietary treatments. Based on these results, PSG can safely constitute 35% of Rainbow Trout diets without any decrease in rearing performance compared with a reference blend containing fish meal.