Comparative evaluation of soybean meal vs. extruded soybean meal as a replacer for fishmeal in diets of olive flounder (Paralichthys olivaceus): Effects on growth performance and muscle quality

A 56-day feeding trial was performed to compare the impacts of replacement of dietary fishmeal by soybean meal (SM) or extruded soybean meal (ESM) on the growth performance and muscle quality of olive flounder (Paralichthys olivaceus) (initial weight: 6.32 +/- 0.01 g). Nine isonitrogenous and isolipidic diets were prepared by substituting 0% (FM, control), 12% (SM12/ESM12), 24% (SM24/ESM24), 36% (SM36/ESM36) and 48% (SM48/ ESM48) of fishmeal with SM or ESM respectively. Results indicated that contents of anti-nutritional factors (trypsin inhibitors, glycinin and beta-conglycinin) in ESM were significantly lower than those in SM. The weight gain rate, specific growth rate, feed efficiency ratio, protein efficiency ratio, activities of intestinal trypsin and lipase, as well as expressions of myogenic regulatory factors (myf5, myod and mrf4) and protein synthesis-related genes (akt, pi3k, tor and s6) in fish muscle decreased linearly and quadratically with increasing replacement levels of dietary fishmeal by SM or ESM. Expressions of mstn and protein degradation-related genes (foxo1, murf1, mafbx, capn2 and ctsl) in muscle showed a trend opposite to growth parameters responding to dietary SM or ESM levels. Significant differences in all the above parameters were observed in the SM36, SM48 and ESM48 groups, accompanied by the declines in contents of most muscle free amino acids. The increase of dietary fishmeal replacement level by SM linearly and quadratically decreased muscle hardness, water holding capacity (WHC), pH and total collagen content. Markedly lower values were found in the SM36 and SM48 groups. However, ESM could replace up to 36% of dietary fishmeal without significantly impacts on above muscle quality parameters. Additionally, reduced myofiber density and sarcomere length were observed in the SM36, SM48, ESM36 and ESM48 groups. However, at the same substitution level, dietary ESM relieved the effects of SM on myofiber morphology to some extent. In conclusion, according to broken-line regression on SGR, the optimal replacing level of fishmeal by SM and ESM for olive flounder was 22.07% and 34.39% respectively. The improved replacement level of fishmeal by ESM, in terms of fish growth, muscle hardness and WHC, may be due to the fact that dietary ESM alleviated the adverse impacts of SM on digestive enzyme activity, muscle growthrelated genes expression, muscle pH, collagen content and myofiber morphology of fish.