Partial substitution of fish meal with soy protein concentrate in commercial diets for juvenile swimming crab, Portunus trituberculatus

Abstract

A 56-day feeding trial was conducted to evaluate the potential use of soy protein concentrate (SPC) as a partial replacement of fish meal in swimming crab Portunus trituberculatus. Four isonitrogenous and isolipidic diets (450 g/kg protein and 80 g/kg lipid) were formulated to contain graded levels of SPC, and fish meal was replaced with SPC at 25%, 50% and 75% level (SPC25, SPC50 and SPC75, respectively). The control diet (SPC0) contained fish meal, soybean meal and krill meal as the main protein sources. Lysine and methionine were added into SPC replacement diets to maintain the same concentrations as the control diet. Each diet was randomly assigned to triplicate groups of 45 juvenile swimming crabs (5.00 ± 0.23 g / crab) that were stocked in the rectangle plastic baskets. The results indicated that the weight gain (WG), special growth rate (SGR), feed efficiency (FE) and daily feed intake (DFI) were not significantly influenced by the dietary replacement of fish meal with SPC (P > 0.05). Crabs fed the SPC75 diet had lower protein efficiency ratio (PER) and molting frequencey (MF) than those fed the SPC50 diet(P < 0.05). The proximate compositions in whole body and muscle were not significantly influenced by the replacement of fish meal with SPC (P > 0.05), however, there was significant difference in the hepatopancreas moisture content (P < 0.05), and crabs fed the SPC0 diet had the highest lipid in hepatopancreas among all treatments (P < 0.05). Moreover, the essential amino acid (EAA), non-essential amino acid (NEAA) and total amino acid (TAA) contents of muscle significantly increased with the dietary replacement of fish meal with SPC levels increasing from 0 to 75% (P < 0.05). No significant differences in hemolymph enzyme activities and characteristics were observed among all treatments (P > 0.05). Crabs fed the SPC0 diet had higher amylase and lipase activities in hepatopancreas than those fed other diets. The hepatopancreas trypsin activity was not significantly influenced by the replacement of fish meal with SPC (P > 0.05). The hepatopancreas alanine aminotransferase (ALT) and aspartate aminotransferase (AST) activities increased when the dietary SPC levels increased from 0 to 25%, and then decreased significantly with the further increase of SPC replacement levels (P < 0.05). The relative expression of genes in target of rapamycin (TOR) pathway, crabs fed the SPC0 diet had higher relative expressions of tor, akt, s6k1, s6, 4ebp1, eif4e1a, eif4e2 and eif4e3 genes in hepatopancreas than those fed other diets (P < 0.05), moreover, there were no significant differences in gene expressions of TOR pathway among crabs fed the SPC25, SPC50 and SPC75 diets (P > 0.05). Based on the two slope broken-line regression analysis of PER against the dietary fish meal replacement with SPC levels, the optimum dietary replacement of fish meal with SPC was determined to be 51.49% for juvenile swimming crab.