Replacing poultry by‐product meal protein with soybean protein isolate in low fishmeal diets for juvenile hybrid grouper (Epinephelus fuscoguttatus ♀ × Epinephelus lanceolatus ♂).

A 7‐week growth trial was conducted to assess the potential of using soybean protein isolate (SPI) to replace poultry by‐product meal (PBM) protein in low fishmeal (FM) diets for juvenile hybrid grouper (Epinephelus fuscoguttatus ♀ × Epinephelus lanceolatus ♂). Seven isoproteic (490 g kg−1) and isolipidic (90 g kg−1) experimental diets were formulated, including six low FM diets and a high FM diet (HFMP). The low FM diets contained 0, 74.8, 149.7, 224.5, 299.4 and 374.2 g kg−1 SPI, replacing approximately 0%, 20%, 40%, 60%, 80% and 100% of the PBM protein (designated as SPIP0, SPIP20, SPIP40, SPIP60, SPIP80 and SPIP100, respectively). Each diet was hand‐fed to triplicate groups of 12 hybrid grouper juveniles (average initial body weight: 14.30 ± 0.1 g per fish). After being fed for 7 weeks, fish fed with HFMP and SPIP60 showed significantly higher (p <.01) weight gain percentage (WG%), protein efficiency ratio and protein productive value than fish fed with other experimental diets. These three parameters were elevated with the increment in PBM protein (PBMP) replacements, reaching the peak value at the 60% replacing level, and then, they were significantly reduced at the 80% or 100% replacing level. Analysis of the two‐slope straight broken‐line model showed that the maximum PBMP replacement level by SPI protein (SPIP) at the intercept value of WG% was estimated to be 60%. Feed conversion ratio showed an opposite tendency of variation to WG%. Fish fed with SPIP80 and SPIP100 displayed lower mRNA levels of hepatic insulin‐like growth factor‐1, hypothalamus neuropeptide Y and head‐kidney nuclear factor erythroid 2‐related factor 2 than fish fed with other experimental diets. Generally, 60% of PBM protein could be replaced with SPI protein in low FM diets for hybrid grouper. [ABSTRACT FROM AUTHOR]