Effects of dietary lysine levels on growth, feed utilization and related gene expression of juvenile hybrid grouper (Epinephelus fuscoguttatus ♀ × Epinephelus lanceolatus ♂)

Abstract

An 8-week growth trial was undertaken to evaluate the effects of dietary lysine (Lys) levels on growth, feed utilization and related gene expression of juvenile hybrid grouper (Epinephelus fuscoguttatus ♀ × Epinephelus lanceolatus ♂). Six isonitrogenous (53.5% of dry matter) and isolipidic (7% of dry matter) were formulated to contain 1.74%, 2.18%, 2.94%, 3.28%, 3.66% and 3.99% dietary Lys levels. Triplicate groups of 15 fish (average initial body weight: 11.44 ± 0.07 g) were fed each experimental diet to apparent satiation by hand twice daily (08:00 h and 16:30 h). Results showed that weight gain % (WG%) were significantly (P < .05) affected by different dietary Lys levels, and fish fed 1.74% dietary Lys had lower WG% than fish fed other dietary Lys levels. Daily feed intake (DFI) of fish fed 1.74% dietary Lys was higher than that of fish fed 2.18%, 2.94%, 3.28%, 3.66% or 3.99% dietary Lys. Feed efficiency (FE), protein efficiency ratio (PER) and protein productive value (PPV) showed similar variation trends as WG%. Hepatosomatic index (HSI) was reduced as dietary Lys levels increased. The protein contents in whole body, muscle as well as liver of fish fed 1.74% dietary Lys were lower than those of fish fed other dietary Lys levels. Fish fed 1.74% dietary Lys showed lower relative mRNA expression levels of insulin like growth factor-I (IGF-1), target of rapamycin (TOR) and S6 kinase 1 (S6K1) in liver than fish fed 2.18%, 2.94%, 3.28%, 3.66% and 3.99% dietary Lys. Based on the straight broken-line analysis of WG% against dietary Lys levels, the optimal dietary Lys requirement for hybrid grouper was estimated to be 2.16% of dry matter (corresponding to 4.05% of dietary protein on a dry weight basis).