Effects of dissolved oxygen and dietary lysine levels on growth performance, feed conversion ratio and body composition of grass carp, Ctenopharyngodon idella

In order to investigate the effects of lysine and dissolved oxygen on grass carp, the grass carp were fed with 13, 15 and 17gkg (-1) lysine diet at about 6mgL(-1) (high dissolved oxygen, HO group) or 3.5mgL(-1) (low dissolved oxygen, LO group) dissolved oxygen level, for 8weeks. The fish were fed to apparent satiation by hand. The results showed that apparent digestibility of protein, energy and dry matter were decreased significantly when grass carp were fed at 3.5mgL(-1) dissolved oxygen, and feed intake (FI) was also inhibited by low dissolved oxygen (P<0.05). Weight gain, protein retention, protein efficiency, feed conversion ratio and amino acid retention of fish at 6mgL(-1) dissolved oxygen level were significantly improved at 3.5mgL(-1) dissolved oxygen level (P<0.05). Weight gain, protein and amino acid retention, and feed efficiency of grass carp at the two dissolved oxygen levels were significantly improved by lysine supplementation (P<0.05). The dietary lysine level and dissolved oxygen of water had an interaction effect on feed conversion ratios (P<0.05). Grass carp fed at low dissolved oxygen level showed lower liver protein and fat contents. Plasma aspartate aminotransferase (AST) activity of grass carp fed at 3.5mgL(-1) dissolved oxygen level was significantly increased compared to 6mgL(-1) dissolved oxygen level (P<0.05). Our results show that low dissolved oxygen level of water is harmful to the liver of grass carp.