A 75-day experiment was conducted with juvenile gibel carp (Carassius auratus gibelio) (4.80 AE 0.01 g) to evaluate effects of dietary chitosan on fish growth performance, haematology, intestine morphology and immune response. Six isonitrogenous (crude protein: 383 g kg À1 ), isolipid (97.5 g kg À1 ) and isocaloric (gross energy: 16.7 kJ g À1 ) diets were formulated to contain 0, 1800, 4000, 7500, 10 000, 20 000 mg kg À1 chitosan, respectively. The results showed that the growth was depressed when the fish fed with 10 000 mg kg À1 chitosan. Serum cholesterol, triglyceride and low-density lipoprotein decreased in 10 000 and 20 000 mg kg À1 chitosan. On day 75, blood leucocyte phagocytic activity respiratory burst and alternative pathway of complement haemolytic activity were enhanced in 4000 mg kg À1 chitosan. The number of goblet cell, intraepithelial lymphocyte of mid-intestine and microvilli height of distal intestine increased at 4000 mg kg À1 dietary chitosan. Dietary chitosan modulated intestine microbiota, depressed pathogen bacteria Aeromonas veronii-like and improved Cellulomonas hominis-like, Bacillus oceanisediminis-like and two uncultured bacterium-like species on day 75. Dietary 7500 and 10 000 mg kg À1 chitosan enhanced the protection against Aeromonas hydrophila infection. In conclusion, oral administration of dietary 7500 mg kg À1 chitosan for 75 days is recommended for the survival of gibel carp.

A 75 days experiment was conducted in a flow‐through system on juvenile gibel carp (Carassius auratus gibelio) (3.43 ± 0.01 g) to evaluate the effects of dietary lysozyme on growth performance, intestine morphology, microbiota and immune response. Four isonitrogenous (crude protein: 367 g kg−1) isolipid (62 g kg−1) and isocaloric (gross energy: 17.92 kJ g−1) diets were formulated to contain 0, 100, 500 and 1000 mg kg−1 lysozyme, respectively. The results showed that specific growth rate (SGR) and feed efficiency (FE) increased at 1000 mg kg−1 lysozyme. Blood leucocyte phagocytic activity (PA) and serum lysozyme (LZM) decreased with dietary lysozyme on day 25, 50 and 75. There were no significant differences in alternative complement pathway (ACP), respiratory burst (ROS), serum superoxide dismutase (SOD), glutathione peroxidase (GSHpx) or malonaldehyde (MDA). After Aeromonas hydrophilia challenge, higher survival was obtained at 500 mg kg−1 group. PA, ROS, SOD, LZM and ACP increased with increasing dietary lysozyme, while MDA reversed. Goblet cells in mid‐intestine and microvilli height in distal intestine increased with dietary lysozyme on day 75. Dietary lysozyme reduced the diversity of intestine microbiota. In conclusion, oral administration of 500 mg kg−1 dietary lysozyme for 75 days is recommended for the survival of gibel carp and 1000 mg kg−1 dietary lysozyme for fast growth.