Effects of practical dietary protein to lipid levels on growth, digestive enzyme activities and body composition of juvenile rice field eel (Monopterus albus)

A 10-week feeding experiment was conducted to evaluate the effects of dietary protein to lipid ratios (P/L) on growth, intestinal digestive enzyme activities and body composition in juvenile rice field eel (Monopterus albus) (initial mean body weight of 65.76 +/- A 1.07 g, mean +/- A SEM). Nine test diets were formulated in a 3 x 3 factorial design to contain three protein levels (350, 400 and 450 g kg(-1)) for each of three lipid levels (40, 80 and 120 g kg(-1)), respectively. Each diet was randomly assigned to triplicate groups of 60 fish per net cage (1.5 x 2.0 x 1.5 m). Results showed the survival was above 96 % and was not affected by dietary treatments. Fish fed the diet with 450 g kg(-1) protein and 40 g kg(-1) lipid showed the best weight gain (WG) (103.95 %) and feed conversion ratio (1.60) (P < 0.05). WG, protein efficiency ratio and energy retention increased with the increasing in lipid at 350 g kg(-1) protein level (P < 0.05). However, WG showed a little decline with increasing dietary lipid when fish fed the diets with 400 and 450 g kg(-1) protein level, but no significant difference was observed (P < 0.05). Hepatosomatic index, visceralsomatic index and intestinal lipase activity increased with the increasing of dietary lipid level irrespective protein level. Intestinal trypsin activity increased with the increasing of dietary lipid level when fish fed the diets with 350 g kg(-1) protein, but showed converse trend when fish fed the diets with 400 and 450 g kg(-1) protein. Serum triglyceride, body lipid and energy were positively correlated with the dietary lipid. Results of the present study showed that the dietary protein/lipid ratio of 450/40 g kg(-1) is considered optimum for rice field eel under culture conditions, and the increase in dietary lipid level has no efficient protein-sparing effect when fish fed the diets with 400 and 450 g kg(-1) protein level.