C.L. Cahu, J.L. Zambonino Infante, P. Quazuguel, M.M. Le Gall
Four groups of Dicentrarchus labrax larvae were fed, from day 10 post-hatching to day 41, with 4 isoenergetic and isonitrogenous (60%) compound diets. H0 diet contained fish meal, which was substituted by fish protein hydrolysate (CPSP G) at a level of 0, 14, 28 and 42% in diets H0, H14, H28 and H42, respectively. Larvae growth was monitored weekly; survival rates were calculated by counting individuals at the end of the experiment. Activities of pancreatic enzymes (amylase and trypsin) and of a cytosolic enzyme (leucine-alanine peptidase) were determined during larval development. Secreted and nonsecreted pancreatic enzymes were assayed in supernatant and larvae homogenate, respectively. The level of secretion was defined as (secreted enzymes/total enzymes) ratio.
The highest survival was obtained in group fed H14 diet (47 +/- 5.0%), and the lowest in groups fed H28 and H42 diets (34 +/- 1.4% and 33 +/- 3.0%, respectively). H0 diet induced an intermediate survival (39+/- 2.1%). Growth of larvae fed H0 and H14 diets (5.5 +/- 2.61 mg and 5.7 +/- 2.77 mg; respectively) was significantly higher than that of larvae fed H28 and H42 diets (3.3 +/- 1.03 mg and 2.6 +/- 0.70 mg, respectively).
At day 41, the group fed the highest level of dietary protein hydrolysate exhibited an amylase and trypsin secretion 20% and 30% respectively lower than those obtained in the 3 other experimental groups. As early as 7 days of compound diet feeding, the specific activity of leucine-alanine peptidase was 4 times higher in H42 group than that of the 3 other groups. This evelated specific activity of cytosolic enzyme was maintained until the end of the experiment. The high level of leucine-alanine peptidase reflected the special feature of young larvae to digest small peptidic chains. Nevertheless, diets containing high levels of protein hydrolysate induced a lowering in larvae growth and survival, which could be related with the lowering in pancreatic enzyme secretion. When incorporated at a moderate level, dietary protein hydrolysates were beneficial for larvae growth and survival.
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