Modifications of digestive enzymes in trout (Oncorhynchus mykiss) and sea bream (Sparus aurata) in response to dietary fish meal replacement by plant protein sources

Three experimental diets in which fish meal was partially (50 and 75%, named PP50 and PP75 diets, respectively) or totally replaced (PP100 diet) by plant protein sources were fed to rainbow trout and gilthead sea bream. We studied the effects of these diets on digestive enzymes in comparison to fish fed fish meal diets (FM). A mixture of vegetal ingredients (corn gluten meal, wheat gluten, extruded peas and rapeseed meal) was used to meet the amino acid requirements of the fish. Over a 12-week trial, four groups of the two species were fed one experimental diet twice a day until they showed satiety. After the growth period, post-prandial protease and α-amylase activities in proximal intestine were measured. Trout fed fish meal as the sole protein source (FM diet) showed a peak in total protease activity 3 h post-feeding; however, trout fed plant protein based diets (PP diets) did not register a peak in activity of these enzymes. In FM fed sea bream, proteolytic activity peaked in the most proximal intestinal segment 6 h post-feeding. This peak was also observed in PP fed sea bream, although the magnitude tended to decrease as the percentage of plant protein increased. This reduction was due to the specific inhibition of chymotrypsin-like bands, although a slight increase in trypsin secretion was detected in the zymograms of this species. The replacement of fish meal by plant protein did not affect α-amylase activity in either trout or sea bream. In sea bream proximal intestine histology was also studied. The PP100 fed group showed shorter folds and smaller goblet cell population when compared to FM fed group. In response to feeding the PP75 and PP100 diets, sea bream and trout showed a significant increase in relative intestinal length. Despite this compensatory mechanism, final weights of the two species diminished; however, this decrease exceeded 20% only in the case of groups fed the PP100 diets.