Effect of dietary lipid content and stocking density on digestive enzymes profile and intestinal histology of rainbow trout (Oncorhynchus mykiss)

The effect of both dietary lipid level and stocking density on digestive function was tested in young rainbow trout under culture conditions. Two dietary lipid levels (14 and 33%) and three culture densities (15, 30 and 40 kg/m3) were assayed in triplicate lots: L14/D15, L14/D30, L14/D40, L33/D15, L33/D30 and L33/D40. Intestinal activities of amylase, lipase, trypsin, chymotrypsin, total protease (at different pHs) as well as some biometric parameters such hepatosomatic index (HSI) and digestivesomatic index (DSI) as indicative of perivisceral fat accumulation, were determined after a 12-weeks experimental period. A histological study of intestine was carried on to evaluate changes in tissue architecture. Fish fed on higher lipid diets grew more and accumulated a higher amount of fat in perivisceral tissue, although higher stocking density reduced this trend. According to digestive capacity, higher lipid intake induced an activation of lipase activity that was not apparent when fish were held under the highest rearing density. By other hand, dietary lipids promoted a significant inhibition of protease activity in fish under high density conditions. These results seem to indicate a clear interaction between both variables. In this study, it was manifested a general effect of dietary lipids level and culture density on digestive profile. These interactions should be taken into account when foreseeing the digestive utilization of a commercial feed at different culture conditions to improve fish production.