Effects of probiotic and prebiotic supplemented diets on digestive enzyme activities in juvenile turbot (Scophthalmus maximus)

The aquaculture production of farmed fish is growing continuously. In order to increase sustainability it is very important to optimize the production process. Especially the assimilation and conversion of feeds can be improved by administering specific and well balanced feed formulations. In this work the effects of five different pro- and prebiotic feed supplements on the growth, feed conversion ratios and digestive enzyme profiles of turbot, Scophthalmus maximus were investigated. Juvenile turbots were raised in aquaculture and fed for 112 days with the different diets. The feed intake was recorded daily. The weight and length were measured every four weeks and samples for the digestive enzyme assays were taken after 112 days for digestive enzyme assays. Casein, starch, two nitroanilide substrates and the fluorogenic substrates amido-methylcoumarin and methylumbelliferol were used to assay enzymes which are responsible for the digestion of major feed ingredients such as proteins, lipids and carbohydrates. The different feed formulations had no significant effects on growth, feed conversion ratios and the enzyme profiles of the turbots. The specific growth rates and feed conversion ratios were optimal. The total proteolytic activity in the stomach was by 80 % due to acid proteinases and by 20 % to alkaline proteinases. In contrast, total proteolytic activity in the gut was mainly contributed by alkaline proteinases (94 %). The activities of the endopeptidases trypsin and elastase were as high as the exopeptidases alanine-aminopeptidase and arginine-aminopeptidase. Activities of the esterase, which hydrolyzes short chained fatty acids were higher than true lipase activities hydrolyzing long chained fatty acids. The amylase activity was high, which indicates that turbots have a great capacity in utilizing alimentary starch supplements. Additionally, turbots possess a moderate chitobiase activity which enables them to digest chitin. In conclusion turbots are very efficient in converting their food. The growth and feed conversion were already at the maximum thus no further improvement could be achieved by the application of the pro- and prebiotics. Furthermore, the feed additives had no detectable effects on the digestive enzyme activities because endogenous digestive enzymes prevail in the digestion of turbot. Besides it was shown that the fish have a high potential to utilize proteins and short chained fatty acids. The presence of chitinolytic enzymes seems to facilitate turbot utilizing chitinous feed supplements such as, shrimp shells and waste which remain after shrimp peeling.

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