Introduction
F ish protein hydrolysates (FPH) are products from either chemical or enzymatic breakdown of fish raw material into single amino acids, peptides and oligopeptides. They are considered a suitable source of protein for animal nutrition because of their balanced amino acid composition and their low molecular weight, allowing higher gut absorption rates and functional properties . The inclusion of FPH in the range of 5-40% in diets has been reported to increase survival and growth rate, decrease malformation rate, increase enzyme activity, modify nutrient transport patterns in the intestine and induce non-specific immune responses in larvae, fry and adult fish. The positive effects of dietary FPH inclusion at the earlier stages of development are related to the fact that the gastrointestinal tract (GIT) is still under development; therefore, providing the fish with medium and small-size peptides through the diet instead of with intact proteins may help in the process of absorption through the enterocytes. Traditionally, this type of studies has been conducted by replacing certain levels of the dietary fishmeal (FM) with similar levels of FPH either as a fraction of the diet or as a fraction of the dietary protein contri bution. In most cases, results have been explained based on the final dietary amino acid composition and/or the water-soluble peptides fractions distributions; h owever, less attention has been given to other nutrients provided by different fish products , which also vary with the dietary source exchange and may play a significant role in the evaluated responses. The present activity aimed to identify key nutrients responsible for variations in the growth rates , the development of haemal lordosis and GIT enzyme activities of E. seabass fry ( 0.2 – 6 g) fed with two different levels of dietary FM (53% and 65%) and FPH (0% and 13%) .
Materials and methods
Fingerlings of E. seabass (0.2 g body weight ) were fed with the 4 experimental diets, in triplicate groups, for 70 days. When fish reached approximately 1.5 g body weight, they were subjected to a swimming challenge test by forcing them to swim against a water current of approximately 17-18 cm/sec for 7 days. Before and after this challenge, samples of fish were taken to document the development of haemal lordosis. At the end of the 70-d feeding trial, and following a recovery period, samples of fish were taken for analyses of growth performance, GIT enzyme activities and haemal lordosis . Data were analysed by means of 2-way ANOVA. In addition, Principal Component Analyses (PCA) were performed to identify potential correlations between the response variables and the dietary nutrients composition.
Results and conclusions
In general, the growth of the fish was not significantly affected by dietary treatments at the end of the 70-d feeding period . However, GIT enzymes such as trypsin, pepsin, leu cine aminopeptidase (LAP) and lipases did respond differently to treatments. Similarly, we found a significant interaction between dietary FM and FPH levels in t he development of haemal lordosis and observed a variation in their levels before and after the swimming challenge test. As expected, the amino acid and peptides distribution profiles are key responsible for the observed parameters; however, the potential role of minerals, trace elements, lipo-soluble vitamins and phospholipids are also defined .