Biochemical composition of early life stages of the European Grayling Thymallus thymallus and effects of weaning on different Artemia nauplii strains and dry microdiets.

The present study was aimed at evaluating changes in biochemical composition of early life stages of the European grayling and the effects of feeding different Artemia nauplii strains and dry microdiets on growth and survival rates during the weaning and post-weaning rearing phases.. The Artemia nauplii strain had a marked effect on fry mortality which was not limited to the first phase of the trial but was maintained over the whole experiment. In particular, at the end of the trial (day-77) larvae fed the AF strain resulted in significantly lower cumulative mortality than those fed the EG counterpart (20.4 vs. 39.5%, P<0.05). This could to some extent be ascribed to the lower hatchability of EG strain relative to the AF one (86.6 vs. 97.4%). In fact, higher mortality in EG-fed fry was associated to a higher incidence of undigested, almost intact Artemia cysts in the gut lumen causing mechanical damage and fungal infection.The MDs here compared did not lead to major differences in term of overall growth or mortality, thus indicating that all were palatable and apparently not limiting in nutrient supply. However, the histological examination revealed enlarged supra-nuclear vacuoles of hindgut epithelium cells in fry fed MDs with the highest lipid content as a result of pinocytosis of macromolecules (lipoproteins) from the gut lumen, that could be ascribed to poor intracellular enzyme activities.