Histological analysis After euthanasia, 6 fish per treatment were dissected and their entire digestive tracts exposed. Samples of anterior intestine were fixed in 4% buffered formalin solution for 24 h. After fixation, the samples were transversally sectioned, dehydrated and embedded in paraffin blocks according to standard histological procedures. Sections of 3 micra were stained for histological purposes with haematoxylin and eosin(H&E). In each animal, 3 transversal sections of anterior intestine were examined with light microscope(Olympus BX51. cell B software) and the follow parameters measured or counted in each section: area, the number of mucosal folds or villi present, villi length(measured since the muscularis mucosa to the basal lamina of the enterocytes and the total number of goblet cell. Afterwards, some elements were related with the section area, calculating villi density(number of villil/area), villi length per area and goblet cell density(number of cells per area and villus) The presence or absence of leucocyte in filtrations in the lamina propria and muscularis mucosae and villi fusion were examined.