Rainbow trout (Oncorhynchus mykiss) is a highly commercial sport and
market fish. Rainbow trout feed formulations are using fish oil, fish meal, grains and
other in-gredients. In recent years the amount of fish meal has been reduced to less than
50 percent. This was achieved by using alternative protein sources such as soybean
meal. The aim of this study was to assess the changes of intestine histology of rainbow
trout fed three different iso-energetic and isoproteic diets with a partial substitution of
fish meal and fish oil with vegetable protein and vegetable oil. The goblet cell number
per μm of intestinal fold appeared to not be affected by diet, while goblet cell diameter
appeared smaller in rainbow trout fed a diet of 31% organic vegetable protein.
Rainbow trout fed the above diet had a smaller submucosa layer. Rainbow trout fed a
diet of 36% vegetable protein and 6% vegetable oil had smaller intestinal folds, but the
brush border height and muscular layer width were unaffected by the diet. Changes in
the rainbow trout intestinal histology following a single meal were studied. Intestinal
samples were taken before feeding and at 6 h and 12 h after feeding. At 6 h after
feeding, the protein synthesis rate in trout is in its peak and the digestion process is in
progress. Goblet cells then secrete their mucous and their size is decreased. The
similarity in the size of the goblet cells before feeding and at 12 h after feeding sugest
that rainbow trout should be fed twice per day and that the second feeding should take
place between 6 and 12 hours after the first feeding. In fish intestine the goblet cells’
post-prandial changes may occur in response to a single meal because they have a
potential role in the digestion process. Research on how dietary composition and, in
particular, plant ingredients modify intestine and liver histology over time and whether
fish can adapt to plant protein and oil feeding are very important for a sustainable fish
supply.
Keywords: Fish meal substitution, Intestinal histology, Rainbow trout, Vegetable
proteins.