Histological Methods to Assess the Effect of Diet and a Single Meal on the Liver and Intestine of Rainbow Trout: Fishmeal and Fishoil Replacement With Plant Protein and Oil

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.