Over the last decades, Norwegian salmon feed industry has been replacing major portion of fishmeal and fish oil in fish feeds with plant ingredients, like protein concentrates, usually with reduced content of antinutritional factors (ANFs). Nevertheless, amino acid imbalance and presence of ANFs are still matters of concern for the industry. Furthermore, plant lipids have an unfavorable ratio of n-3 : n-6 and does not provide the recommended EPA and DHA to the fish. Hence, ingredient composition of feed may have an effect on the health of the fish. Dietary administration of probiotics can influence the mucosal immune system of the fish. Gills are the main organs for respiration but they play also an important role as first line defense of the fish. Therefore, the microscopic structure of the gills and specifically the number and size of mucous cells, is considered a health indicator of fish, which may be affected by several extrinsic factors, including feed ingredients.
A feeding trial was conducted with Atlantic salmon (mean weight of 146.97 ± 4.9g SD). The fish were fed three types of feeds with different basal diets with or without probiotics. The ingredient composition of the diets were, Diet 1: fish meal/ fish oil based, Diet 2: a commercial-like diet dominated by plant ingredients (plant : marine is 70:30), and Diet 3: a fish meal/ fish oil based diet in which soybean meal replaced 20% of the fish meal. Dietary probiotic was cultured in the laboratory and vacuum coated on the diets. Gills samples were collected from 12 fish per treatment and fixed in 4% formalin. After paraffin embedding, tissue sections of 4 μm were prepared and stained with Hematoxylin and Eosin (H&E) and Alcian Blue – Periodic Acid Schiff (AB-PAS). Microphotographs (n=50 secondary lamellae/ fish) from five different filaments per fish were acquired using a camera attached to a light microscope. Quantitative analysis of gills morphology was performed with Image J (1.52a); total mucous cell area per epithelium area (ME), and number of mucous cells per epithelium area (NE) were determined.
The results showed significant differences in ME and NE. Both parameters were significantly influenced by diets and probiotics. Fish fed Diet 2 and 3 had significantly more ME and NE compared to those fed Diet 1. Addition of probiotics to all the diets increased the ME and NE. This study revealed that the ingredient composition of diets directly influences the gill mucous cell area and number of gill mucous cells, while dietary administration of probiotics increased both the number of mucous cells per epithelium area and the total area of mucous cells per epithelium area.