Dietary effects on growth, skin morphology and stress marker genes of Atlantic salmon (Salmo salar L.) smolts

Abstract

The transfer of smolts from freshwater to saltwater is a challenging period during the commercial production of Atlantic salmon. The fish undergoes profound behavioural, developmental, and physiological changes. Additionally, the first protective barrier against environmental disturbances and osmotic stress is the salmon skin. This thesis aimed to examine how diets affect Atlantic salmon smolts' skin morphology and stress marker genes. Also, the dietary effect on growth and the response to seawater transition was examined. To understand how freshwater diet could affect the smolts, the fish were fed either a marine or plant-based diet from start-feeding (30g) until seawater transfer. Thereafter, both dietary groups were mixed and fed a commercial diet for 16 days after saltwater transfer. The data collection included examination of the skin, which involved photographing the whole fish, and skin sampling for histology analyses and gene expression. Also, the fish was weighted, and length was measured. Fish were sampled just before and 16 days after saltwater transfer. The fish fed a marine diet in freshwater had a higher body weight compared to the fish fed the plant diet in both fresh- (112 vs. 81g) and saltwater (118 vs. 89g). Furthermore, the dietary effect on skin morphology showed that the fish fed a plant-based diet in freshwater had a larger epidermis area and thicker epidermis in saltwater than those fed a marine diet in freshwater. The epidermis was limited influenced by expression levels of stress marker genes. However, the response to saltwater transition showed a reduced number of mucus cells, apically (relative to internal) mucus cells and condition factor. Scale loss was not significantly affected by saltwater transition or dietary composition.