Parr–smolt transformation and dietary vegetable lipids affect intestinal nutrient uptake, barrier function and plasma cortisol levels in Atlantic salmon

For Atlantic salmon, the gastrointestinal tract is the site of food digestion and nutrient uptake, a regulatory site for ion and water balance as well as a barrier against invading pathogens. During the parr–smolt transformation and subsequent seawater (SW) transfer, major changes occur in the intestine. A global shortage of fish oils (FO) for feed production is estimated to appear within a few years, and vegetable oils (VO) are being considered as alternatives for FO in fish feed production. However, VO influences the fatty acid composition of the polar lipids of cell membranes in the intestine which can disturb intestinal functions. A VO-based diet during the parr–smolt transformation, which is a sensitive developmental period, may cause adverse effects. Therefore, Atlantic salmon parr were fed either sunflower oil (SO) or FO as the major lipid source during out-of-season light controlled parr–smolt transformation. At three time points gill Na+,K+-ATPase activity and plasma levels of cortisol and growth hormone were assessed. Intestinal epithelia were sampled for assessment of nutrient absorption and bacterial translocation using an Ussing chamber in vitro system. While both dietary groups showed plasma hormone profiles indicative of successful parr–smolt transformation, the SO-fed fish had consistently increased cortisol levels compared to the FO-fed fish. Translocation of pathogenic bacteria increased, probably due to disturbed barrier functions, during the parr–smolt transformation. However, the fish fed the SO-diet maintained a higher barrier function compared to FO-fed fish, an effect that may be beneficial to these fish. Nutrient uptake was less affected by smoltification. Fish fed the SO-diet had higher uptake rates of amino acids and free fatty acids during mid-smoltification than fish fed the FO-diet. The combined effects of barrier function and nutrient uptake may suggest a positive effect of including vegetable lipids in the diet during the parr–smolt transformation. However, the vegetable lipid diet also seemed to act as a stressor and elevated the basal cortisol levels, which may be of concern in the context of general fish health and welfare.