Introduction
Atlantic salmon (Salmo salar) is an anadromous salmonid of high economical value globally. Since it is a nutritious food product, Atlantic salmon contributes significantly to economic and employment security in many countries worldwide. With aquaculture being one of the most rapidly expanding food production sectors, the global salmon production in 2019 reached approximately ~2.6 million tons. For salmons, entry into seawater is a crucial part of their life cycle and a critical step in production. A high pathogen pressure in the marine environment combined with a suppressed immune system of the fish results in a higher susceptibility to infections and diseases. Fish welfare is recognized to be essential for high product quality and a good health status. Stressed fish commonly have poor health and performance. Monitoring fish health status by measuring gene expression of biomarkers could be a useful tool to take measures for stress reduction.
Microalgae are considered promising functional feed ingredients, as they have beneficial effects on immune status. However, knowledge on the immune and stress effects of microalgae diets in Atlantic salmon remains limited. Previous research revealed the serious impact feed additives can have on gut health and systemic physiological parameters in salmonid fish. This study aims to investigate the influence of microalgae diets as probiotic feed and the transfer into marine farming systems on the expression of genes functioning as biomarkers for fish health status.
Material & methods
Atlantic salmon were randomly assigned to 18 experimental tanks (28 smolts per tank) in a recirculating aquaculture system (RAS) of Aquaculture Research Station Büsum, Germany. The tanks were filled with brackish water (∼ 12 psu). After 8 weeks, the fish were transferred from brackish water to seawater (∼ 32 psu). The fish were fed manually to apparent satiation twice daily. The diets were supplemented with different microalgae (8%). The microalgae used were Spirulina, Chlorella broken, Schizochytrium, Chlorella intact and Tetraselmis. In order to analyze the fish health status, RNA was isolated from five different organs (head kidney, spleen, gills, intestine, liver). Subsequently, the Biomark HD/Fluidigm test system was used for multigene expression analysis.
Results
In our study, we observed inter-individual differences in gene expression between fish fed with the same microalgae diets. In addition, following expectations, the different immune-related organs showed a different immune response. Overall, we recorded no major down- or upregulation of immune-related genes.
Discussion and conclusion
The present study demonstrates that microalgae do not have adverse influences on the immune status of Atlantic salmon and could serve as feed additive in the feeds of Atlantic salmon.