Improve salmon robustness in seawater: optimized seawater transfer time and adequate feeding strategies for swimming trained Atlantic salmon

The objective of this study is to develop a protocol to produce more robust smolts in the freshwater phase and to optimize the timing for seawater transfer. Smoltification is a critical event in the life of Atlantic salmon which allows the salmon to adapt to a life in the sea. However, in farms a mismatch between smoltification and seawater transfer frequently occurs, which might be explained by insufficient smoltification markers available. This mismatch can result in low fish performance in the first months at seawater and in some cases mortality, representing a major concern for farmers and consumers, since it compromises fish welfare and impairs the productivity of Norwegian salmon farms. Therefore, the identification of suitable biomarkers as well as improve salmon robustness is essential to increase salmon adaptation to the seawater life. To improve the salmon robustness, one strategy that has shown to have positive effects is the active use of swimming training. Thus, to produce more robust salmon we will use interval swimming exercise combined with a suitable feeding strategy to ensure an optimal nutritional status of the fish. Two trials, with a total of 834 Atlantic salmon (Trial 1: 480 and Trial 2: 354 fish) are planned in this study. All fish will be PIT tagged to keep track of the individual fish health and growth rate. To decrease the stress from the handling we will use light anaesthesia. For tissue and blood collection, fish will be euthanized with an overdose of NaCO3-buffered tricaine methanesulfonate. The fish will be subjected to a stress event after 1 month in seawater, cyclic hypoxia combined with increase temperature, to access their robustness. We plan to use the data collected in this study to develop an in silico model so that the three R's (replacement, reduction and refinement) can continue to be implemented. The facility and all personnel involved in the trial have the necessary authorization and training to carry out the experiment according to Norwegian and European legislation. Fish health and feed intake will be constantly monitored and if signs of abnormal behavior or infection are observed, the fish will be euthanized following the present ethical guidelines.