The effect of Atlantic salmon smolts quality, robustness and welfare on resistance towards a challenge with Piscirickettsia salmonis

Mortalities due to infectious diseases is a long-term issue causing important economic losses to the Atlantic salmon industry worldwide. Producing better quality and more robust smolts before transfer to seawater cages may be an important measure to reduce mortalities and improve welfare. Exercise training fish while in fresh water tanks has proven to improve not just the cardiorespiratory capacity of fish, but also their growth potential and to stimulate the immune system thus increasing the survival rate when faced to an infection. In this study, we evaluated how subjecting the fish to exercising conditions by swimming in the tanks at water speeds considered optimal could improve their robustness and welfare, as well as their resistance to a P. salmonis controlled challenge. Compared to the control (untrained group), trained fish showed a better growth and better fin quality among other production-relevant parameters. Transcriptomic analyses indicated that training promoted an over-expression of genes involved in the cellular arm of the immune system. Furthermore, there was a massive effect of training on the cardiac muscle transcriptome, including genes that could be grouped into functional mechanisms such as metabolism, cellular and immune processes. Exercise training also promoted healthier liver and gills as shown by histopathological observations. Finally, the exercise regime did not improve resistance on the disease challenge with P. salmonis. Two groups of Atlantic salmon pre-smolts were subjected to different swimming speeds: 0.4 or 1.0 body-lengths per second for eleven weeks, after which fish from both groups were marked and set on a common tank for the disease challenge with P. salmonis infected shedders (cohabitation model). The challenge lasted for 60 days. During the training and challenge phases, samples were collected for transcriptomic and histological analyses.