LAUSR

Abstract The salmon louse (Lepeophtheirus salmonis) is a significant problem in Atlantic salmon aquaculture. The industry is forced to use harsh antiparasitic treatments, including hydrogen peroxide (H2O2) baths, to treat the fish. We have utilized in vivo, semi in vivo, and in vitro methods to study the effects of exposures to H2O2. Assessment of external welfare indicators and plasma levels of cortisol, glucose, lactate, and ions did not reveal systemic impacts. Transcriptome analyses showed acute changes in the skin, with transient activation of several transcription factors known as generic stress markers of Atlantic salmon. Post-treatment (24 h), genes involved in the neutralization of free radicals were induced. Expression changes of regulators of cell cycle and cell differentiation were linked to microscopic observations in tissue sections and scale explants cultures. Histology showed adverse effects of H2O2 on skin morphology followed by compensatory recovery, which was not completed after 24 h. Decreased proliferation of cells was observed in keratocyte cultures. Exposure to H2O2, lowered pH (6.8), and elevated temperature (16 °C) markedly decreased the migration of keratocytes, which may indicate a reduced ability to repair skin and compound effects with H2O2. The study adds to the understanding of the adverse effects of hydrogen peroxide and promotes new models in Atlantic salmon skin research.