The color change and stress response of Atlantic salmon (Salmo salar L.) infected with Aeromonas salmonicida

•We investigated the impact of pathogen on fish from the color change response.•The contrast of fish challenged by pathogen were quantified.•The physiological changes caused by pathogen were studied synchronously.•The physiological mechanism underline the color change behavior was discussed. Fish challenged with pathogens often show behavioral disorders. The present study investigated the influence of Aeromonas salmonicida infection on the body color changes of Atlantic salmon (Salmo salar L.). In both conditions of a white background and transfer from the black background to the white background, the body color contrast of the fish infected with A. salmonicida (100 μL bacterium solution 8.2 × 109 CFU ml−1 in the dorsal muscle) declined significantly following an initial increase compared with the control group (100 μL 0.9 % NaCl); the greatest differences were observed on day 11, at 57.6 % and 52.7 % respectively (P