Photometrically Controlled Quantification of Opercular Melanin Spot Fading in Confined Atlantic Salmon

Atlantic salmon aquaculture faces persistent welfare challenges from handling, confinement, disease and delousing, yet established welfare indicators typically require capture or invasive sampling and signal problems only after they have developed. Melanin-based pigmentation in salmonids is associated with stress responsiveness, but quantitative methods for measuring stress-associated fading of individual spots are lacking, and longitudinal photometry in repeatedly imaged individuals is vulnerable to between-session drift in illumination and camera settings. We present a computer vision framework that quantifies opercular melanin spots while explicitly controlling for image acquisition. Opercular regions were detected with YOLOv8, registered by projective transformation, and spots segmented with point-prompted fine-tuned SAM 2 model. Images were linearized and normalized to a common sensor exposure using EXIF metadata prior to feature extraction, and all effects were further adjusted for residual per-image exposure differences. Twenty-one salmon were imaged before and after 18 h confinement (676 matched spot pairs); plasma cortisol confirmed the challenge (W=14, p < 0.001). Eight control fish were imaged once as a reference group. Confinement was associated with increased spot grayscale intensity, reduced spot-to-background contrast and reduced spot area, with closely matching estimates on both opercula. Control fish were indistinguishable from pre-stress fish in contrast (p=0.28) but differed markedly from post-stress fish (d=1.04, p=0.002). Since control fish were imaged at a single timepoint, elapsed time cannot be excluded as a contributor. The framework provides a photometrically controlled, non-invasive basis for quantifying pigmentation dynamics in salmon.