Divergent Healing Trajectories Among Common Mechanical Skin Wound Types in Post-Smolt Atlantic Salmon (Salmo salar Linnaeus, 1758).

Mechanical wounds are a common form of skin damage in Atlantic salmon (Salmo salar Linnaeus, 1758) aquaculture and can predispose fish to secondary infections. Such wounds often arise from routine farming practices, including delousing operations. Although visually similar, different types of mechanical injury may trigger distinct healing processes, which remain poorly understood. To investigate this, post-smolt Atlantic salmon were subjected to three wound models: scale loss, superficial wounds involving the epidermis and dermis, and deep wounds extending through all skin layers. Responses were assessed at 2 days and 5 weeks post-wounding to capture different phases of skin wound healing. The injuries were characterised using serum parameters, stereomicroscopy, histology, immunohistochemistry for cell proliferation, quantitative polymerase chain reaction (qPCR), and RNAscope analysis of T cell markers. Despite their initially similar appearance, scale loss and superficial lesions followed markedly different healing trajectories. Fish with scale removal mostly recovered to the pre-damage state within 5 weeks, whereas superficial damage elicited a delayed healing response similar to that of deep wounds. Each wound type exhibited distinct epidermal and hypodermal alterations, highlighting differences among wound types that should be considered by fish health personnel and farmers.