Skin wounds and injuries are frequently occurring in farmed fish, particularly in more intensive production systems. Any disruptions of the skin, such as mucus removal, scale loss or deeper incisions, are negatively correlated with barrier functions and disease resistance. In this review, the current knowledge on mechanically induced wounds in fish is dedicated to five areas of focus: (i) The anatomy and functions of fish skin, (ii) Techniques to inflict mechanical skin damage in farmed and laboratory fish for experimental purposes, (iii) Healing mechanisms of deep wounds, (iv) In vitro models for wound healing studies and (v) Wound care, with focus on factors that may enhance or delay skin regenerative processes. The aim of the review was to presents key points for a better understanding of skin resilience and fish robustness, with focus on Atlantic salmon, Salmo salar, in Norwegian coastal production environment.

Skin biopsies (5 mm) taken from behind the dorsal fin on Atlantic salmon post-smolts were followed over a 2 month period. The healing process was dominated by hemostasis, acute inflammation, and epidermal repair the first 14 days post wounding (dpw), as shown through imaging, histological evaluation, and transcriptomics. Most of the immune genes showed decreased expression after two weeks, approaching the levels of intact skin, as also reflected in sections where reduced inflammation in the wound bed was observed. Transcriptional events suggest recruitment of lymphocytes to the wound site during the acute phase, with activation of humoral responses from 14 dpw and onward. From the histology, a more adherent mucus was observed that correlated with altered transcription of glycosyltransferases. This may indicate different properties and functions of the mucus during the wound healing process. Wound contraction started between 14 and 36 dpw. The occurrence of these events was concurrent with granulation tissue formation, melanocyte migration and up-regulation of genes involved in extracellular matrix formation. The presented description of the wound healing processes in Atlantic salmon gives insight into comparative ulcerative biology in mammals and fish and provides both novel and updated knowledge that can be applied for improved best operational practices for fish welfare in aquaculture.