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.

Background: Hyperthermia has been shown in a number of organisms to induce developmental defects as a result of changes in cell proliferation, differentiation and gene expression. In spite of this, salmon aquaculture commonly uses high water temperature to speed up developmental rate in intensive production systems, resulting in an increased frequency of skeletal deformities. In order to study the molecular pathology of vertebral deformities, Atlantic salmon was subjected to hyperthermic conditions from fertilization until after the juvenile stage. Results: Fish exposed to the high temperature regime showed a markedly higher growth rate and a significant higher percentage of deformities in the spinal column than fish reared at low temperatures. By analyzing phenotypically normal spinal columns from the two temperature regimes, we found that the increased risk of developing vertebral deformities was linked to an altered gene transcription. In particular, down-regulation of extracellular matrix (ECM) genes such as col1a1, osteocalcin, osteonectin and decorin, indicated that maturation and mineralization of osteoblasts were restrained. Moreover, histological staining and in situ hybridization visualized areas with distorted chondrocytes and an increased population of hypertrophic cells. These findings were further confirmed by an up-regulation of mef2c and col10a, genes involved in chondrocyte hypertrophy. Conclusion: The presented data strongly indicates that temperature induced fast growth is severely affecting gene transcription in osteoblasts and chondrocytes; hence change in the vertebral tissue structure and composition. A disrupted bone and cartilage production was detected, which most likely is involved in the higher rate of deformities developed in the high intensive group. Our results are of basic interest for bone metabolism and contribute to the understanding of the mechanisms involved in development of temperature induced vertebral pathology. The findings may further conduce to future molecular tools for assessing fish welfare in practical farming.

In the recent years, challenges faced in salmonid farming with regard to deformities have helped to improve our knowledge on skeletal biology and development of bone health in salmonids. Different nutritional, genetic and environmental factors are associated with skeletal deformities in salmonids and other fish species. Minerals are a group of essential nutrients having a vital role to play in skeletal development, growth and remodelling. The knowledge generated thus far on the structural and functional importance of minerals in salmonid bone health is largely restricted to phosphorus. A brief account of dietary phosphorus‐related bone deformities encountered in salmonids, critical life stages for the development of bone deformities, recent developments in the understanding of their aetiology and dietary phosphorus levels required to improve bone health are presented. The effect of increased dietary level of phosphorus in reducing bone health disorders under conditions of improved feed conversion efficiency and use of triploids in salmon farming are illustrated with help of metadata analysis from literature. With regard to the role of other essential minerals (macro‐ and micro‐) impacting bone health in salmonids, data available are extremely limited and hence information on other fish species and mammals is summarized in relation to bone development and incidence of deformities. The need to improve mineral bioavailability, utilization and reduce effluent mineral load is presented in brief with example from Norwegian salmon farming. Refinements in mineral requirement recommendations for salmonids and advanced methodologies for studying aetiology of skeletal anomalies, bone mineral status, skeletal development and deformities are also discussed.

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.

In this study, we look closer at how high fish densities influence wound repair mechanisms in post-smolt Atlantic salmon. The fish were wounded with a 5 mm skin punch biopsy needle and stocked at two different densities, a high fish density (100 kg/m³) treatment and a low fish density treatment (20 kg/m³) serving as the control. The healing wounds were followed for 57 days with samples taken 1, 3, 7, 14, 36, 43 and 57 days post wounding. The transcriptomic analysis suggests that high fish density enhance inflammation and represses cell proliferation, tissue secretion and collagen synthesis in the healing wounds. The histological analysis further showed delayed epidermal and dermal repair in the high fish density treatment compared to control. The overall wound contraction was also altered by the treatment. In conclusion, high fish density enhances immune responses and delay tissue repair, which ultimately results in delayed wound healing.