Rapid temperature-dependent wound closure following adipose fin clipping of Atlantic salmon Salmo salar L. M Andrews 1 , M Stormoen 1 , H Schmidt-Posthaus 2 , T Wahli 2 and P J Midtlyng 1 1 Centre of Epidemiology and Biostatistics, Faculty for Veterinary Medicine and Biosciences, Norwegian University of Life Sciences, Oslo, Norway 2 Centre for Fish and Wildlife Health, Institute of Animal Pathology, University of Bern, Bern, Switzerland Abstract Three groups of Atlantic salmon were kept at a constant temperature of 4, 10 and 14 °C. The adipose fins were removed; six fish/group were sampled at 11 subsequent time points post-clip- ping. Samples were prepared for histopathological examination to study the course of re-epitheliza- tion. A score sheet was developed to assess the regeneration of epidermal and dermal cell types. Wounds were covered by a thin epidermal layer between 4 and 6h post-clipping at 10 and 14 °C. In contrast, wound closure was completed between 6 and 12 h in fish held at a constant temperature of 4 °C. By 18 h post-clipping, superficial cells, cuboidal cells, prismatic basal cells and mucous cells were discernible in all tempera- ture groups, rapidly progressing towards normal epidermal structure and thickness. Within the observation period, only minor regeneration was found in the dermal layers. A positive correlation between water temperature and healing rates was established for the epidermis. The rapid wound closure rate, epidermal normalization and the absence of inflammatory reaction signs suggest that adipose fin clipping under anaesthesia consti- tutes a minimally invasive method that may be used to mark large numbers of salmon presmolts without compromising fish welfare. Keywords: animal welfare, epidermis, healing, integ- ument, marking, skin. Introduction Atlantic salmon, Salmo salar L., aquaculture has been growing steadily over the past few decades par- ticularly in Norway, Scotland, the Faroe Islands, Canada and Chile. Due to the floating net pen-rear- ing technology, escapee-ism has become a relatively common occurrence (Zhang et al. 2013). This has triggered concerns especially among salmon anglers and stakeholders of wild salmon populations of the potential negative effects that farm escapees may have on wild stocks (Jonsson & Jonsson 2006; Ska- ala, Wennevik & Glover 2006). To characterize the perceived hazard, the term ‘genetic pollution’ has been used 1 , inferring that contributions to the reproduction of river stocks from farmed escapees are negative because they may reduce the genetic var- iability of the wild salmon populations (Clifford, Mcginnity & Ferguson 1998; Hindar et al. 2006). Besides general measures to reduce the incidence of farm escapes, Norwegian stakeholders requested visi- ble marking of farmed fish to allow for removal via angling and also during management operations to cultivate and enhance natural river stocks (Skaala et al. 2014). This situation prompted the Norwegian Seafood Industry Association (FHL) to call for studies to identify the most effective methods to distinguish between wild and cultured Atlantic salmon 2 . Genetic testing may be used; however, this tech- nique is not suitable for discrimination in the field Correspondence M Andrews, Centre of Epidemiology and Bio- statistics, Faculty for Veterinary Medicine and Biosciences, Norwegian University of Life Sciences, Postboks 8146 Dep, Oslo N-0033, Norway (e-mail: melanie.andrews@nmbu.no) 1 http://en.wikipedia.org/wiki/Genetic_pollution (accessed Feb- ruary 22, 2014). 2 http://fhl.no/dette-er-tiltakene-mot-romming/ (accessed March 09, 2014). 1 Ó 2014 John Wiley & Sons Ltd Journal of Fish Diseases 2014 doi:10.1111/jfd.12261