Evaluation of large‐scale marking methods in farmed salmonids for tracing purposes: Impact on fish welfare

Escaped farmed Atlantic salmon (Salmo salar) is considered a threat to wild salmon. In order to take action to reduce the impact of escapees, methods to distinguish escapees from wild fish and to trace them back to their origin are in demand. This paper gives an overview of available methods and discusses the impact on fish welfare, both in the short‐ and long‐term. Adipose fin clipping, freeze branding, different external and internal tags, as well as natural and chemical marks are considered. All marking procedures that involve handling of fish have an impact on fish welfare. Spraying with pigments and most externally attached tags significantly reduce the welfare, both on short‐term and long‐term perspectives. Although the use of natural or chemical marks, like exposure to stable isotopes via egg immersion or vaccination, involves no or no extra handling, subsequent analysis may require killing the fish after catch. Large‐scale marking with implanted or external tags could represent higher risks of human errors and reduced fish welfare, as opposed to small‐scale marking. In general, the knowledge about effects of marking on fish welfare is limited for most of the available methods, in particular regarding other effects than mortality and growth reductions.