Red Skin Disease in Wild Atlantic Salmon – a Severity Field Guide
Red Skin Disease (RSD) is a term given to an emerging disease of wild Atlantic salmon characterised by haemorrhaging along the underside of the body, and occasionally the lower flanks. Milder symptoms of ventral haemorrhaging have been recorded in some populations of salmon for decades, but the severity and prevalence appear to have increased markedly in the last few years.
The most consistent characteristic of the condition appears to be distinct and pronounced ventral haemorrhaging in fresh run Atlantic salmon. This haemorrhaging is described as petechial or pinpoint, expressed as rounded reddened spots on the skin.
The cause and impact of RSD are currently unknown and detailed histopathological descriptions are still being established. Until then, the following field guide has been produced to standardise clinical reporting of RSD-like lesions to support ongoing monitoring of this condition across salmon rivers. It also serves to distinguish this emerging condition from other, frequently observed skin lesions in migratory salmonids.
This index is not intended as a definitive diagnostic guide, but simply to standardise reporting of RSD. This document may evolve as our understanding of RSD develops and will be updated accordingly alongside a more detailed case description.
The images in this guide show the ventral surface of salmon exhibiting varying severities of ventral haemorrhaging associated with RSD, along with descriptions to assist the assessment of individual fish.
To distinguish RSD-like lesions from other skin conditions, there is a page at the end of this guide showing a range of images of wild Atlantic salmon with common symptoms and infections - none of which are considered to be consistent with RSD. Whilst secondary infections may be a typical progression of RSD-like lesions, these growths often mask underlying characteristics and hinder a reliable assessment of this condition.
Severity and characteristics of RSD
Based on observations from rivers across the UK and Ireland, it appears that RSD-like lesions can vary widely both in the extent and severity of ventral reddening. As such, the categorisation of mild, moderate, and severe lesions is based on the coverage of ventral regions of affected fish and the severity of haemorrhage/inflammation. The contribution of these two factors to the assignment of RSD severity is demonstrated in the diagram below.
As an Alliance of six organisations, we will build on the existing work of our partners and maximise our impact by taking a coordinated approach and vital action in order to halt and reverse the decline of wild Atlantic salmon.
The goal of the Missing Salmon Alliance is to build an evidence-base to influence national and international decision-makers to regulate activities that adversely impact wild Atlantic salmon.
The Missing Salmon Alliance
The MSA is comprised of the following members:
Game & Wildlife Conservation Trust, Atlantic Salmon Trust, the Angling Trust with Fish Legal, The Rivers Trust and Fisheries Management Scotland.
https://www.missingsalmonalliance.org