New case study: tackling complex gill disease in salmon farming
Identifying a standardised and evidence-based definition of complex gill disease
Gill disorders in salmon remain a challenge for fish farmers. Across marine farms, inflamed and damaged gills have been observed which can lead to respiratory issues and reduced growth. These problems arise from a mix of factors, e.g., bacteria or viruses, combined with environmental stresses like harmful plankton, poor water quality, or sudden temperature changes.
When no single cause can be pinpointed, the condition is called Complex Gill Disease (CGD). By 2017, the term was widely used but poorly defined, making diagnosis and management inconsistent. This project set out to clarify what CGD means and improve how it is monitored and managed. There were two main goals:
- Understand CGD patterns and risk factors.
- Examine how the gill microbiome relates to health.
Valued at £2.4m, the partners included SRUC, University of Glasgow, Loch Duart, Grieg Seafood Shetland (now Scottish Sea Farms), Bakkafrost Scotland, Cooke Aquaculture Scotland, Mowi Scotland, Wester Ross Fisheries (now Mowi Scotland), Nevis Marine, Moredun Research Institute, Salmon Scotland.
Experts agreed CGD is a gill disorder showing general signs of damage, e.g., thickened tissue, fused gill structures, inflammation, or bleeding, without an obvious single cause. Seven types of gill problems were identified, including those linked to parasites, viruses, bacteria, harmful plankton, and toxins.
Researchers ran a study across eight salmon farms, collecting detailed data on gill health, water conditions, and potential pathogens. They introduced practical diagnostic guidelines and confirmed which tests work best. PCR testing was highly accurate for detecting pathogens, while microscopic tissue checks were best for confirming disease.
A microbiome study revealed that the mix of bacteria on salmon gills changes over time and can be influenced by hatchery systems and even the changing seasons. While no single bacterial pattern predicted disease, fish with CGD had less diverse microbiomes.
The project delivered the first clear definition of CGD, practical diagnostic tools, and new knowledge about gill health. Farmers can now monitor gill disease more effectively, interpret test results with confidence, and manage environmental risks better, supporting healthier fish and more sustainable salmon farming.
The full title of this project is ‘Examining complex gill disease in Scottish salmon’.
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