The Hidden Cost of Mouth Rot: What New Research Means for B.C. Salmon Farming

A recent study published in Aquaculture Journal provides an important reminder that fish health challenges are more than biological problems, they are economic ones as well.

The study, Economic Modelling of Mouth Rot in British Columbia Atlantic Salmon Farming, examined the economic impact of bacterial stomatitis, commonly known as mouth rot or yellow mouth, in farmed Atlantic salmon. The disease is associated primarily with Tenacibaculum maritimum and is most commonly observed during the early months following seawater entry.

The results are significant: the researchers estimated a USD $10.58 million annual burden across the B.C. industry. 

The model incorporated losses associated with mortality and antibiotic treatment, while accounting for the feed costs that are avoided when fish are lost. But other potential impacts, like reduced growth, additional labour, management disruption and longer-term effects on production performance may not be captured in that figure. And this distinction matters because mouth rot can affect feeding and growth as well as survival, meaning the economic harm could be far larger.

Mouth rot is not a new issue for B.C. salmon producers. It has been documented in the province for decades and remains one of the more important bacterial health challenges facing farmed Atlantic salmon.

The disease is particularly challenging because its occurrence appears to be influenced by multiple factors rather than simply the presence of the bacterium. Research has associated outbreaks with factors including fish condition, life stage and environmental conditions such as temperature and salinity. 

B.C. isolates also represent multiple Tenacibaculum lineages, highlighting the complexity of the disease.This means that simply identifying T. maritimum is not necessarily enough. The industry needs a better understanding of when the pathogen becomes pathogenic, which strains pose the greatest risk, and what factors make fish more susceptible to disease.

There are several potential avenues for reducing the impact of mouth rot. No single intervention is likely to solve the problem, but a combination of approaches could substantially reduce reliance on antibiotics and the economic losses associated with outbreaks.

1. Earlier and more targeted diagnostics

Routine molecular surveillance, including qPCR, can help identify the presence and abundance of Tenacibaculum species. However, the next step is moving beyond simple pathogen detection toward pathogen characterization and risk prediction.

2. Vaccine development

A commercially available vaccine for Atlantic salmon remains one of the biggest gaps in the current mouth rot toolbox.

However, research is progressing. Experimental challenge models have been developed specifically to reproduce Tenacibaculum infections in Atlantic salmon at Onda, providing an important platform for evaluating vaccines and other interventions.

3. Functional feeds and nutritional support

Functional feed ingredients, including immunostimulants, probiotics, prebiotics, marine-derived bioactives and other ingredients with antimicrobial or immune-supporting properties, could potentially help improve the ability of fish to withstand infection.

The opportunity is to develop evidence-based nutritional strategies that complement existing disease-management programs, particularly during high-risk periods such as the transition to seawater.

4. Better understanding of environmental and management triggers

The presence of a pathogen does not necessarily mean disease will occur. Research indicates that environmental conditions, fish condition and other stressors can influence the severity of tenacibaculosis.

The goal would be to identify high-risk windows before an outbreak develops, allowing management interventions to occur earlier.

B.C. already has many of the pieces needed to address the challenge: established fish-health monitoring, diagnostic infrastructure, research expertise and a long history of studying Tenacibaculum. The Province requires aquaculture operators to maintain fish-health management plans and report key health and production parameters, while diagnostic testing is available through government and third-party laboratories.

The next opportunity is to connect those capabilities more closely with predictive diagnostics, controlled challenge studies, vaccine development and functional nutrition research.

For an industry operating under increasing pressure to improve fish welfare, reduce antimicrobial use and maintain production efficiency, mouth rot is a good example of why the next generation of fish-health solutions will need to focus not simply on treating disease but on predicting, preventing and mitigating it before it becomes an economic event.

As the B.C. aquaculture sector continues to evolve, investing in the science behind prevention could ultimately be far less costly than continuing to manage the consequences of disease after it occurs.

For producers, researchers and technology developers alike, the question is increasingly shifting from “How do we treat mouth rot?” to “How do we stop it from becoming an outbreak in the first place?”