Salmon under stress: Strengthening the first line of defense

In salmon aquaculture, health starts at the surface. Gill, skin and gut form a dynamic mucosal barrier—an interface constantly challenged by the surrounding environment. When these barriers fail, the costs mount quickly. Shifts in water quality, handling stress and exposure to potential pathogens all put pressure on these delicate tissues, often leading to wounds and inflammation, poor performance and increased risk of disease outbreaks.

As the industry advances in developing strategies to safeguard animal health, functional nutrition has become a key driver of resilience. Among the solutions most commonly applied and under continuous scientific development are yeast cell wall (YCW)-based ingredients that are documented to help fish maintain stronger mucosal barriers and remain in good health.

Two independent research trials by Lallemand Animal Nutrition recently explored how a novel YCW product influenced salmon facing two very different challenges: Amoebic Gill Disease (AGD) and tenacibaculosis.

Tackling Amoebic Gill Disease (AGD)

Amoebic Gill Disease, caused by Neoparamoeba perurans, is one of the most persistent threats to salmon health worldwide. The parasite colonizes gill tissue, causing lesions that impair oxygen uptake and compromise overall fish performance. Current interventions—freshwater or hydrogen peroxide bathing—are operationally intensive, biologically stressful and present practical challenges for producers.

To test whether nutritional intervention could mitigate AGD severity, researchers in Scotland conducted a 10-week trial with post-smolt Atlantic salmon (200-250g) at 13°C and 33 ppt salinity. Fish received either control or YCW-supplemented diets in quadruplicate tanks. After five weeks baseline under optimal conditions, salmon were exposed to standardized N. perurans challenge, followed by freshwater treatment, then a second pathogen challenge—a protocol designed to mirror real-world production cycles and stress.

The study employed gill scoring adapted from Gjerde et al. (2019), evaluating each gill side for a more complete representation of lesion development and disease severity.¹

The results were encouraging. Salmon receiving the functional diet grew faster and converted feed more efficiently—showing a 20% improvement in specific growth rate (Figure 1A) and a 32% better feed conversion ratio (Figure 1B). Even more striking was the difference in gill health: the YCW-fed group showed far fewer advanced lesions, with up to six times more fish retaining only mild signs of infection (Figure 1C).

The findings suggest that nutritional support targeting mucosal function could help improve resilience of fish vis-à-vis external stressor and reduce the severity of AGD. This helps fish maintain normal performance even under challenge.

Figure 1. A) Specific growth rate (SGR), B) Feed conversion ratio (FCR) and C) Proportion of gill score lesion’s category at the end of the trial

Tenacibaculosis challenge in Canada

A second study in Canada focused on Tenacibaculum maritimum, a filamentous bacterium known to cause severe skin and fin lesions in marine species. This condition, tenacibaculosis, represents a significant health challenge in aquaculture and requires proactive strategies to reduce its impact on fish welfare and production.

A parallel 15-week trial in Canada examined whether similar mucosal health benefits could be observed against bacterial pathogens. Juvenile Atlantic salmon (initial body weight 40g) at 12°C received control or YCW-supplemented diets in quadruplicate, fed to visual appetite over the trial’s duration. Fish were reared in ideal conditions for five weeks then subjected to a standardized T. maritimum challenge by immersion.

The trial continued for nine weeks post-challenge, during which survival and the development of gross pathological lesions on external mucosal surfaces were monitored at frequent intervals.

Here again, diet made a difference. Fish fed the YCW supplemented diet had a 71% survival rate, compared to 58% in controls (Figure 2A). The prevalence of moderate to severe skin lesions dropped by around 30%, suggesting the ingredient helped fish maintain stronger mucosal integrity and resist secondary damage (Figure 2B).

Figure 2. A) Survival probability and B) Skin score lesions at the end of the second trial (15 weeks)

Building resilience from within

Together, these studies highlight the potential of yeast based functional nutrition to support animal health by maintaining mucosal integrity in seawater salmon.

Mucosal health sits at the crossroads of nutrition, immunity and welfare. Supporting these natural barriers through targeted dietary strategies could not only reduce disease losses but also minimize dependence on chemical interventions and associated side effects. While more work is needed to unravel the precise biological mechanisms— how YCW compounds interact with the gut and skin immune system and overall barrier function—the findings are clear: feeding the mucosa is a definite contributor to fish health and welfare (Figure 3).

Figure 3. Salmons’ first line of defense: How nutrition strengthens mucosal health

These trials document the benefits of a novel yeast-based functional ingredient at mitigating the severity and associated mortalities, of distinct mucosal diseases in seawater Atlantic salmon. This was associated with stronger skin and gill barriers under optimal conditions (data not shown), and notably, improvements in growth and feed efficiency were also observed.

These promising results indicate that this functional ingredient can enhance fish resilience against mucosal diseases in the Atlantic salmon industry.

References

¹Gjerde, B., S. A. Boison, M. L. Aslam, M. Løvoll, H. Bakke, S. Rey, and M. Lillehammer. “Estimates of Genetic Correlations between Susceptibility of Atlantic Salmon to Amoebic Gill Disease in a Bath Challenge Test and a Field Test.” Aquaculture 511 (2019): 734265. https://doi.org/10.1016/j. aquaculture.2019.734265.