Mucosal health for fish welfare begins at the surface level. New research from Scotland and Canada demonstrates how gill and skin health serve as reliable indicators of overall fish vitality and adaptive capacity. Two breakthrough trials reveal that maintaining mucosal tissue integrity through targeted nutrition offers a welfare foundation approach, shifting aquaculture from reactive disease treatment to proactive health maintenance.
R&D Project Leader in Aquaculture
Lallemand Animal Nutrition
In fish, mucosal surfaces are the body’s first line of defense against environmental challenges such as sudden variations in water quality and temperature, and exposure to pathogens and non-infectious harmful agents. These barriers are critical to keeping fish healthy and productive. Intensive farming often subjects fish to repeated stressors that compromise gut, skin and gill barriers.
There are three major categories of insults that can compromise health and performance: Infectious and non-infectious agents such as bacteria, parasites, viruses, toxic algae, and plankton; environmental threats like water quality, heat stress, and rapid temperature changes; and management practices such as transfers, handling, treatments, and net-cleaning operations.
Recent research from Lallemand Animal Nutrition has focused on understanding how maintaining mucosal tissue integrity can support salmon health when faced with common aquaculture pathogens. Two independent trials, conducted in Scotland and Canada, examined the relationship between mucosal surface health and fish performance, salmon in particular, when exposed to different pathogens. These studies provide insights into how nutritional interventions targeting mucosal surfaces may serve as a reliable strategy to support overall fish welfare and adaptive capacity.
SUPPORTING GILL HEALTH IN AMOEBA-PRONE ENVIRONMENTS
Amoeba gill disease (AGD) is caused by the marine ectoparasite Neoparamoeba perurans, which colonizes and induces gill damage, resulting in significant losses in the Atlantic salmon industry across major producing regions. Current solutions include freshwater or hydrogen peroxide bathing, which can be operationally and biologically challenging.
A 10-week trial was performed in Scotland to test the potential contribution of a novel yeast-based functional ingredient for mitigating the incidence of AGD in Atlantic salmon under controlled conditions. Yeast cell wall (YCW) tested in the mentioned trials is a combination of specifically selected inactive yeasts containing Saccharomyces cerevisiae and Cyberlindnera jadinii strains and their derivatives. This combination helps promote a synergistic effect on the immune response through multi-receptor activation and resolution of inflammation without the risk of immune fatigue. It also helps maintain skin health by increasing skin mucus excretion, supporting antimicrobial activity, and tissue regeneration. This approach represents a shift from reactive treatment to proactive welfare maintenance through mucosal health support.
Selected salmon had an initial body weight of 200-250g and were maintained at 13±1°C in 33 parts per thousand (ppt) salinity conditions. Fish were fed a post-smolt diet mimicking commercial recipes that were formulated and tested as control or supplemented with the yeast supplement in quadruplicate. Salmon were reared for five weeks under ideal conditions then exposed to a first standard AGD-challenge followed, two weeks later, by a freshwater bath and a second standard AGD-challenge.
The trial employed a gill scoring system to evaluate lesion development and severity. Results suggested that nutritional intervention significantly influenced both growth performance and gill tissue response to pathogen challenge.
Fish receiving the functional diet showed 20% improvement in specific growth rate and 32% enhancement in feed conversion ratio (FCR) compared to controls (p<0.05 for both parameters; Figure 1A and B). Over the trial’s duration, the gill score progressively increased from light to moderate and advanced lesions. At the end of the trial, the proportion of advanced and heavy lesions was significantly reduced in the supplemented group (two-fold and six-fold respectively, p<0.01). Conversely, the proportion of fish showing only light lesions was significantly higher in the YCW-supplemented group (Figure 1C).
A) Specific growth rate (SGR) improvement, B) Feed conversion ratio (FCR) enhancement, and
C) Distribution of gill lesion severity categories at trial conclusion
Statistical significance: * p<0.05, ** p<0.01
BACTERIAL PATHOGEN RESPONSE
Building on the AGD findings, a second trial examined whether similar mucosal health benefits could be observed against bacterial pathogens. Tenacibaculum maritimum (T. maritimum) is a gram-negative filamentous bacterium ubiquitous in aquatic environments, which causes high mortality in various marine aquaculture species. Tenacibaculosis is associated with ulcerative skin, tissue necrosis, mouth erosion, tail and fin rots, and necrosis of the gills and eyes. To date, no consistently effective methods are available due to the multiplicity of strains and antibiotics used. Other preventive strategies aiming at maintaining fish healthy could help prevent expansion of such bacteria challenge.
A 15-week Canadian trial tested the same yeast-based diet used in the Scottish study. A standard commercial diet for Atlantic salmon with an initial body weight of 40 g, maintained at 12±1 °C, was either supplemented with YCW or not as the control. Diets were tested in quadruplicate and fed to visual satiation over the trial’s duration.
Fish were reared in ideal conditions for five weeks during which smoltification occurs. At the end of this period, fish were confronted with a T. maritimum immersion challenge for over five hours. The trial evaluated the diet’s effect on survival and reduction of lesions on some external mucosal surfaces such as skin, mouth, and gill. Mortality was recorded daily, and skin, mouth, and gill lesions were scored before and at four time points after the bacterial challenge.
Results suggested improved survival probability from 58.5% to 71.2% in fish receiving the experimental diet (p<0.05; Figure 2A).
Scoring of mouth, gill, and skin disease allows researchers to evaluate the disease propagation and severity. In this trial, gill and mouth were not the most representative mucosal surface of the bacterial infection, representing a study limitation. Nevertheless, notable results were observed on skin ulcers. 63 days after the challenge, fish fed YCW-supplemented diets showed a 30% decrease in moderate and severe skin lesions, along with a 16% increase in light lesions (p<0.05; Figure 2B).
A) Survival probability over 15-week trial period and B) Skin lesion severity distribution at 63 days post-challenge
Statistical significance: * p<0.05
TISSUE INTEGRITY AS WELFARE FOUNDATION
Both trials support the hypothesis that YCW supplementation may serve as an effective functional feed ingredient to not only improve performance but also help prevent the propagation of various undesirable microorganisms including amoeba and bacteria. In the first trial, fish demonstrated a better tolerance to infection which was associated with improved growth performance. In the second trial, reduced skin lesion severity corresponded with higher survival rates.
These findings contribute to growing understanding of mucosal tissue health as a fundamental component of fish welfare. The consistent pattern of improved tissue integrity across different pathogen challenges suggests that mucosal surfaces serve as reliable indicators of overall physiological status and adaptive capacity. This welfare foundation approach represents a paradigm shift from treating disease symptoms to maintaining the biological barriers that support fish health.
The research suggests that nutritional strategies targeting mucosal health for fish welfare can influence how they respond to routine aquaculture challenges. Rather than simply treating disease symptoms, nutritional approaches that support tissue integrity may provide more sustainable solutions for maintaining fish welfare in commercial production systems. The observed improvements in growth rates, feed efficiency, and survival outcomes indicate that investing in nutritional solutions that benefit on mucosal health may provide returns for commercial operations while supporting improved fish welfare standards.
About Dr. Camille Houdelet
As the Research and Development Project Leader in Aquaculture at Lallemand Animal Nutrition, Dr. Camille Houdelet leads the development and evaluation of microbial-based solutions to support fish health and performance, with a strong focus on functional ingredients and host-microbiota interactions. Her work bridges applied research and innovation to deliver science-driven nutritional strategies for sustainable aquaculture.
