RESISTANCE TO RAINBOW TROUT FRY SYNDROME FOR UNTREATED AND TREATED RAINBOW TROUT Oncorhynchus mykiss FAMILIES
Introduction
Rainbow Trout Fry Syndrome (RTFS), caused by Flavobacterium psychrophilum, is a globally important disease in rainbow trout aquaculture, particularly affecting fry and juvenile fish with high mortality (Hesami et al., 2011; Muñoz-Atienza et al., 2019). For larger fish mortality rates are low, but morbidity rates can be high (Madsen & Dalsgaard, 1999; Nilsen et al., 2011). Its impact on production can be substantial, leading to significant economic losses in commercial aquaculture. No commercial vaccine is available, and disease control relies mainly on antibiotics, raising concerns about environmental impact and antimicrobial resistance. These challenges are especially significant in organic aquaculture, where treatment options are limited. EU targets for increased organic production and reduced antimicrobial use highlight the urgent need for alternative, preventive disease management strategies in trout farming. The heritable nature of resistance to RTFS is known. Today it is however unknown if disease resistance for treated fish can be considered the same trait as disease resistance for untreated fish in a natural outbreak situation.
Materials and Method
Rainbow trout (Oncorhynchus mykiss) from 210 full- and half-sib families were sampled at an organic farm in Denmark during a natural RTFS outbreak in 2025. Fish from each of the 210 families were stocked into three untreated and three untreated tanks. The presence of Flavobacterium psychrophilum was confirmed by a veterinarian through microscopic identification of the bacteria in blood samples. During the outbreak, deceased fish were sampled daily. Data collection of deceased fish included date of death, weight, length, and tissue samples for genotyping.
Results and discussion
Mortalities for each day for treated (1, 3, 5) and untreated tanks (2, 4, 6) are shown in Figure 1. The outbreak lasted on average 47 days, from mid-September to early November. Mortalities were on average 9% for treated fish and 54% for untreated fish. Further analysis will be done to estimate genetic correlations between treated and untreated fish.
This study provides insights into genetic components impacting resistance to RTSF in untreated and treated rainbow trout after a natural disease outbreak. These findings are important in order to know if selecting for disease resistance for treated fish will be effective for improving resistance in untreated fish.
Figure 1. Mortalities for treated (1, 3, 5) and untreated tanks (2, 4, 6).
Acknowledgement
This work was part of the TROUTGANIC funded by the Green Development and Demonstration Programme (GUDP) under the Danish Ministry of Food, Agriculture and Fisheries, The project is part of the Organic RDD 8 programme, which is coordinated by ICROFS.
References
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Madsen, L., & Dalsgaard, I. (1999). Reproducible methods for experimental infection with Flavobacterium psychrophilum in rainbow trout Oncorhynchus mykiss. Diseases of Aquatic Organisms, 36, 169-176. https://doi.org/10.3354/dao036169
Muñoz-Atienza, E., Távara, C., Díaz-Rosales, P., Llanco, L., Serrano-Martínez, E., & Tafalla, C. (2019). Local regulation of immune genes in rainbow trout (Oncorhynchus mykiss) naturally infected with Flavobacterium psychrophilum. Fish & shellfish immunology, 86, 25-34.
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