The role of water quality indicators and management factors in the prevalence of bacterial diseases in rainbow trout (Oncorhynchus mykiss) farms in Lorestan Province, Iran

Background: Bacterial pathogens are a major constraint in rainbow trout (Oncorhynchus mykiss) aquaculture, leading to recurrent losses worldwide. Although water quality is often considered a primary driver of outbreaks, evidence increasingly suggests that farm-level management practices may play an even more decisive role. However, this relationship has rarely been tested using field-based molecular data in Iranian aquaculture systems. Our study addressed this gap by hypothesizing that specific management risk factors, such as water source, biosecurity, and stocking density, influence pathogen prevalence more than natural variations in water quality. This approach aimed to provide a more integrated understanding of disease dynamics and identify practical prevention targets. Methods: The study occurred in various rainbow trout farms in Lorestan Province. Researchers recorded water quality parameters like temperature, dissolved oxygen, and hardness. They also noted management factors such as water sources, stocking density, quarantine measures, hygiene, and staff training. Researchers identified bacterial isolates from sick fish using PCR. They analyzed the data with one-way ANOVA to compare mean values and used logistic regression to explore the link between management and water quality factors and disease occurrence. Results: Molecular PCR analysis revealed that approximately ۶۰% of rainbow trout samples in Lorestan Province were free of bacterial infection, while over ۴۰% were positive for at least one pathogen. Among the identified bacteria, Lactococcus garvieae (۳۰%) and Streptococcus agalactiae (۱۷%) represented the most prevalent agents, followed by Aeromonas hydrophila (۱۲%) and Streptococcus iniae (۱۲%), whereas Yersinia ruckeri and Streptococcus dysgalactiae were detected with lower frequency. Regional differences were evident: S. iniae was dominant in Borujerd, Y. ruckeri in Aligoodarz, and mixed infections were most common in Dorud. Pearson’s chi-square test indicated highly significant associations for A. hydrophila and Y. ruckeri across counties (p<۰.۰۱), while other pathogens showed a more uniform distribution. Water quality parameters varied significantly between sites (p<۰.۰۱), with dissolved oxygen ranging from ۷.۰–۸.۸ mg/L, temperature ۱۵.۰–۱۷.۹ °C, pH ۷.۶–۸.۸, ammonia ۰.۱۹–۰.۶۸ mg/L, and nitrite ۰.۱۹–۰.۴۶ mg/L. Despite these variations, regression analysis confirmed that water quality alone was not directly associated with bacterial outbreaks. Instead, management-related factors—including the use of river water, high stocking density, and lack of quarantine—demonstrated the strongest correlation with disease occurrence. These findings highlight that biosecurity and farm management practices are more decisive than physicochemical water parameters in controlling bacterial diseases in rainbow trout aquaculture. Conclusion: The findings show that management practices have a greater impact than water quality factors on bacterial diseases in rainbow trout farms. We recommend improving farm hygiene, biosecurity, and stocking control. Transitioning to recirculating aquaculture systems (RAS) can also help reduce disease rates and support sustainable production.