Monitoring of parasitic and bacterial pathogens in farmed olive flounder in Wando, South Korea
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This study monitored five major pathogens in cultured olive flounder (Paralichthys olivaceus) in Wando, South Korea, from May to September 2025, with emphasis on their relationship to rearing water temperature. A total of 521 fish were collected monthly from ten farms. The parasites (Trichodina spp. and scuticociliates) were observed under microscopy and three bacteria (Vibrio harveyi, Edwardsiella piscicida, and Streptococcus parauberis) were identified by culture-based methods. Parasitic diseases were the most frequently detected and showed consistently high occurrence in most sampled fish across all rearing temperatures above 15°C. Most scuticociliate-infected fish showed external lesions such as fin erosion and skin ulceration. Among bacterial pathogens, V. harveyi was detected throughout the survey period, while E. piscicida peaked during August to September and S. parauberis increased as temperatures declined. Co-infections between parasites and bacteria occurred in 40% of sampled fish (207/521), indicating that co-infections with parasite and infectious bacteria are highly prevalent among farmed olive flounder in Wando. This study provides fundamental data for identifying disease occurrence patterns in farmed olive flounder in Wando and also assessed their association changed water temperature.
Abstract
This study monitored five major pathogens in cultured olive flounder (Paralichthys olivaceus) in Wando, South Korea, from May to September 2025, with emphasis on their relationship to rearing water temperature. A total of 521 fish were collected monthly from ten farms. The parasites (Trichodina spp. and scuticociliates) were observed under microscopy and three bacteria (Vibrio harveyi, Edwardsiella piscicida, and Streptococcus parauberis) were identified by culture-based methods. Parasitic diseases were the most frequently detected and showed consistently high occurrence in most sampled fish across all rearing temperatures above 15°C. Most scuticociliate-infected fish showed external lesions such as fin erosion and skin ulceration. Among bacterial pathogens, V. harveyi was detected throughout the survey period, while E. piscicida peaked during August to September and S. parauberis increased as temperatures declined. Co-infections between parasites and bacteria occurred in 40% of sampled fish (207/521), indicating that co-infections with parasite and infectious bacteria are highly prevalent among farmed olive flounder in Wando. This study provides fundamental data for identifying disease occurrence patterns in farmed olive flounder in Wando and also assessed their association changed water temperature.