Disease monitoring of cultured Platichthys stllatus (starry flounder) in the Pohang, Gyeongsangbuk-do
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The Korean aquaculture industry has long focused on Paralichthys olivaceus and Sebastes schlegelii, highlighting the need for diversification of farmed species. In response, the cultivation of Platichthys stellatus has become increasingly important in this industry to diversify aquaculture species. However, as its aquaculture scale expands, mass mortality events have also increased. Despite this trend, disease outbreaks in farmed P. stellatus have been rarely reported to date. This study investigated the causes of mass mortality and the potential emergence of pathogens in P. stellatus farms located in Pohang, Gyeongbuk, by collecting fish monthly from three farms from April to December 2024. Parasitological examinations detected Trichodina spp. However, no significant correlation was observed between its occurrence and monthly mortality rates, suggesting it was not the primary cause. For viral analysis, PCR was performed in parallel with cytopathic effect (CPE) assays using fish cell lines susceptible to aquatic viruses. However, no viral pathogens were detected, and no CPE was observed in any of the cell lines tested. In contrast, bacterial analysis using 16S rRNA sequencing identified Aeromonas salmonicida, Streptococcus parauberis, and Vibrio scophthalmi as predominant isolates. Phylogenetic analysis of the vapA gene confirmed A. salmonicida strain AD112405 as A. salmonicida subsp. masou- cida and its pathogenicity was verified through a challenge test. These findings demonstrate that A. salmonicida subsp. masoucida is a causative agent of mass mortality in farmed P. stellatus. The results provide important insights for developing effective disease management strategies in the expand- ing P. stellatus aquacultre industry.
Abstract
The Korean aquaculture industry has long focused on Paralichthys olivaceus and Sebastes schlegelii, highlighting the need for diversification of farmed species. In response, the cultivation of Platichthys stellatus has become increasingly important in this industry to diversify aquaculture species. However, as its aquaculture scale expands, mass mortality events have also increased. Despite this trend, disease outbreaks in farmed P. stellatus have been rarely reported to date. This study investigated the causes of mass mortality and the potential emergence of pathogens in P. stellatus farms located in Pohang, Gyeongbuk, by collecting fish monthly from three farms from April to December 2024. Parasitological examinations detected Trichodina spp. However, no significant correlation was observed between its occurrence and monthly mortality rates, suggesting it was not the primary cause. For viral analysis, PCR was performed in parallel with cytopathic effect (CPE) assays using fish cell lines susceptible to aquatic viruses. However, no viral pathogens were detected, and no CPE was observed in any of the cell lines tested. In contrast, bacterial analysis using 16S rRNA sequencing identified Aeromonas salmonicida, Streptococcus parauberis, and Vibrio scophthalmi as predominant isolates. Phylogenetic analysis of the vapA gene confirmed A. salmonicida strain AD112405 as A. salmonicida subsp. masou- cida and its pathogenicity was verified through a challenge test. These findings demonstrate that A. salmonicida subsp. masoucida is a causative agent of mass mortality in farmed P. stellatus. The results provide important insights for developing effective disease management strategies in the expand- ing P. stellatus aquacultre industry.