Environmental <scp>RNA</scp>‐Based Surveillance of Viral Hemorrhagic Septicemia Virus (<scp>VHSV</scp>) in Olive Flounder (<scp><i>Paralichthys olivaceus</i></scp>) Aquaculture: Detection Dynamics and Risk Assessment

ABSTRACT Viral hemorrhagic septicemia virus (VHSV) is a major pathogen in aquaculture worldwide, causing significant outbreaks through waterborne transmission. This study aimed to evaluate the effectiveness of environmental RNA (eRNA)‐based surveillance for detecting VHSV in aquaculture systems, focusing on the relationship between infection dynamics, water temperature, and fish weight. Over an 8‐month period, VHSV prevalence was assessed in fish and outlet water samples from six olive flounder farms in Jeju Island, South Korea. Higher detection rates were observed at lower temperatures (&lt; 18°C) and in smaller fish, with 18.7°C and 158 g identified as critical susceptibility thresholds. Controlled laboratory experiments further confirmed a strong correlation between VHSV shedding and infection intensity, with a qPCR CT value of 23 (~10 6 copies mg −1 ) serving as a reliable indicator for high‐risk infections. The eRNA method successfully detected VHSV in farms with severe infections, demonstrating its potential for noninvasive disease surveillance, although sensitivity was limited at low infection rates. These findings provide actionable insights for early detection, biosecurity enhancement, and sustainable aquaculture management, particularly in temperature‐sensitive regions like Jeju Island. This study supports the integration of eRNA‐based surveillance into routine disease monitoring, contributing to both aquaculture sustainability and environmental health.