Release Dynamics of Edwardsiella anguillarum and Fish‐Derived Materials From Experimentally Infected Red Seabream ( Pagrus major )

Edwardsiellosis, a disease caused by Edwardsiella anguillarum , poses a serious threat to red seabream ( Pagrus major ) aquaculture. To understand the transmission cycle of E. anguillarum in aquaculture environments, we analysed the release dynamics of the pathogen and fish‐derived materials from red seabream experimentally infected via intraperitoneal injection, immersion or cohabitation. Environmental DNA (eDNA) concentrations of E. anguillarum in rearing water increased as infection progressed and showed positive correlations with red seabream eDNA across all infection methods. Size‐fractionation demonstrated that although > 50% of E. anguillarum eDNA was in the fraction < 3.0 μm but ≥ 0.4 μm, 30%–40% was also detected in the 3‐ and 20‐μm fractions. These larger fractions contained 80% of red seabream eDNA. Furthermore, viable E. anguillarum and its DNA were detected in external tissues of surviving fish (gill filaments, skin mucus and distal intestine), with no significant differences in quantities between immersion and cohabitation groups. These results collectively suggest that E. anguillarum released from infected red seabream enters the surrounding water together with fish‐derived materials, with multiple external tissues contributing to this release, and further suggest that a portion of the bacteria may persist in the environment attached to tissue particles.