Formalin‐inactivated vaccines are widely employed as a primary preventive strategy against red sea bream iridoviral disease (RSIVD), which poses a substantial economic threat to the aquaculture of rock bream ( Oplegnathus fasciatus ). However, conventional quantitative PCR (qPCR) cannot differentiate infectious virions from noninfectious vaccine residues or viral debris, limiting accurate assessment of vaccine efficacy. Therefore, the present study aimed to evaluate the protective efficacy and viral shedding dynamics of a formalin‐inactivated vaccine using propidium monoazide (PMAxx)‐based viability qPCR (vqPCR). Vaccination demonstrated robust protection, achieving a relative percent survival of approximately 80% in immersion challenges and significantly reducing viral shedding into seawater. Notably, comparative analysis revealed that conventional qPCR significantly overestimated viral risk in vaccinated fish by detecting noninfectious DNA artifacts. Although vaccination did not confer sterilizing immunity, it suppressed infectious viral replication to sublethal levels, effectively preventing horizontal transmission to naïve cohabitants. Furthermore, disease progression and shedding kinetics were temperature‐dependent, occurring more rapidly at 25°C than at 20°C. Overall, these findings highlight that vaccination induces functional sterilization of shedding and underscore the need to adopt vqPCR for accurate epidemiological risk assessment in aquaculture populations.

In open marine aquaculture, rearing diverse fish species in adjacent net pens facilitates the waterborne spread of red sea bream iridovirus (RSIV). This study systematically evaluated the horizontal transmission dynamics of RSIV in black sea bream (BSB, Acanthopagrus schlegelii ) and Japanese sea bass (JS, Lateolabrax japonicus ) under immersion and cohabitation conditions. After viral exposure, BSB exhibited high survival rates despite significant viral loads and persistent shedding, effectively acting as an asymptomatic reservoir. Conversely, JS functioned as a highly susceptible host, experiencing 0% survival alongside rapid, high‐level viral replication and dissemination prior to mortality. These divergent epidemiological roles highlight the critical need for species‐specific biosecurity and disease management strategies in shared marine aquaculture zones.

Coastal sea-cage aquaculture environments support dynamic microbial communities, yet their viral components remain poorly characterized. This study profiled DNA viral-like reads recovered from total metagenomes of iron-flocculated seawater concentrates (operationally termed DNA virome) at a rock bream (Oplegnathus fasciatus) sea-cage farm off Geoje Island, Korea, at five time points between July 2024 and May 2025 using long-read Nanopore metagenomics. Multivariate analyses of DNA virome composition indicated temporal patterning in virome composition; samples collected within the cages tended to group together. The DNA virome was dominated overall by bacteriophages of abundant marine bacteria and by viruses infecting picoeukaryotic phytoplankton, and it showed marked temporal turnover during the investigated year: Micromonas-associated viruses became prominent in December, whereas Ostreococcus- and Bathycoccus-associated viruses increased during seasonal rewarming in May, suggesting that environmentally driven changes in microbial host communities may contribute to temporal virome turnover. Pelagibacter phage abundance was positively correlated with that of its potential host Pelagibacter ubique, and both Synechococcus T7-like virus and Synechococcus virus were positively correlated with Parasynechococcus marenigrum, associations that are consistent with, but do not demonstrate, taxon-specific virus–host relationships. Nanopore reads assigned to infectious spleen and kidney necrosis virus, now classified within the species Megalocytivirus pagrus 1, were recovered from samples collected in September 2024 (12,960 bp) and May 2025 (340 bp). Average nucleotide identity analysis indicated that the seawater-derived sequence was most closely related to red seabream iridovirus subtype II, the subtype predominant in Korean cultured marine fish. The seawater virome results and the bioinformatic pipelines developed in this study together may provide a non-invasive approach for tracking temporal virome dynamics and for environmental screening of viral pathogen DNA in sea-cage aquaculture.