# Viral Encephalopathy and Retinopathy in Dusky Groupers (Epinephelus marginatus, Lowe 1834) from Two Marine Protected Areas of the Northern Mediterranean Sea

## TL;DR

Wild dusky groupers in two Mediterranean marine protected areas died from a virus linked to warm seawater temperatures.

## Contribution

The study confirms betanodavirus as a cause of mortality in wild dusky groupers and links outbreaks to rising seawater temperatures.

## Key findings

- Betanodavirus strains caused mortality in dusky groupers in two geographically distant marine protected areas.
- High seawater temperatures coincided with disease outbreaks, suggesting thermal stress triggers infections.
- Genetically similar viruses were found across distant sites, indicating possible long-distance spread via vectors or environment.

## Abstract

Marine protected areas are designated regions set aside for the long-term conservation of marine life, habitats, and ecosystems. Several fish populations can benefit from the protection of these areas, including groupers, increasing their numbers and sizes. Wild dusky groupers are large predatory fish in the Mediterranean Sea, valued for their ecological role and for recreational and commercial fishing. Like other marine species, they can be affected by viral diseases that cause mass mortalities, such as viral encephalopathy and retinopathy caused by betanodaviruses. This study investigated mortality events among wild dusky groupers in two protected areas along the coasts of Italy and the Principality of Monaco in 2018–2019. Pathological and virological investigations confirmed that betanodavirus strains were responsible for the groupers’ death episodes. Closely related viruses were found in fish over 150 km apart, suggesting that the virus can spread across long distances, possibly via mobile vector species or environmental transport. Disease outbreaks coincided with periods of unusually warm seawater, indicating that rising temperatures may increase infection severity. These findings improve understanding of viral diseases in wild fish and highlight the importance of monitoring fish health and environmental conditions in marine protected areas.

Betanodavirus infection poses a significant threat to marine fish species in the Mediterranean, affecting both aquaculture and wild populations. Despite increasing evidence of viral circulation in farmed and wild fish, data on natural outbreaks in wild groupers remain limited. This study investigated mortality episodes in wild dusky groupers (Epinephelus marginatus) within two marine protected areas (MPAs): Portofino MPA (Liguria, Italy) and Larvotto MPA (Principality of Monaco) during 2018–2019. Pathological examinations and virological diagnostics confirmed that the causative agents were betanodavirus strains belonging to the RGNNV genotype. Phylogenetic analyses revealed high genetic similarity among viral strains detected at geographically distant sites and across host species, suggesting potential regional connectivity mediated by mobile vectors or environmental transport. Seawater temperature analysis indicated that extreme and prolonged high-water temperatures were prodromal and coincided with observed outbreaks, supporting a role for thermal stress in triggering outbreak onsets. These findings highlight the susceptibility of wild dusky grouper populations to betanodavirus and underscore the interplay between host behavior, environmental conditions, and pathogen dynamics. The study emphasizes the importance of integrated health surveillance strategies within and around MPAs to monitor fish health and environmental parameters, thereby conserving wild fish populations and biodiversity.