First Report of Tenacibaculum maritimum Infection in Grouper ( Epinephelus spp.) Aquaculture in Taiwan

Between April 2024 and 2025, six independent outbreaks of ulcerative skin disease occurred in cultured groupers ( Epinephelus spp.) across southern Taiwan and Penghu County during winter‐to‐spring seasonal transitions at water temperatures ranging from 19°C to 24°C. Affected fish presented with extensive epidermal erosion and deep dermal ulceration on the operculum, lateral trunk and caudal peduncle regions. Histopathology revealed marked integumentary breakdown characterised by epithelial sloughing, deep dermal necrosis and dense basophilic mats of filamentous bacteria tracking along necrotic subcutaneous myofibres. Wet‐mount microscopy and Gram‐stained lesion smears consistently revealed sheets of long, slender, Gram‐negative rods across all cases ( n = 18). Eighteen representative isolates exhibiting pale‐yellow, flat, adherent colonies with irregular margins were recovered on Marine Agar formulated with natural seawater (salinity 30‰–35‰). Species‐specific PCR and near‐full‐length 16S rRNA gene sequencing (1505 bp; > 99.5% identity to the type strain NCIMB 2154 T ) confirmed the aetiology to be Tenacibaculum maritimum . Physiological profiling revealed a narrow salinity tolerance window (30‰–45‰), in contrast to the broader range of classic temperate strains (10‰–35‰). This study represents the first definitive isolation and molecular/physiological characterisation of T. maritimum in farmed groupers in Taiwan, expanding its host spectrum and geographical range into subtropical mariculture zones.