First Report on <i>Vibrio harveyi</i> Infection in Black Rockfish ( <i>Sebastes schlegeli</i> ): Skin Ulcers, Histopathology, and Immune Response

In August 2023, a severe skin ulcer outbreak occurred in net‐caged black rockfish ( Sebastes schlegeli ) of Nanhuangcheng Island, Bohai Sea. The causative bacterium, Vibrio harveyi , was identified through 16S rDNA and gyrB sequencing. Artificial infection experiments confirmed its high pathogenicity with a 7‐day LD50 of 5.41 × 10 6 CFU/mL. Histopathological analysis revealed hepatocyte vacuolization, erythrocyte aggregation in the spleen leading to splenic congestion and reducing the contrast between red and white pulp regions, glomerular atrophy, renal damage, intestinal epithelial necrosis, and muscle fiber disintegration. Immunoenzymatic assays showed elevated alkaline phosphatase (AKP), acid phosphatase (ACP), and lysozyme (LZM) activity. Additionally, qRT‐PCR analysis revealed downregulation of IL‐12B and upregulation of TNF‐ α , CASP10, and RAFEA. Antibiotic susceptibility testing showed sensitivity to minocycline and doxycycline, but resistance to erythromycin and azithromycin. This study provides novel etiological and histopathological insights into V. harveyi infection in black rockfish, highlighting the bacterium’s impact on host tissues and immune response.