The abundance of the fish pathogenic bacterium Aeromonas salmonicida in different parts of the marine environment was determined in a fish farm stocked with Atlantic salmon (Salmo salar) suffering from furunculosis. By application of highly specific monoclonal antibodies and immunofluorescence techniques, the bacterium was found in high abundances (4.3 × 103 cells/mL) at the air-water interface. Aeromonas salmonicida was also registered in high numbers in the sediments beneath the farm, and in moderate to low numbers in the water column. When samples were collected in the environment outside the fish farm, the number of A. salmonicida was below the detection limit in surface samples, but the bacterium could be detected in the water column in samples collected downstream to the farm. The high number of A. salmonicida found in the lipid-rich air-water interface is discussed, taking into consideration the high hydrophobicity of the cell surface of the bacterium and the physical and ecological conditions in this specific habitat. Key words: immunofluorescence, total bacterial counts, surface microlayer, furunculosis.
