Vibrios associated with mortality in cultured plaice Pleuronectes platessa fry

Fifty two bacterial strains, identified as Vibrio spp., were isolated from diseased plaice fry. The most numerous group comprised V. anguillarum (26/52), of which 3 isolates belonged to serogroup O2a, 16 corresponded to serogroup O18, and 7 isolates were non-typeable. All serogroup O18 isolates had identical ribotype patterns. Fourteen isolates were identified as V. splendidus biotype I (n = 11) or V. splendidus-like (n = 3). Seven isolates were V. fluvialis, representing the first isolation of this species in Denmark and the first description of V. fluvialis associated with diseased fish. All V. fluvialis isolates had identical ribotype patterns, indicating the presence of a single clone. The last 5 isolates belonged to 2 different, unidentified Vibrio species (n = 2 and 3, respectively). Although all isolates were recovered from diseased plaice fry, their exact role as pathogens for the fry is as yet uncertain. Selected isolates were tested for virulence to salmon and turbot. When injected into juvenile salmonid fish, the recorded LD50 values were higher than 10(6), indicating that their virulence was relatively low. However, virulence seemed to deteriorate upon subculturing, and therefore, the strains may have been more virulent upon primary isolation from the plaice fry.