The objective of this study was to analyse the reasons of carp Cyprinus carpio, silver carp
and bighead hybrids (Hypophthalmichthys molitrix × H. nobilis) mortality registred in ponds in April, 2015.
A total of 25 fish samples without (n = 10) and with (n = 15) clinical signs of disease were collected along
with water samples. The results of water quality analysis showed the high level of permanganate oxidation –
2.4–2.8 times more than the normal ratio, of nitrite nitrogen – 1.5–2 times more, and of nitrite nitrogen – 1.4–
1.5 times more. The concentration of oxygen at the near bottom layer was in the range of 2.0 mg/dm3. As the
result of parasitological investigations monogenea (Dactylogyrus hypophtalmichthys, D. extensus), trematoda (Diplostomum spathaceum) and mollusk (Unio pinctorum) were detected. Sixty two different bacterial
isolates were identified in water (30) and fish (32). Bacterial characterization was realized by morphological
and enzymatic properties. For additional identification MALDI TOF mass-spectrometry and PCR were used.
The detected microorganisms belonged to genera Aeromonas and Yersinia. The highest indexes of bacterial
contamination were registered in bighead for A. hydrophila (5 × 103 CFU/g) and in carp – for Y. ruckeri
(2 × 103 CFU/g). All fish strains of A. hydrophila and Y. ruckeri were sensitive to ciprofloxacin, tetracycline,
and chloramphenicol. 90 % of aeromonas and 100 % of yersinia strains were resistant to furasolidone. The
infection of carp by Yersinia ruckeri was detected and observed in Southern Russia for the first time.
