The article reflects the negative impact on the microbiocenosis of water, fish and the environment of the uncontrolled use of antibacterial drugs, under the influence of which foci of late autumn aeromonosis (Moscow region) and summer pseudomonosis (Rostov region) are formed in fish farms, opportunistic microorganisms: Aeromonads, Pseudomonads, Acinetobacteria, Moraxella, coliform bacteria change their morphological characteristics (capsules are formed in coliform bacteria and Moraxella, the swarming phenomenon characteristic of Proteus - in Acinetobacteria) and increase their virulence. Along with this, there is a large number of resistant strains of bacteria, antibiotic-dependent mutants in those farms where several courses of antibacotherapy were conducted, the active substance of which is ciprofloxacin, morphological changes in the skeleton were noted in fish, the spine acquired an S-shaped shape, and pinpoint hemorrhages appeared on the liver, like from a needle prick. Resistant strains acquire not only epizootic, but also epidemiological significance, especially for people working in the field of fish farming and forage production, because resistant strains by conjugation can transfer their R-factor of multiple resistance to any strains, which has now become a worldwide problem, according to WHO. In such a situation, the main method of struggle is prevention, increasing the immune-physiological status of fish by using probiotics, high-quality compound feed, creating conditions for a comfortable fish keeping. If a course of antibiotic therapy was carried out during the outbreak, then it is imperative to carry out a 7-10-day course of feeding with a probiotic.

The aim was to determine the effective dosages of Levofloxacin, an antibacterial drug new for aquaculture,   for the treatment of bacterial haemorrhagic septicaemia in carps and improvement of the physiological state of fish following disease and antibiotic therapy.The methods: the experiments were carried out in aquaculture on juvenile carps infected with a mixed culture of pathogens: highly pathogenic strains of Aeromonas sp., Acinetobacter calcoaceticus, and Citrobacter. Three dosages of Levofloxacin were tested: 250, 500, and 1,000 mg/kg of ichtyomass with feed for 7 days. After the antibiotic therapy, a course of feeding was carried out with SUB-PRO probiotic at a dose of 0.4 g/kg of feed   for 10 days.Scientific novelty: application of a broad-spectrum antibiotic Levofloxacin, an antibacterial drug new for aquaculture, in fish.The results: positive results were obtained with Levofloxacin, an antibiotic new for aquaculture, in the treatment of bacterial haemorrhagic septicaemia. The advantage of the drug is the inhibitory effect on a wide range of bacterial agents and the absence of negative effects on the physiological state of fish. An effective dosage  of Levofloxacin (1,000 mg/kg of ichtyomass for 7 days) was established under experimental conditions. The positive dynamics on the blood parameters of the probiotic drug “SUB-PRO” after the disease and antibiotic therapy was revealed, which provided an increase in the resistance of the fish organism.The practical significance: the findings will be crucial in the development of practical recommendations for  the treatment of bacterial haemorrhagic septicaemia of fish in aquaculture.

Aquaculture is a fast-growing sector in Russia. To grow fish to a greater extent than for other agricultural objects, it is necessary to preserve and maintain natural habitat conditions. Minor changes in the aquatic environment, such as changes in pH, temperature fluctuations, and the presence of organic pollutants can lead to a decrease in the number of fish. Use of antibiotic drugs for the prevention and treatment of diseases in fish farming can lead to a change in the microbiocenosis of fish and the hydroecosystem, the appearance of antibiotic-resistant strains of pathogenic microorganisms and a change in the immunophysiological status of the fish organism. To date, a promising and effective therapeutic and prophylactic agent in fish farming is the use of probiotics, which have pronounced antimicrobial activity against pathogenic and conditionally pathogenic microorganisms, immunocorrecting and anti-inflammatory effect. As a result of our research, we have established the effectiveness of the use of preparations Subalin containing Bacillus subtilis and Zoonorm containing live bacteria Bifidobacterium bifidum in the cultivation of carp. A positive effect on the increase in body weight of fish was revealed, the level of contamination of parenchymal organs of fish with bacterial flora decreased, specific and nonspecific resistance increased.