Fish Diseases and Management

7.6.3. Bioremediators and Other prophylactic measures

Bioremediators

Bioremediators on the other hand are environmental probiotics, which alters the environmental parameters by the reduction of toxic and unwanted compounds by breaking them down to much simpler and harmless compounds.

Environmental probionts or bioremediators improve water quality by nitrification, removal of toxic substances such as ammonia, hydrogen sulphide and nitrite from the culture systems. They would efficiently recycle the waste material by decomposing the complex organic substances and generate nutrients in the form of simple inorganic compounds. Of the several strains of bacteria, gram-positive Bacillus spp. are generally more efficient in converting organic matter back to CO₂. Other bacterial species of importance towards bioremediation are: Nitrobacter, Pseudomonas, Enterobacter, Cellulomonas, and Rhodopseudomonas spp. Bioremediators (biocontrol agents) apart from improving water quality parameters, modify or manipulate the inherent microbial communities and reduce the pathogenic microbes. The cell densities of these organisms are greatly influenced by the carbon-nitrogen ratio of the sediment and the addition of carbon source could increase the proliferation of heterotrophic bacteria compared to the pathogenic bacteria.  Ammonia build up of in the culture systems are removed by nitrifiers by oxidation of ammonia to nitrite and subsequently to nitrate.

Other prophylactic measures

Other prophylactic measures include management aspects aimed at controlling disease incidences. Selection of healthy juveniles and avoiding contamination during culture is an effective step for disease control. Specifically, quarantining the stocks that are imported or transferred over a wide geographic area has to be practised. Disease avoidance by using specific pathogen free (SPF) stock is a very useful method for disease prevention. Development and raising of specific pathogen resistant (SPR) strains through selective breeding of animals known  is one of the potential approaches, in disease prevention. 

Use of biosecurity measures like (preventing the entry of infected host and carriers), closed or semi - closed recycle system with reduced water exchange and increased water-reuse culture systems, disinfection of contaminated water before discharge in case of an outbreak, maintaining optimum stocking density, use of virus-free broodstock to prevent vertical transmission of the viral diseases, avoiding too much organic loading and regular monitoring of  the health status of the cultured animals by suitable diagnostic methods are the recommended measures for a disease free aquaculture system.

Despite the best biological and management measures undertaken for controlling the diseases, prevention of disease outbreaks sometimes become impossible in an aquaculture farm or hatchery. In such conditions, chemotherapy would become essential with chemotherapeutants that are eco-friendly and less toxic. Antibiotic application has to be done with caution in controlled condition after identifying the correct antibiotic through antibiogram assay. Other chemical disinfectants like formalin, chelated copper compounds, quaternary ammonium compounds (benzalkonium chloride – BKC), bleaching powder and potassium permanganate are used at appropriate concentrations and in a rational manner for treating specific cases of infection.