The Role of Humic Acids in Aquaculture: A Review

Water conditions that offer balanced values are ideal environments for fish. Sudden or fluctuating changes in water parameters such as pH, oxygen, and especially temperature interact and destabilize other relevant parameters and cause stress to the fish. When fish encounter such stressful situations, their defense mechanisms do not work properly, which makes them easily susceptible to pathogenic microorganisms and secondary infections. Because such an impact that affects the physical condition, physiological state, and behavioral status of fish, deteriorates general welfare. Various antibiotics and similar medications have been used in controlling or eliminating stress-related diseases to overcome such adverse conditions. However, despite the proper use of these medications, various measures have been taken and the use of some antibiotics has been limited or stopped, as the efficiency of treatments decreased over time, and some antibiotics caused antibiotic-resistant bacteria in fish. As an alternative approach, the use of various natural or synthetic substances has become popular and widespread today in commercial and experimental applications to take preventive measures against pathogens and provide a solid background to fish welfare. Humic acids, one of the substances belonging to the natural environment of fish are among the natural and organic components used for this purpose. Since humic substances are part of the natural habitat of fish, the defense mechanism of fish does not recognize this foreign matter as a threat, so that the fish maintain its usual activities without any reaction. Humic acids that generally known to have positive effects on many organisms, also enhance some vital parameters of fish such as healing of lesions, inhibiting pathogen load, boosting the immune system, and promoting growth when used as a feed additive. Moreover, humic acids have been reported to reduce inverse effects of stress on physiological and histopathological parameters of fish, toxic effects of heavy metals and organic pollutants, and to repair the disease-based injuries faster. In this review, the latest knowledge on the effects of humic substances on two of the primary problems of aquaculture systems, which are fish diseases and relevant pathogens, is discussed.