Recirculating Aquaculture Systems (RAS) have become one of the most widely used cultivation systems in the aquaculture industry due to their ability to provide greater control over water quality and higher production volumes per area.
However, a phenomenon has caught the attention of scientists—the darkening of Nile Tilapias (Oreochromis niloticus) raised in recirculating aquaculture systems.
In this regard, scientists from Cairo University, the University of Tripoli, Beni-Suef University, the Agriculture Research Center, among other organizations, conducted a study with the aim of unraveling the hidden etiologies behind this phenomenon and evaluating the immunological reactions triggered by the invasive agent responsible for chronic irritation.
Additionally, the research explores effective treatment strategies to mitigate the impact on the fish population.
Methods
The researchers collected a total of 100 Nile Tilapia specimens from a private farm in the Kafrelsheikh Province, located in the northern part of the Nile Delta in Egypt. The affected fish exhibited a peculiar black skin coloration, which disappeared when removed from the tank water.
The comprehensive investigation included macroscopic examinations, parasitological assessments of skin and gill scraps, and blood biochemical tests on both infested and non-infested control fish.
Parasites as the Cause of Black Coloration
The study revealed that the monogenean parasite Gyrodactylus cichlidarum was the predominant parasite in the infested fish, causing abnormal black skin coloration and disrupting the immune system. The infested fish showed a significant increase in cortisol levels, lysozyme activity, and various liver enzymes compared to the control group.
“We can conclude that the severe monogenic cutaneous infestation, along with a relatively large number of them in the biological filter, could have resulted in the expected abundant excretion of metabolites that caused the tilapia skin to induce a cascade of cellular recruitments at the affected site, as well as floods of chemical mediators throughout the generalized bloodstream,” report the scientists.
According to the study’s results, the transient change in black skin coloration among Nile Tilapias is due to the fish’s immune response to chronic exposure to different stages of the monogenean and its metabolites throughout the production cycle.
The scientists did not consider this infestation as a case of direct parasitism but rather an event of opportunistic indirect colonization triggered by various factors related to inadequate biosecurity at the farm level.
Treatment Strategies
Treatment trials were moderately successful. A triple treatment plan was implemented, starting with a 7-day application of 0.7 g/m3 copper sulfate pentahydrate, preceded by a 40% hydrogen peroxide solution at 1.5 ml/m3 for the same period.
After the initial treatment, a maintenance dosage of 0.095 ml/m3 of a mixture containing 15% glutaraldehyde and quaternary ammonium compounds (QACs) was administered for 3 days. As a supportive immunostimulant regimen, a weekly dosage of vitamin C (0.45 g/m3) and Saccharomyces cerevisiae (0.45 g/m3) was added to the tank water to improve the general health of the fish.
Conclusion
This study sheds light on the previously unknown phenomenon of color change in Nile Tilapia, attributing it to the presence of the monogenean parasite Gyrodactylus cichlidarum.
The alteration of the immune system in infested fish underscores the importance of understanding both the etiology and immunological reactions. The implemented treatment strategy, involving a combination of copper sulfate, hydrogen peroxide, and a glutaraldehyde/QAC mixture, showed promising results in reducing the monogenean count and restoring normal skin coloration.
The researchers conclude that the triple parasiticide/immunoenhancer treatment strategy allowed for the cessation of mortality and the restoration of normal skin color in tilapias with notable degrees of success.
The addition of immunostimulant regimens further contributes to a holistic approach in the management and improvement of the health of the affected fish population.
Contact
Alaa Eldin Eissa
Department of Aquatic Animal Medicine & Management
Faculty of Veterinary Medicine, Cairo University
Giza, 11221, Egypt
Email: aeissa2012@cu.edu.eg
Reference (open access)
Eissa, A., Attia, M.M., El Zlitne, R.A. et al. The puzzling etiologies of transient black discoloration in Nile Tilapia (Oreochromis niloticus) intensively cultured under RAS system. Aquacult Int (2023). https://doi.org/10.1007/s10499-023-01328-9
