Freshwater eels are among the most intensively traded aquaculture species globally, known for their taste and cultural significance, particularly in Asian cuisine. The high market value of this commodity makes it an ideal choice for aquaculture investment. Having high demand in international markets and often considered as luxury seafood item, eels command steep prices driven by strong demand and limited supply.

One of the most pressing threats to eel farming is the occurrence of infectious disease, which results in significant financial losses when an outbreak occurs. This has prompted researchers to develop methods to minimize financial losses while still optimizing profits. Unfortunately, relatively little literature exists regarding diseases associated with tropical freshwater eels.

Researchers from the DA-Fisheries Biotechnology Center (FBC), hosted by DA-NFRDI, worked to address these challenges with a recent study on disease outbreaks in cultured freshwater eels, highlighting both the risks and the critical role of science in sustaining this high-value commodity.  As part of their study on “Development of culture techniques in captivity of freshwater eels, Anguilla sp.” in 2018, researchers documented a case of mass mortality in tank-cultured tropical freshwater eels (Anguilla marmorata and Anguilla bicolor pacifica) caused by the parasite Ichthyophthirius multifiliis, commonly known as “ich.”

“Ich”-y problem

Juvenile freshwater eels (A. marmorata and A. bicolor pacifica) were collected from the Cagayan River and stocked in concrete circular tanks at three different densities: 1,000, 2,000, and 3,000 pieces per cubic meter. By day 16 after stocking, mortalities were already observed in tanks with the highest density (3,000 pieces per cubic meter).

Water samples and moribund eels were collected and examined. Using DNA analysis, the parasites found on the body and fins of the eels were confirmed to be 99.94 percent similar to Ichthyophthirius multifiliis, verifying an “ich” infection. Infected eels showed clear signs of stress, including sluggish movement, excess mucus production, and white spots on their body, fins, and gills. Beyond indicating poor health, these symptoms also translate to economic losses due to reduced quality and survival.

A key finding of the study was the strong link between stocking density and disease outbreaks. Eels stocked at the highest density experienced earlier and more severe infections, with a mortality rate reaching 61.48 percent, significantly higher than those kept at lower densities. This has important implications for eel farming. While higher stocking densities are often used to increase production, the results show that overcrowding can accelerate disease spread and ultimately reduce total yield.

For a high-value species like freshwater eels, even small drops in survival can lead to major financial losses. The findings highlight the need for farming practices that carefully balance productivity with fish health.

Salty solution to the infection

Beyond identifying the cause of the outbreak, researchers also tested practical ways to control it. Infected eels were treated using three approaches: a 100 ppm formalin bath, a 5 ppt saltwater bath, and a 10 ppt saltwater bath, each applied for one hour. Formalin is a commonly used chemical treatment in aquaculture, while saltwater immersion offers a simpler and more accessible alternative. After treatment, the eels were transferred back to freshwater tanks for recovery.

Results revealed an important difference. Although widely used, formalin proved toxic to glass eels, leading to additional mortality shortly after exposure. In contrast, the 5 ppt saltwater bath recorded 100 percent survival during treatment and significantly reduced mortality afterward. The 10 ppt saltwater bath showed lower survival (about 80 percent), and the eels became stressed and lethargic, with deaths observed shortly after exposure.

The finding is particularly relevant for small-scale farmers, as saltwater treatments are more affordable, safer, and easier to apply compared to chemical options. Disease outbreaks such as “ich” can significantly reduce the profitability of eel farming, especially in systems that rely on wild-caught juveniles and intensive culture practices. Without effective management, these risks can slow the growth of the industry.

As global demand for eels continues to rise, there is an increasing need for efficient and sustainable production systems. For the Philippines, building a competitive eel aquaculture sector requires not only resources, but also strong scientific support. Study shows that research plays a crucial role in solving real-world production challenges like disease control. In a market where every surviving fish carries high value, science is not just helpful; it is essential. ### (Vanessa Mae Escaño)

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Reference: Hilario, J. A., Choresca Jr, C. H., Danting, M. J. C., Ca-As, C. G. P., Gente, A. A., Magbanua, F. L. T., & Oclos, M. T. T. (2020). A case report of Ichthyophthirius multifiliis infection causing mortality on tank-cultured tropical freshwater eels, Anguilla marmorata and Anguilla bicolor pacifica. Aquaculture Research, 51(11), 4814-4818. https://doi.org/10.1111/are.14787