Mitigating Fish Diseases in the Zambian Fisheries Sub-Sector

By Loziwe Njobvu-Chilufya

The Fisheries sub-sector in Zambia plays an important role for the Zambian population, directly and indirectly, as a source of employment and improved food and nutrition security. This sub-sector comprises of two main branches, namely Capture fisheries and Aquaculture. Capture fisheries involves the harvesting of naturally occurring fish in water bodies such as rivers and lakes. Conversely, aquaculture involves the farming of fish in captivity (ponds and cages).

Fish is an important source of unique essential nutrients like micronutrients such as iron, zinc and calcium as well as omega-3 fatty acids which have proven to have numerous health benefits.

Fish stocks in the natural fisheries have over time been subjected to several factors that threaten their potential for increased biomass, such as indiscriminate use of illegal fishing methods and gears. In comparative terms, there is a tendency for tragedy of the commons in Zambia’s capture fisheries due to their open access nature, whereas aquaculture farms are managed with a deeper sense of ownership by the respective fish farmers. Given a general observed decline in fish populations in the capture fisheries, the initiatives by the Zambian government to step up aquaculture is well placed, as increased fish production from aquaculture is expected to fill the demand gap for fish from the capture fisheries that has reached their potential possible.

Figure 1: Fishing in capture fisheries: Setting of nets

Figure 2: Fish farming (Aquaculture): Fish pond production

Figure 3: Fish farming (Aquaculture): Cage production on Lake Kariba

Diseases in Capture Fisheries

Fish, just like terrestrial animals, do get diseases. Unfortunately this is a sad reality that has befallen Zambia in the recent past. In the capture fisheries, the first case of fish disease was reported in 2007 after fishermen spotted a number of fish with deep ulcerations on the river banks of the Zambezi in Sesheke district. Upon investigation by a team of officers from the Department of Fisheries in collaboration with the Veterinary School of Sciences at UNZA, it was established that the fish had Epizootic Ulcerative Syndrome commonly known as EUS. EUS is caused by anoomycete fungus Aphanomyces invadans which causes significant lesion and ulceration of the skin and necrosis of muscle leading to mortality.

Figure 4: EUS infected fish

EUS outbreaks have been observed to occur during periods of low temperatures. Since the first incident in the Zambezi River, the spread of EUS outbreaks have subsequently been reported in the Kafue River (2010), Chongwe River (2011), Bangweulu swamps (2014), Chambeshi and Kalungu Rivers (2015) and Lake Itezhi-tezhi (2020). Control of EUS or any disease in natural waters poses a very big practical challenge. Nonetheless, its spread can be controlled by restricting the movement of fish, fishing gear and equipment from infected areas to uninfected areas. Restricting movement of fish would involve issuance of a disease test certificate before transportation of fish. Regular monitoring and surveillance in capture fisheries can increase knowledge and data on diseases therefore leading to more preparedness in case of a disease outbreak.

Diseases in Aquaculture

Promotion of aquaculture in Zambia has led to the rapid growth of the subsector attributed to intensified production. Production of fish in captivity in high densities creates a favorable environment for disease outbreaks. Consequently this sector has not been spared from disease outbreaks. Heavy mortalities (ranging between 0.3 and 15% of the nine (9) affected cages) of fish diseases were recorded in 2014 by commercial fish farms on Lake Kariba. Analysis of moribund (sick) fish were indicative of bacterial infections (Streptococcus and Aeromonas) which are prominent pathologic bacteria of fish in stressful conditions. Symptoms of a diseased fish include skin hemorrhage, bulging eyes, bloated abdomen, open wounds, loss of appetite, and presence of fungi on the skin, gasping for air at the surface, chewed or eaten away gills or fins and abnormal amount of mucous covering the body.

Stress compromises the fish's natural defenses so that it cannot effectively protect itself from invading pathogens. Causes of stress include poor water quality, overstocking, nutritionally deficient diet, erratic feeding, rough handling of fish, incident of predators and wild fish, over fertilization and poor sanitation.

Diseases in pond production have not been so prevalent. Reported cases of fish deaths have most often been established to be as a result of dissolved oxygen deficiency or poor handling of fingerlings during transportation.

Disease prevention is better than cure. Therefore, good management practices and Biosecurity should be heightened in aquaculture to prevent the introduction of pathogens. Good management practices encompasses promoting practices that reduces stress on fish such as correct stocking densities, good water quality and use of nutritionally balanced feed.

Biosecurity measures can be applied to prevent all known possible introduction and spread of pathogens. This can be achieved by stocking healthy fingerlings (Quality fingerlings), using quality water, disinfection of equipment, employee hygiene, isolation and destruction of moribund and dead fish, screening of cages and ponds from predators and restricting access to culture facilities.

What to do When You Experience Fish Death

Early reporting to authorities at suspicion of disease or upon seeing dead fish is encouraged. The following steps can be taken upon suspicion of fish disease in cultured fish; stop or reduce feeding, remove and slaughter promptly all moribund fish in ponds or cages at early stage of infection to prevent outbreak, improve pond hygiene by diluting the water in the pond with fresh water, reduce stocking levels and aerate the water if possible. This should be followed by reporting the case to the nearest district office.

Stakeholders of capture fisheries are equally encouraged to immediately report suspicions of fish diseases or observed fish deaths to the nearest district office.

Mitigation Measures by the Department of Fisheries

Worth noting is that the desired benefits to be derived from the fishery resource, such as nutrition, income generation and/or employment, are hampered by the prevalence of diseases in fish. For instance, community members in fisheries where EUS set had mixed feelings whether or not to consume EUS infected fish as its appearance was too unpalatable in some instances. Other players in the value chain, such as fishers and traders were adversely affected by the “natural” post-harvest fish loss (PHFL) caused by EUS as the fish could hardly be sold/bought, thereby inducing, to some extent, poverty among the stakeholders. In addition, heavy mortalities due to fish diseases recorded by commercial fish farms caused big financial losses for aquaculture farmers.

In light of the devastating impact of fish disease on the fishing industry, the occurrence of these fish disease outbreaks both in wild fish and aquaculture has awakened decision makers and stakeholders to the importance of promoting biosecurity and improved health management practices to prevent and control diseases in this sector. Successful fish health management begins with disease prevention rather than treatment.

This realization necessitated the development of the National Aquatic Animal Health Strategy (NAAHS) supported by the Zambia Aquaculture Enterprise Development Project (ZAEDP) in collaboration with the Departments of Fisheries, Veterinary Services and the University of Zambia. The strategy is a guiding document towards a collective approach in managing diseases in the sector. If the sector has to realize its potential, it is necessary to adopt practices that will support the sustainable development of the sector.

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