• Fish disease under the microscope

    To help small-scale farmers prevent and manage fish disease,
    WorldFish is researching conditions that can decimate fish stocks,
    recommending better management practice training and promoting quality seed.

Aquaculture is the fastest-growing food-producing sector. Today, almost half of all seafood eaten is farmed. By 2030, demand will reach 232 million metric tons meaning aquaculture production will need to double.

Fish disease is a threat to that growth with more than USD 6 billion lost annually due to the problem (World Bank 2014).

While disease occurs in both wild and farmed fish populations, it is more frequent and detrimental in aquaculture systems. Preventing and managing fish disease is critical to minimizing production losses and increasing aquaculture productivity.

What is fish disease?

Signs of fish disease include abnormalities and symptoms such as a fish not feeding, ulcers on the body, or cloudy eyes. The most common indicators of disease are fish coming to the surface and gasping for oxygen, or higher than usual levels of sickness or death in an aquaculture system.

Unlike in livestock, where the clinical signs of disease are more evident, the symptoms of fish disease can be harder to see when fish spend most of their time underwater.

Do wild and farmed fish catch the same diseases? They can, explains Senior Scientist Aquaculture at WorldFish, Mohan Chadag, but disease occurs more often in aquaculture systems.

Fish disease more frequent in aquaculture

Fish disease is more frequent in aquaculture than wild fisheries, where outbreaks are rare. In the oceans or seas, where waters are sparsely populated, an infectious agent may not spread quickly and easily. In comparison, aquaculture populations are reared in close proximity at high densities, enabling pathogens to spread quickly and easily.

Pathogens are infectious agents that cause fish disease. Pathogens are always present in an aquaculture system, but not always at sufficient levels to produce a disease. “If the infectious dose increases, if stress increases, if fish can’t defend themselves, then disease will occur,” explains Mohan Chadag, Senior Aquaculture Scientist at WorldFish.

“A pathogen can produce several hundreds or thousands of infectious agents [offspring], and in an aquaculture system the majority of them will immediately find fish to infect. The higher density of fish results in a high contact ratio and transmission rate leading to death and disease outbreaks,” explains Chadag.

Like humans, fish can get sick when stressed. Fish are sensitive to changes in their environment, and in aquaculture systems they are under more stress then in the wild. Stress factors can include low oxygen levels, temperature changes, poor water quality, and salinity fluctuations. When a fish is stressed, the effectiveness of its immune system is reduced, making it more susceptible to disease.

In developing countries, neighboring farms often share the same water resources, which contributes to the spread of disease explains Mohan.

The big impact of disease on small-scale farms

In developing countries, fish disease often spreads quickly between small-scale aquaculture farms. Often, hundreds of neighboring farmers use and discharge water into the same shared water body. For example, a farmer will drain their pond in an effort to rid it of a disease, but the infected water is then used by neighboring farmers. In this context, preventing the spread of fish disease is difficult.

The impact of fish disease on small-scale farmers varies. A minor disease can cause poor fish growth or low-level mortalities, resulting in lower production, while the worst diseases can cause a farmer to lose his whole crop, such as the white spot disease in shrimp.

“Most farmers perceive mass outbreaks and deaths to be more damaging because the total loss is easy to see,” explains Mohan. “But in fact, low-level mortalities have greater impact. A couple of fish may die every day, but over the full growing season this is a lot of lost fish.”

Diseased fish that survive can be hard to sell. Consumers are hesitant to buy fish that have signs of sickness such as bulging eyes or an ulcer, leading to reduced sales for a farmer.

Together, these production and economic losses caused by fish disease make it hard for farmers to maintain their livelihoods.

"For fish to be a major,
sustainable food source, disease must be kept at bay"

(Mohan Chadag, Senior Aquaculture Scientist at WorldFish)

A farmer’s best defense: good farm practices

Ali Akkas has been farming shrimp in Bangladesh in his family’s 40m2 gher, a modified rice-fish field, for the last four years. In 2012, the white spot virus infected his crop. Within a few days the whole gher became contaminated and all the shrimp died.

There is no one-size-fits-all approach to managing fish disease explains Mohan. “The best weapon a farmer has is to be aware of basic biosecurity principles and adopt good management practices.”

“If a neighboring farmer experiences a disease outbreak, a farm should close their system to prevent the infected water from entering,” says Mohan. “If a farmer drains his pond after a disease outbreak, they should sun dry it for two weeks and add ample lime to reduce the risk of disease re-occurring.”

“If a farmer leaves a dead fish in their pond, it will disintegrate and other fish will nibble at it. Farmers should remove dying fish every day; proper disposal of diseased fish is crucial,” he adds.

To help farmers manage disease, WorldFish provides practical training to fish and shrimp farmers in Bangladesh and Egypt. The training covers the whole aquaculture process including pond preparation, stocking rates, quality seed, feed management, water quality and more. Farmers are taught that they do not need to know the cause of a disease to manage it because they can still eliminate or manage the risk factors.

In 2012, Ali received practical training on aquaculture as part of the USAID-funded Aquaculture for Income and Nutrition (AIN) project. “Now most of the time we stock PCR-tested post-larvae only and follow improved management practices like using a nursery, maintaining proper stocking density and maintaining proper depth of water in the gher,” he says.

The results have been positive. “Our yields have increased and now we are making better profits from our business. In 2014, I got 100 kg shrimp and 120 kg prawn from my gher. In 2015, I have partially harvested 100 kg and there is another 50 kg shrimp and 150 kg prawn in my gher.”

In Egypt, more than 2700 fish farmers have received training since 2012 through the Improving Employment and Income through Development of Egypt’s Aquaculture Sector.

In Bangladesh, over 40,000 farmers have received training in better management practices (BMPs) through two USAID-funded projects: AIN, and the Cereal Systems Initiative for South East Asia (CSISA).

Biosecurity is the set of preventative steps that fish farmers follow to prevent and manage the spread of disease in their crop explains Mohan.

Access to quality shrimp seed helps Bangladeshi farmers

Disease is a major constraint for shrimp aquaculture worldwide. In Bangladesh, polymerase chain reaction (PCR)-tested shrimp post-larvae is one major intervention helping farmers fight shrimp disease.

“The white spot syndrome virus is one of the major problems for shrimp culture in Bangladesh,” explains Amitosh Shen, a District Fisheries Officer in Cox’s Bazaar. Outbreaks of this highly lethal and contagious infection can destroy entire populations of shrimp farms within a few days. Research shows that farmers using PCR-positive seed are more likely to develop disease in their shrimp population.

Through the AIN project, WorldFish is working with the Department of Fisheries to support hatcheries to test shrimp and then sell only PCR-negative seed, which is free of the white spot syndrome virus. Using quality brood stock enables farmers to ensure their starting shrimp population is disease-free. “The Bangladesh government has included in the Hatchery Act 2010-11 that hatcheries must test all post-larvae and certify them as virus free before selling to farmers,” says Amitosh.

PCR-tested seed has been successful in reducing disease since it was introduced to Bangladesh in 2004. A survey of farmers found that between 2011-12 and 2014-15 households using PCR-tested seed had a 26% increase in production yield on average. In 2014-15, over 448 million PCR-tested post-larvae were distributed to farmers.

WorldFish is currently trialing a new intervention in Bangladesh, specific pathogen free shrimp post-larvae, to further help farmers prevent and manage disease in their farms.

Shrimp is an important commodity in Bangladesh. Preventing the outbreak and spread of diseases such as the white spot syndrome is vital, says Mohan.
2700 Egyptian fish farmers have been trained
in better management practices
40,000 Bangladesh fish farmers have been trained
in better management practices

Protocols can reduce disease transmission

Just like trade and people move freely across the globe, so too do fish. Fish and fish products are traded for aquaculture, stocking programs, human consumption, animal feed and bait. “The increasing movement of live aquatic animals increases the risks of pathogens, pests and infectious diseases being shared across countries,” says Mohan.

Global standards exist to reduce the risks associated with the spread of aquatic animal diseases. When introducing improved fish strains to a new country, WorldFish follows strict guidelines and protocols set by the World Organization for Animal Health (OIE) to ensure the transfer is done safely and responsibly. This involves a number of measures at the pre-border, border and post-border stages, including quarantine checks and health certifications.

“With GIFT [genetically improved farmed tilapia], we don’t think there are any OIE-listed fish pathogens that are likely to be introduced with tilapia,” explains Mohan. “But any introduction, irrespective if there is a known pathogen or not, should follow these guidelines because there is always an element of risk of live animals carrying live pathogens.”

Minimizing the spread of fish disease through global trade is achieved by following transfer standards explains Mohan.

Case study: Tilapia death in Egypt

In Egypt, domestic aquaculture provides 60% of total fish consumed. Tilapia production makes up 75% of all aquaculture production, providing one fish per week for every person in Egypt.

Over the last two years in Egypt, many farmed tilapia populations have died from disease during the hot months of July and August. Prior to 2013, this had never happened.

“We used to have limited losses in winter,” explains fish farmer Mohamed Besheer Hassan Awad from El Hamoul, Kafr El-Sheik, who has been using better management practices on his farm since 2013. “But we have recently witnessed a large mortality problem in summer, which is a very unusual thing to happen.”

Pre-harvest deaths in aquaculture result from the complex interplay of factors such as seed source and quality, poor nutrition, and hostile environmental conditions. To pinpoint the cause of recent mortalities in Egypt, WorldFish is collaborating with the International Livestock Research Institute (ILRI) and Merck Singapore.

ILRI scientists visited Egypt in September 2015 and undertook rapid epidemiological assessments. They collected field samples to identify mortality risk factors and understand the role of infectious agents. Research findings from these studies will help to develop farm-level interventions and better management practices to minimize the impact of disease in tilapia farming.

Future research

“For fish to be a major, sustainable food source, disease must be kept at bay. Billions of dollars have been lost to fish disease. This is big money. We need to continue research into fish disease, by way of investing in laboratories, diagnostics, people and resources,” says Mohan.

WorldFish continues to promote good management practices as the first line of defense, and is researching new interventions such as rapid diagnostic tools and vaccines.

“A lot of work is being done on rapid pond side diagnostics and simple tools that farmers can use to make early predictions or to identify underlying reasons for mortality,” explains Mohan. “We’re also collaborating with agencies to see if we can develop vaccines, or trial vaccines developed elsewhere, for use in the developing countries that we work.”

Efforts are underway to determine the impact of climate change on fish disease in aquaculture and evaluate the disease resistance of improved strains of tilapia.

Managing disease to ensure future fish demand is met

To meet global demand for fish, aquaculture production will need to double by 2030. Disease is recognized as one of the limiting factors to increasing production. “We cannot afford to lose fish to disease,” asserts Mohan. “Even if you’re losing 5-10% to disease and we can save that, then that itself is a big production gain,” asserts Mohan.

“Globally, there is no aquaculture system that is free from the risk of disease,” explains Mohan. “Unless we are able to manage disease, minimize its impact, and bring down the prevalence and incidence of diseases we will not be able to meet our expected production targets. So overall fish health management is very, very important if aquaculture is to meet future fish food needs.”

Related publications

Health management in marine fish and shrimp hatcheries with special reference to immunostimulants (an overview)

Egyptian Government expresses efforts to provide aquaculture industry with high quality fish and to prevent diseases outbreak. The production of larvae and fry is still unpredictable for some species, owing to the lack of control of the microbiota in the rearing systems. Using conventional approaches such as the use of disinfectants and antimicrobial drugs, have had limited success in the prevention or cure of aquatic disease.

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