Keeping a lid on disease

By the bank of the river Lo in Vietnam, a man bends down to lift the lid of a floating bamboo cage. Inside the cage are grass carp, a popular species that can be raised at high density on grass and other vegetation, and which are highly valued by consumers. However, as this farmer inspects his fish he realizes that, as last year, they are showing signs of disease. He does not know which disease they suffer from but he is familiar with the behavioural and physical signs, which indicate that his fish are not in their best health. The farmer can harvest his fish prematurely and they can then be consumed within the household or even sold. However, his carp have not yet reached their optimum size and sick fish are only worth half as much as healthy ones.

J.Turnbull, Institute of Aquaculture, Stirling, UK

As in other regions of Southeast Asia, cage aquaculture is a popular activity in the Lo River Region of Vietnam, particularly for those with little or no land. Successful cage culture can result in benefits approximately five times greater than livestock. But disease outbreaks have been found to be a major constraint to producing a successful harvest, especially during the wet season when water turbidity increases and environmental conditions deteriorate. Over 80% of reported fish losses are due to disease and more than half of farmers in the Lo Region found that disease was most common in smaller fish (<250g in weight). Poor farmers are particularly affected by this problem as most of them were found to stock smaller fish. In addition, a single production cycle is intended to last two years, but few farmers (~20%) are able to achieve this either because of disease or because fish have to be harvested prematurely to provide income for the household.

With a rising incidence of disease, the number of cages has dramatically declined over the past few years. In addition, an appraisal of cage culture carried out in 1999 revealed that obtaining a government loan, to build cages or purchase fish, is a complicated process and this current financial system is seen as another major constraint for farmers. Despite these setbacks many households continue to maintain 1-2 bamboo cages close to the family home. The majority of farmers stock grass carp, which has a high domestic value, particularly in rural areas where selling 1kg of fish generates the same income as one full day spent fishing or three days making handicrafts. However, more recently some farmers have begun to stock their cages with a native carp, Ca Bong, which appears to be less susceptible to disease. Most importantly, cage culture integrates well with other livelihood activities, including keeping livestock, growing fruit and vegetables, making handicrafts, and fishing so farmers are not totally dependent on their stock even if they become diseased. Indeed, the majority of farmers view it as a mean of accumulating savings as they can be marketed more flexibly compared with livestock. In addition, for some, cages provide a place to live: a shelter built on top of the cages becomes accommodation for the household and, unlike shared pond aquaculture, conflict is avoided as ownership of caged fish is not disputed.

However, although disease is a major factor in the success or failure of cage culture, in the diverse range of freshwater systems used in Vietnam and also in Thailand, it has been revealed that farmers lack specific knowledge of the types of disease which affect their stock. Many of the infections that occur within these inland systems are opportunistic pathogens, which means that fish in good condition can withstand a degree of bacterial challenge but readily succumb to virulent pathogens, resulting in dramatic mass mortalities. To offset losses, farmers often increase stocking densities thereby promoting stress-related diseases and exacerbating the situation.

Strategies for improved diagnosis and control of bacterial diseases in small-scale aquaculture in South-East Asia are currently being developed and assessed by a DFID-funded project in collaboration with local institutes and farmers in Vietnam and Thailand. Farmers have reacted very positively to the methodologies used and are enthusiastic to receive as much information as possible. Information is currently being produced in a range of formats (leaflets, magazines, newspaper articles) but most importantly farmers and extension officers are being given the opportunity to feed back their thoughts to researchers through a series of meetings which will help to focus the methods developed on farmers' needs.

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