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Health management practices for cage aquaculture in Asia - a key component for sustainability

by the Fish Site Editor
01 April 2007, at 1:00am

By Zilong Tan, Cedric Komar and William J. Enright and published by Intervet. The intensification of aquaculture and globalization of the seafood trade have led to remarkable developments in the aquaculture industry. Nevertheless, the industry, particularly Asian aquaculture (> 90% of world production), is paying a price for this unprecedented growth in terms of deterioration in environmental and health conditions.

Abstract

The industry has been plagued with disease problems caused by viral, bacterial, fungal and parasitic pathogens. In recent years, disease outbreaks are becoming more frequent in the region and the associated mortality and morbidity have caused substantial economic losses.

Asian aquaculture is characterized by an enormous diversity of species, with several dozen marine species being farmed. Consequently, more resources are needed to understand the basic epidemiology of diseases in the various species. In Asia, some disease-causing agents have been described but comparative studies between isolates from different geographical locations and fish species are generally not available. Epidemiology data are scarce, as are basic data on the immune systems of Asian fish species. This hampers development of effective strategies for disease control. Also, most farming is operated on a small scale and technical support, including disease diagnosis and training, is often lacking at farm level.

Increased trade of live aquatic animals and the introduction of new species for farming, without proper quarantine and risk analysis in place, result in the further spread of diseases. In Asia, most individual fish farms produce several species of fish. Trash fish are widely used as feed. Fry are often wild caught or derived from wild-caught broodstock. Furthermore, legislation for and implementation of farming licenses and zoning policies are not in place in most Asian countries. Coupled with a ‘gold rush’ mentality, this often results in too many fish and too many farms in a concentrated area, which in turn promotes disease transmission. The combination of all these factors, together with the diversity of organisms in tropical waters, leads to a truly challenging disease situation.

At present, many farmers still focus more on treatment than prevention. Irresponsible use of antibiotics and chemicals in aquaculture can lead to residue problems, an increasing consumer concern, and to the development of drug resistance among the bacterial pathogens. In Asia, with the exception of Japan, few fish vaccines are yet commercially available. The major advantages of prophylactic vaccination over therapeutic treatment are that vaccines provide long-lasting protection and leave no problematic residues in the product or environment.

Asian aquaculture will continue to grow at a fast pace due to both area expansion and production intensification. Under these conditions, the prevalence and spread of infectious diseases will unavoidably increase as a result of higher infection pressure, deterioration of environmental conditions and movement of aquatic animals. Accordingly, the effective control of infectious diseases has become more and more important in the cultivation of aquatic animals. Good health management is the “silver bullet” for disease control. Collectively, this includes the use of healthy fry, quarantine measures, optimized feeding, good husbandry techniques, disease monitoring (surveillance and reporting), sanitation and 2 vaccination, and proper control and biosecurity measures when diseases do occur. Overall, the emphasis must be on prevention rather than treatment. Remember, ‘one gram of prevention is better than a kilogram of cure’.

Introduction

Today, aquaculture is the fastest growing food-producing sector in the world compared with terrestrial animals and 90% of world aquaculture production is in Asia. However, from the time man started to culture fish, fish diseases have changed from being an interesting phenomenon to an important socio-economic problem. Infectious disease is considered to be the industry’s single most important cause of mass mortalities and economic losses. Health problems have two fiscal consequences on the industry: loss of productivity due to animal mortality and morbidity, and loss of trade due to food safety issues.

Estimates from various organisations have indicated that approximately a third to a half of all fish and shrimp put into cages or ponds are lost due to diseases before they reach marketable size. The actual economic losses in the aquaculture industry worldwide are estimated to be in excess of US$9 billion per year, which is roughly 15% of the value of world farmed fish and shellfish production. Despite being long established, diseases and associated economic losses in aquaculture are a huge problem in the Asia (Bonadad-Reantaso et al., 2005). According to Wei (2002), outbreaks of bacterial diseases caused losses of over US$120 million to the fish aquaculture industry in China between 1990 and 1992. In 1994, marine fish diseases caused industry losses of US$114.4 million in Japan alone (Arthur and Ogawa, 1996). In addition, within a 3-month period, Koi herpes virus (KHV) infection of common carp led to losses of approximately US$5.5 million for Indonesian farmers in one area alone (Bondad-Reantaso, 2004). IntraFish Media reported in 2004 that, “the FAO recently sent out an alert in a press release about the dangers some of these diseases can pose not only for human health but they can also paralyze regional food producing sectors and leave thousands of farmers and producers out of work and with no income. Asia has particularly been mentioned where millions of people live off fishing or aquaculture or both”. Thus, disease is undoubtedly one of the major constraints to production, profitability and sustainability of the aquaculture industry.

The aquaculture industry in Asia is characterized by an enormous diversity of fish species and most Asian farms operate on a small scale where technical support, including disease diagnosis and training, is lacking. Consequently, treatment is generally decided without proper disease diagnosis and antibiotics are often improperly used. This has led to residue problems and the development of bacterial drug resistance. Moreover, poor husbandry methods are still in practice in many places, e.g., the use of trash fish as feed, or fry sourced from the wild or derived from wild-caught broodstock. These practices open a door for pathogen infections. In addition, the increased trade of live aquatic animals and the introduction of new species for farming, without proper quarantine and risk analysis in place, have resulted in the spread of diseases within and between countries. The combination of all these factors has led to a truly challenging disease situation in Asian aquaculture where disease prevention is difficult (Tan and Grisez, 2004).

Norwegian salmon farming is often taken as an example of how things should or could progress in aquaculture. However, the production of fish in tropical and subtropical areas is quite different. Differences involve not only the species cultured, but also (and mainly) the scientific knowledge that is available on reproduction, husbandry, feed requirements, diseases and immunology specific to the farmed species. Taking these differences into account, the knowledge that has been gathered in salmon health management can be used to more efficiently advance the relevant science in this region.

As Asian aquaculture will continue to grow at a fast pace due to both area expansion and production intensification, the prevalence and spread of infectious diseases will unavoidably increase as a result of higher infection pressure. In order to become sustainable, the industry must undergo changes and pay more attention to health management strategies.

In this paper, an overview is given about the current situation regarding health management practices in Asia. Recommendations for improvement are discussed.

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March 2007

the Fish Site Editor