Prawn Health Management and Disease Control to Sustain Hatchery and Pond Production Systems

Prawn Health Management and Disease Control to Sustain Hatchery and Pond Production Systems

Chapter One of Prawn Health Management and Disease Control to Sustain Hatchery and Pond Production Systems

INTRODUCTION

BACKGROUND TO THE STUDY 

Total world harvest of prawns in the Africa was 2.1% in 1981. In 1989 it had risen to 26%. However, disease and pond environment problems are causing difficulties. During the early 1990s, total production dropped in all the Prawn producing countries. Losses due to disease in Indonesia have been estimated at US$300 million, and in the Philippines only 65% of prawn hatcheries are operative also due to diseases. Bacteria in the genus Vibrio have been implicated as a major cause of the problem. An outbreak of vibriosis can kill 70-90% of juvenile prawns, and significant production losses can result from low-level chronic mortalities and growth depression. Most hatcheries rely on antibiotics to achieve acceptable survival levels. This encourages the emergence of resistant strains of bacteria, and therefore, is not sustainable. Also, aerosols containing antibiotic-resistant bacteria or antibiotics themselves pose health hazards to hatchery workers, and disposing medicated water presents environmental risks. Aquaculture has been an important sector in the introduction, transfer and spread of aquatic diseases in the prawn aquaculture.
The introduction of exotic pathogens along with newly introduced aquatic animals has too often resulted in severe socio-economic and ecological impacts (Klinger and Floyd, 2002). However, good prawn hatchery management practices, including rigorous screening of brood stock for pathogens and routine diagnostics and treatment of fry and fingerlings, will do much to reduce the possibilities of stocking unhealthy seed and the spread of disease into new areas (Arthur and Subasinghe, 2002). Losses of more than $70 million due to disease-induced mortality and impaired growth are incurred annually in Andhra Pradesh. Ectoparasitic diseases account for 70 per cent of the problems, while bacterial and fungal diseases account for 27.5 per cent and 2.5 per cent problems, respectively (Rao et al., 1992). Fungal infections are common among many prawn species and can prove fatal if not treated early. Incidence of white spot viral disease had caused high mortalities and severe damage to the prawn culture industry in Indonesia and India (Hatai, 1994; Anon, 1994). A global estimate of disease losses in aquaculture by the World Bank in 1997 was of the order of US$ 3 billion per annum. The major health problems were identified in prawn farming but were not able to quantify either health-related losses or the health management costs incurred by the farmers (Subasinghe, 2001; Mohan and Bhatta, 2002).
The high risk of disease transmission and parasite infestations among species has increased the level of uncertainty which the farm managers have to contend to develop the industry (Pozio and Rosa, 2005). Scarfe et al.(2005) have observed that aquaculture bio-security programs addressing the aquatic animal pathogens and diseases have become an important focus for the aquaculture industry. Most of the farmers are aware about the consequences of diseases on growth, survival and final production, but only a small percentage is able to identify the problems and quantify disease-related losses. The majority of farmers depends on friends, consultants, sales persons or pharmacists for advice on diagnosis and medication. The advice of feed and chemical sales persons is likely to be biased with their marketing interests (Mohan and Bhatta, 2002; MacRae  et al., 2002). Hence, assessment of the economic impact of disease in aquaculture is vital to develop farmer-oriented primary prawn health management packages and in determining the optimal investment for prawn disease control (Faruk et al., 2004). Under this backdrop, the present study is been conducted to analyze prawn health management and disease control to sustain hatchery and pond production systems.

STATEMENT OF THE PROBLEM 

Continuous exchange of a small proportion of the water is the normal way of maintaining good water quality in prawn aquaculture. However, some farmers change water more suddenly every two weeks, and in much larger proportions, because this tends to make the prawns moult. The more that moult (and are therefore soft-shelled) at the same time, the less potential losses there may be due to cannibalism. Low dissolved oxygen should be suspected if prawns begin to crawl out of the ponds or congregate at the edges of the pond in daylight. If this problem occurs, flush the pond. Very high pH levels in freshwater prawn ponds can cause prawn mortalities, both because of the direct effect of the pH itself and because of the greater solubility of waste ammonia at high pH. High pH is often caused by dense phytoplankton blooms. Major problems that may arise during culture are mortality of the stock due to low dissolved oxygen in the pond water. Heavy plankton bloom, very low water level and lack of water exchange leads to low dissolved oxygen levels. Continuous rainy/cloudy days precipitate this problem. Immediate water exchange or aeration of ponds during night hours prevents this problem. Development of bottom algae due to high transparency of water is another problem during monoculture of prawns. To avoid this problem always maintain transparency in 30-40 cm range by frequent fertilization. Predation is one of the greatest problems for any aquaculture enterprise, including freshwater prawn farming. Pond management is the major key to prawn health management.