Traditional aquaculture practice at East Calcutta Wetland: the safety assessment.

Author:Raychaudhuri, S


Cities consume resources and produce both solid and liquid waste. Waste may normally be defined as any material or energy which cannot be economically used, recovered or recycled at a given time and place. Thus every city of the world must find a way of dealing with its waste. But what if this waste can be treated as a useful resource instead of a hazard to the public? That is exactly the practice developed by the 20,000 natural ecologists, i.e the farmers as well as the fisherman of East Calcutta Wetland. It is because of them that sewage has taken on a new meaning for Calcutta. Natural processes coupled with informal practices contribute significantly to wastewater treatment and promotes recycling. ECW's resource recovery system, developed by local people through the ages, has saved the city of Calcutta from the costs of constructing and maintaining wastewater treatment plants. Calcutta currently has no sewage treatment plant for its 12 million inhabitants. The recycling system that has developed over a century operates with minimum use of technology and engineering (1).

Calcutta's raw wastewater-fed fish pond system, called "Bheri" as well as the Munich's treated wastewater-fed fisheries is frequently cited as showpieces of wastewater-fed fish production. At ECW these are operating very successfully since 1930 and are the worlds largest ensemble of wastewater fisheries covering an area of approximately 4,000 ha and providing employment to more that 4000 families (2). Though both treat wastewater but the architecture as well as associated purification mechanism of Bheri is quite different from any other wastewater fed fisheries elsewhere. These are shallow, 50 to 150cm deep, flat bottom ponds which are prepared following certain specific procedure (3). The shallow depth, immense sunlight, constant aeration due to stirring as well as continuous supply of input (in large bodies) along with algal photosynthetic oxygen release, growth of plankton, introduction of fish at a specific point of plankton growth, the liming of the floor of the pond during preparation and the water hyacinth surrounding the edges together result in the reduction of coliform count up to 96 to 99% and metal content upto 25 to 99% in the Bheri (4). Being shallow Bheri receives ample solar energy which is trapped by a dense population of plankton which in turn is consumed by the fishes. The plankton play a significant role in degrading the organic matter, but its overgrowth becomes a problem for pond management by causing algal bloom. It is at this critical phase of the ecological process that the fish plays an important role by grazing on the plankton. The two fold role played by the fishes is indeed crucial--they maintain a proper balance of the plankton population in the pond and also convert the available nutrients in the wastewater into readily consumable form (fish) for humans.

Many industries, especially electroplating, battery and plastic manufacturing release heavy metals such as cadmium and zinc in wastewater (5-6). Mobilization of heavy metals in the environment due to industrial activities is of serious concern due to the toxicity of these metals in human and other forms of life (7-8). Removal of toxic heavy metals from industrial wastewater is essential from the standpoint of environmental pollution control (9). Conventional techniques of controlling metal-contaminated water (for example, chemical treatment) are costly in terms of equipment and chemicals. Moreover, they require intensive management and long-term maintenance (10). There have been extensive studies on the extent of metal accumulation in different parts of the fishes (11-17)

In this study we attempt to analyze the extent of metal accumulation using EDXRF in the muscles of two specific types of fishes from Bheri. One of them, Labeo rohita, is a surface feeder and the other, Cirrhinus mrigala, is a bottom feeder. The analysis of these two layers in...

To continue reading