Poisoned wetlands, toxic fish

Fishermen use wastewater in Kolkata to rear fish (Image Source: India Water Portal)
Fishermen use wastewater in Kolkata to rear fish (Image Source: India Water Portal)
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The East Kolkata Wetlands, often referred to as the ‘Kidneys of East Kolkata’, are located on the eastern periphery of Kolkata Metropolitan area and cover 12,500 hectares of area. The entire geographical region of EKW is dominated by water bodies along with 254 sewage-based fishing grounds (5852.14 hectares), agricultural land (4718.56 hectares), productive farming zone (602.78 hectares) and urban and rural settlement areas (1326.52 hectares) respectively.

What is so unique about EKWs?

Chakraborty and Das Gupta in their report inform that the city of Kolkata generates an estimated 750 mililitres of sewage every day. This wastewater is converted into food and used in fisheries and agriculture across the sprawling 12,500 hectares of wetland area, and the process through which the waste gets converted into food is unique as well.

The wastewater from the city is led by underground sewers to pumping stations in the eastern limit of the city and then pumped into open channels. This sewage is then drawn by the local fishery owners into the fish ponds or bheris directly from the tributary wastewater canals.

Bheris are a unique feature of the Kolkata wetlands, and are shallow fishponds fed by naturally treated wastewater rich in algae, which allows for low-cost fish cultivation. Organic waste gets loaded in the bheris at the rate of 20 to 70 kg per hectare per day. It is allowed to stay in the sun and these sewage filled fishery ponds act as solar reactors.

The heat generated by the sun is adequate for photosynthesis to take place and helps in the growth of a dense plankton population in the bheri, which in turn grows on the organic matter in the wastewater. This high growing plankton serves as food for the fish population who thrive on this nutrient rich plankton. The fish play a two fold role – they maintain the balance of the plankton population in the pond and convert the available nutrients in the wastewater into readily consumable form (fish) for humans.

Organic pollution in the wastewater is thus reduced by 80 percent and the coliform bacteria in the wastewater are reduced by 99.9 percent in these ponds. Channels drain out the effluents and slurry from the treated wastewater, that is then used to grow rice and vegetables. Around a quarter of the city’s fish and vegetables are grown from the bheris.

The wetlands help in providing cheap food and vegetables for the city, but also support the livelihoods of about 1,18,000 people.

Pollution and deterioration of pisciculture

Informal cultivation of fishes and crops in wastewater has been practised in Asia since along time informs this paper titled 'Assessment of the bioaccumulation pattern of Pb, Cd, Cr and Hg in edible fishes of East Kolkata Wetlands, India' published in the Saudi Journal of Biological Sciences.

Fish are an important part of the diet of people living in the lower Gangetic delta. Easy availability of large variety of fishes, that are rich in omega fatty acids, protein and low density lipoprotein are key factors for health of populations and help in providing the necessary nutrition for people living in the area.

However, increasing urban spaces, agricultural activities, pollution of ecosystems, industrialisation, oil-spills, overfishing are affecting the aquaculture sector negatively. Besides, pesticides, industrial effluents and heavy metals are also a major nuisance leading to the deterioration of pisciculture mainly in developing economies such as India.

Fish are very sensitive to water pollution and bioaccumulation due to direct contact with water. Heavy metal contamination of water not only affects the health of fish negatively, but also the people who consume the fish.

Heavy metal pollution of wetlands

The paper discusses the implications of bioaccumulation of toxic heavy metals on aquaculture practices in the East Kolkata Wetlands. Heavy metal toxicity can have devastating implications on the ecological health of freshwater as well as marine ecosystems. Environmental toxicants such as heavy metals can accumulate in the fish body through osmosis and persistent trace metals such as lead, chromium, cadmium, arsenic and mercury are also known to have a negative effect on health of humans when they consume fish affected by heavy metals.

For the study, toxic heavy metal bioaccumulation pattern was studied for selected fish species namely Rohu, Catla and Nile Tilapia cultivated in the bheries of the East Kolkata Wetlands over the four seasons namely, pre-monsoon, monsoon, post-monsoon, and winter for three consecutive years namely 2016, 2017 and 2018.

Heavy metal accumulation in fish

  • The study found the presence of toxic metals such as chromium, lead, cadmium and mercury in Rohu, Catla and Nile Tilapia fish. Heavy metal accumulation trends increased over the three years and monsoon season showed maximum accumulation trends followed by post monsoon, winter and pre monsoon respectively.
  • The accumulation was least in the pre-monsoon season owing to low rainfall, high evaporation rates, soil dryness, and lesser rates of heavy metal dissolution in surrounding waters.
  • The leaching of nutrients from water, soil and run-off from adjoining industrial and highly urbanised areas along with wastewaters from point and nonpoint sources were responsible for heavy metal contamination of the wetlands and accumulation of the metals in fish.
  • The persistence and the degree of heavy metal toxicity in water and sediments was found to depend on factors such as water flow, water speed, frequency of tides, monsoonal run-off.
  • The bulk of monsoonal run-off received by the East Kolkata Wetlands from point and non-point sources was also a main source of heavy metal load in the wetland.
  • High monsoonal run-off along with lowering of pH levels lead to acidification. This increased dilution of toxic heavy metals in soil or water and the heavy metal locked up in sediments dissolved in water and got into the bodies of fish.
  • Fish uptake of heavy metals happened through gills, fins, and muscles through osmosis leading to precipitation in the body parts. The bioaccumulation pattern of toxic heavy metals in commonly consumed fish were higher than normal values.

This heavy concentration of toxic heavy metals in fish rings an alarm bell with respect to ecological health of these wetlands as well as the people's health who consume this fish, argues the paper. The paper proposes increasing awareness among fish farmers through local government backed knowledge dissemination facilities and mechanisms such as diversion of monsoonal influx of waste water and preliminary treatments before being used for aquaculture practices to reduce pollution loads in the wetlands.