Title : Prospect and challenges of Sewage Fed Pisciculture : East Kolkata wetlands (Ramsar) experiences
Abstract:
Waste treatment function of the East Kolkata wetland is considered as the main function of this Wetlands of international importance is being compromised due to increase in the presence of heavy metals along with the organic nutrients in sewerage supplied to EKW fishponds. It is observed predominantly Kolkata city?s industrial effluents are also mixing in the Sewage. Thus, Wetland function of treating raw sewage is getting affected by settling of heavy metals and sludge of Industrial pollutants in the fish farm beds and also lead to bioaccumulation of heavy metals in fish and plant species. Presence of heavy metal contaminants in wetlands pose health risk to the producers and consumers dependent on resource harvest from wetlands.
Wastewater of the city is discharged into the fish farms locally known as bhery. This wetland system exhibits immense potential in remediating the water quality by reducing the high amounts of biochemical oxygen demand (BOD) and chemical oxygen demand (COD) it receives from the inflow on a daily basis. Each hectare of the shallow waterbody has the ability to remove 237 kg of BOD per day.
The bhery being shallow allows full vertical circulation of water to the surface where algal blooms occur. A solar radiation that is about 250 Langley’s a day, allows sufficient photosynthesis which augment reoxygenation to allow for efficient BOD and pathogen/faecal coliform reduction. Factors that play role in water purification are the shallow ponds acting as stabilization ponds, abundant water hyacinth that absorbs heavy metals, sun light penetrating to the bed of the water body and other microbial components that help in bioremediation (Raychaudhuri et al, 2008).
The commercially important fish and shellfish in the EKW include 79 species of fish (Chandra, Raghunathan and Mao 2020), 11 species of prawns, 3 species of crabs and 20 species of molluscs. Among the fish species, 17 are culture species and 41 are wild species.
Traditionally, aquaculture in the EKW was predominantly of indigenous Indian Major Carps (IMC), (Rohu (Labeo rohita), Catla (Catla catla) and Mrigal (Cirrhinus mrigala), and sometimes minor carp Bata (Labeo bata). In the 60s, exotic fish species were introduced, which included Common carp (Cyprinus carpio), Silver carp (Hypophthalmichthys molitrix), Grass carp (Ctenopharyngodon idella) and Mozambique tilapia (Oreochromis mossmbicus). The Nile tilapia (Oreochromis niloticus) has replaced Mozambique tilapia in recent times. In limited quantities, walking catfish (Clarias batrachus) and striped catfish (Pangasianodon hypophthalmus) are also produced, and attempts made to introduce Seabass (Lates calcarifer) enhance farm incomes 27 ornamental fish species of 21 families have also been recorded from EKW (Mahapatra and Lakra 2014). Previously the fish fauna in the EKW system comprised both brackish water and fresh water forms. reported occurrence of 80 species of fishes from the Salt Lake.
Further it is observed entry of 4 exotic species (Crocodile fish) in EKW in recent years. Exotics Clarius guriepinnus and Pangasius sutchi have also been recorded from the aqua culture farm. Further, 63 Indigenous species Nandous nandus and Xenentodon cancila which were abundant during 1980’s has not been recorded at present. The rapid spread and population increase of suckermouth armoured catfishes belonging to the genus Pterygoplichthys (Loricariidae) in EKW in recent times is of increasing concern, because of the notable possibility that these non-native catfishes are adversely affecting fish germplasm and commercial fishery of this unique ecosystem.
But this unique sewage fed fisheries is facing number of problems which requires improvement of fish culture technology-optimization of yield, improvement of Pond design keeping in mind comprehensive production system in the realm of treatment of sewage and epidemiologically sound fish quality.
