World Aquaculture - September 2023

30 SEPTEMBER 2023 • WORLD AQUACULTURE • WWW.WAS.ORG aquaculture (LEISA), fostering the production of aquatic proteinrich food (Edwards 2008). EKW, previously known as the Waste Recycling Region (WRR), has been an example of the wise use of wetlands when defined as “their sustainable utilization for the benefit of humankind in a way compatible with the maintenance of the natural properties of the ecosystem” (Ramsar Convention Secretariat 2000). Rapid urbanization leads to inevitable scarcity of freshwater and increasing amounts of sewage discharge from domestic sources. However, sewage has potential for the future because it can render three important services: (1) water conservation as it mixes with freshwater to increase water levels, (2) nutrient recovery from organic-laden sewage that fertilizes freshwater to promote East Kolkata (now Calcutta) Wetland (EKW), the largest sewage fed aquaculture system in the world, started its journey in the 1930s (Bunting et al. 2010). Kolkata city discharges an average of 1300 mL/d (million liters/day) of municipal sewage through more than 14000 km of drains and canals (Figure 1), but 320 mL/d is available to feed 130 sewage-fed fish farms, covering a total area of 4000 ha. Municipal sewage is used to fertilize ponds, popularly known as “bheris,” for an average daily production of 50 t of fish. This amounts to more than 15,000 t of fish annually for consumers (Ghosh, 2018). The reuse of organic wastes emanating from municipal domestic sources has been increasingly important, including as an alternative source of fertilizer for low-external-input sustainable Sewage Fed Aquaculture in East Kolkata Wetland R. N. Mandal, Farhana Hoque, S. Adhikari, B. N. Paul, and D. N. Chattopadhyay FIGURE 1. A long narrow canal carrying sewage from Kolkata city to a ‘Bheri.’