|Studying Water Pollution through Fish Assessment (Indian Institute of Sciences)|
Lakes and tanks are known to be the ecological barometers of the health of a city. They regulate the micro climate of any urban centre.
Bangalore district has about 141 lakes. The government is spending millions for development of these lakes. Rapid urbanisation has lead to the loss of wetland habitat through encroachment, bad management, pollution from sewage, and waste and litter disposal activities. These factors have seriously affected the survival of tanks and lakes and have posed serious threat to the flora and fauna supported by them.
Sankey Lake is among the few lakes in the northern part of the city with continuous drains. It is also the main source of ground water to this part of Bangalore. It harbours a rich biodiversity which includes birds, fishes, aquatic plants and microbes. The presence of a biotically diverse and beautiful botanical garden and a forest nursery adjacent to the lake increases the ecological value. The Bangalore City Corporation is also developing a park at the north western end.
The 'Sankey Lake,' situated in the heart of Bangalore City (Lat.:13° 00'24" - 13° 00'41"N; Long.:77° 33'53" - 77° 34'5"E; altitude: 921 m MSL, maximum water spread area 12 ha, maximum depth 23 ft, average depth 9 ft), is a 500 year old, perennial water body and supports a significant biotic community. Since the beginning of 1982, drainage of industrial effluent and other domestic sewage into the lake has been stopped and the lake is expected to be free from noticeable pollution.
Long-term studies on hydrology and microbial ecology, conducted during the last decade, have indicated that Sankey Lake has high potentiality for development of inland fisheries practices . The average annual photosynthetic profile suggests the significance of the heterotrophic food chain in sustaining the higher trophic levels. With a mean fish production of 859 metric tonnes/year, the present fish production efficiency works out to 0.43 per cent. Since the lake is still mesotrophic and is amenable to management measures, a higher target fish production appears quite feasible [1, 2].
Sankey Lake was studied for its hydrobiological characteristics by many agencies like the Central Inland Fisheries Research Institute. the Central Institute of Freshwater Aquaculture, Bangalore University, University of Agricultural Sciences and the Indian Institute of Science from time to time but a comprehensive study was not carried out by these agencies covering all aspects of ecology [1,2,3,4]. This lake forms a nucleus for research investigations and an ideal breeding ground for commercially important fishes like Etroplus suratensis, Murrels (Channa marulius), Catfishes (Heteropneustes fossilis), small palaemonid prawns and the commercial variety, Macrobrachium malcolmsonii. This lake harbours Tilapia along with many other smaller fishes. In the past, the lake was stocked at times with small numbers of freshwater prawns (Macrobrachium malcolmsonii). The results were satisfactory, thus providing evidence of its potential for freshwater prawn culture as well.
Cage culture is the growing of fish in an enclosure of fish netting material, such as nylon, to monitor growth, productivity, and survival of different species like Catla, silver carp, common carp, Tilapia and Mrigal. In the first ever cage culture experiment (from fry to fingerlings stages) conducted in this lake for four months, common carp fry at a stocking density of 2.13 lakh/ha, showed a survival of 97.5% and 88.0% for silver carp. Cage culture (fingerlings to table size) of common carp, Catla has shown a production range of 92 to 225 t/ha with a survival rate from 80 to 100%. Experiments of cage culture with peninsular carps like Labeo fimbriatus, L. calbasu were also undertaken in this lake. Artificial breeding of the cage reared L. calbasu and common carp was also tried. A viable hybrid of common carp, calbasu, has been produced for the first time in this country from cage reared fishes of Sankey Lake .
Heavy washing of clothes by dhobis and continued entry of domestic sewage in some areas are posing pollution problems. Rapid growth of human population, proliferation of buildings, roads and vehicular traffic in Bangalore have taken a heavy toll of wetlands. Further, encroachment, disfiguring by brick/tile industries, waste disposal activities and bad management have threatened the very existence of many of the valuable and productive wetland habitats in the city, thereby posing serious threat to the flora and fauna supported by them. Although there is wide public concern about wise use of wetlands, lack of knowledge of the ecological conditions of these habitats has caused many losses. The loss of environmental benefits could be very crucial in a situation which the Bangalore city faces today [3,4,5]. Sankey Lake is the only wetland which has withstood the changes in the growth of Bangalore even though it is located in the heart of the city. As a result of human activity over the years, accumulations of silt and clay have led to changes in the pattern of sediment-water exchange. Dissolved oxygen is not a limiting factor and thus the water has promoted the growth of 21 species of phytoplankton.
Although, the human activity has resulted in the enrichment of nutrients in the sediment, their level in water still remains low due to poor exchange of nutrients in the sediment .
Comparison of plankton species made from three different studies
during 1981-84, 1982 and 1989 indicated that both the phyto and zoo plankton
species richness has been increasing over the years [1,3,4].
In the 1989 survey, though Microcystis was observed to be dominant among the phytoplankton species, an increased number of Myxophyceae forms is considered ecologically significant. A total number of 27 phytoplankton and 28 zooplankton species have been recorded from this lake. The 1989 survey  covering 97 sites listed 58 plankton and 55 zooplankton species for the Bangalore area. This means that nearly half the plankton species richness of Bangalore is found in this lake which is right in the heart of urban Bangalore. From the point of view of the lake waters, the progression in plankton species richness from 1981 to 1989 probably indicates an improving situation .
Nevertheless, this ancient lake which reflects the cultural heritage of the city, has an annual average photosynthetic value ranging from 0.81-1.42% and has a mean fish production level of 36.57 gm-2 . The important role of Sankey Lake needs to be highlighted, which has been maintaining the ground water level in the surrounding areas, that includes Malleswaram, Palace Orchards, Rajamahal Vilas, Vyalikaval, Palace Gutthalli and Yeshwantpur. Careful observation of the lake morphology, hydrodynamics and sedimentation, show that the water from Sankey Lake that seeps through the soil is not lost but recharges the natural underground reservoir which in turn supplies water to wells and bore wells in all the areas mentioned [1,2,3]. This lake has great fish potential, supports human environmental needs and contributes to climatic stability. Thus the multiple contribution of the Sankey Lake to the city's economy through ground water recharge and the ecological role of the tanks needs to be recognized.