Introduction

Wetlands (lakes, ponds, tanks, areas of marsh, fen, peat land, whether natural or artificial, permanent or temporary, with water that is static or flowing, fresh, brackish or salt, including areas of marine water) in urban landscape aid in (i) moderating microclimate, (ii) recharge groundwater resources, (iii) remediation – kidneys of the landscape remove contaminants, (iv) sustaining food (fish, fodder etc.), (iv) recreation and habitat for array of diverse organism, (vi) infiltration and mitigate floods and loss to human life s and properties, (vii) enhance the aesthetics of the landscape and support many significant recreational, social, and cultural activities, aside from being a part of region’s cultural heritage.

Wetland ecosystems are the most productive ecosystems in the biosphere. Lakes provide the essential components for the sustenance of life on earth, such as water (agriculture, drinking, etc.), food (protein production, fodder, etc), biodiversity (diverse flora and fauna), energy (fuel wood, etc), recreation (tourism), transport, water purification, pollutant sink, flood control and climate stabilizers (Ramachandra, 2008). Lakes and ponds naturally support a healthy population of aquatic life. Lentic ecosystems (still waters that include wetlands, ponds, tanks, lakes, etc.) have three zones – littoral, limnetic and benthic. The littoral zone is near shore area where sunlight penetrates all the way to the sediment and extends from the shoreline to the innermost rooted plants. Littoral zone is populated with frogs, snakes, snails, clams, and a variety of adult and larval insects. The limnetic (pelagic) zone is the open water down to the depth of light penetrations. This zone contains phytoplankton (diatoms, green and blue green algae, etc.), zooplankton (protozoa, microcrustaceans, arthropods, etc.). It is also inhabited by a variety of larger swimming organisms including fish, amphibians and larger insects. The benthic zone (bottom of the lake) is covered by fine layers of mud consisting mostly of decomposers (Ramachandra and Solanki, 2007). Watershed based planning and resource management is a strategy for more effective protection and restoration of aquatic ecosystems and for protection of human health. The watershed approach emphasizes all aspects of water quality, including chemical water quality (e.g., toxins and pollutants), physical water quality (e.g., temperature, flow and circulation), habitat quality and biological health and biodiversity (e.g., species abundance, diversity etc.) (Ramachandra, 2005). Wetlands ensure water security evident from higher groundwater availability in the vicinity. Valuation of wetlands goods and services highlight its role in supporting the local livelihood  to the extent of Rs 10500 per hectare per day. Water and food security provided by these ecosystem necessitate its conservation and sustainable management to maintain intergenerational equity.

Various threats faced by lakes include encroachment of lakebed, flood plains and lake itself; encroachment of rajakaluves/storm water drains and loss of interconnectivity; topography alterations in lake catchment; unauthorised dumping of municipal solid waste and building debris; sustained inflow of untreated or partially treated sewage and industrial effluents; removal of shoreline riparian vegetation and pollution due to enhanced vehicular traffic etc. (Ramachandra et al., 2013). The land use analyses of greater Bangalore showed 925% growth in built-up area during the last four decades (1973 to 2014) with the decline of vegetation by 78% and water bodies by 79%. The analyses of the temporal data of greater Bangalore revealed an increase in urban built up area of 342.83% (during 1973–1992), 129.56% (during 1992–1999), 106.7% (1999–2002), 114.51% (2002–2006) and 126.19% from 2006 to 2010 (Ramachandra et al., 2012). The elevated nutrient levels through sustained inflow of untreated sewage brings about undesirable changes in lakes like (a) unpleasant odor and taste, (b) excessive algae growth, (c) reduced water clarity, (d) release of toxins from blue-green algae, (e) low dissolved oxygen, (f) changes in fish population or large scale fish mortality (Ramachandra et al., 2015). Sustained inflow of untreated effluents waste from industries, domestic sewage into water bodies triggers stress in aquatic ecosystems.  Enrichment of nutrients especially compounds of nitrogen and or phosphorus in water bodies referred as eutrophication, leads to an accelerated growth of algae and higher forms of plant life to produce an undesirable disturbance in trophic levels in the aquatic ecosystem and to the quality of the water concerned (EPA, 2001).

1.1 Ulsoor lake or Halasuru Lake derives its name from the locality it is situated, namely, Ulsoor, close to MG Road. It is located on the eastern side of the city with the spatial extent of over 50 ha (123.6 acres) with an average depth of 5.8 m; 3 km of shore length and located at 900 m of elevation. This lake is built by the family of Kempe Gowda-II in 17th and 18th centuries for drinking and irrigational purposes and is located in the middle of the city. During the early 19th century, this lake was the major source of drinking water for the cantonment area and troops. The catchment area of Ulsoor Lake is 1.5 km2. The lake has a recreational complex with a swimming pool. There is also a garden at one end of the lake and there are many islands that are spread across the Ulsoor lake and tourists reach these islands via boating.

1.2 Fish mortality in lakes: Fish kill is an indicator of environmental stress, declining of aquatic ecosystems health and water quality problems. Furthermore, investigations of fish kill will indicate the spatial and temporal distributions of pollutants (e.g. nutrients, pesticides, trace metals, oils) and problems (e.g. hypoxia, anoxia) in aquatic environment (Kibria, 2015). Weather patterns, water temperature, salinity, pH, ammonia, nitrite, carbon dioxide, turbidity, dissolved oxygen, pesticides, stress, thermal shock, toxic algae, toxins, fish community structure and disease outbreak (due to the presence of viruses and bacteria in the water) are the factors that triggers fish kill/mortalities. Fish also become stressed during a low DO period and become more susceptible to viral or bacterial infections (Njiru, 2015). Fish can suffer from various types of protozoan (Ichthyophthiriasis, Costiasis, Trichodiniasis, Epistylis and Myxosporidians infestation), fungal (Saprolegniasis, Branchiomycosis and Icthyophonus disease), bacterial (Fin and tail rot fin diseases, Ulcer/Columnaris disease, Dropsy and Eye disease), viral (Spring viremia and Fish pox), crustacean (Argulus infestation and Learnaea disease) and helminth diseases (Sharma et al., 2012).