Mumbai’s Powai lake is dying, becoming toxic for its fish | mumbai news | Hindustan Times
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Mumbai’s Powai lake is dying, becoming toxic for its fish

Mumbai city news: Powai lake is home to carp, eels and other varieties of fish, apart from a small colony of crocodiles, but has already lost many native species.

mumbai Updated: Jun 15, 2017 12:56 IST
Badri Chatterjee
Dissolved oxygen is an important measure of water quality, as it is a direct indicator of a water body’s ability to support life.
Dissolved oxygen is an important measure of water quality, as it is a direct indicator of a water body’s ability to support life.(Prashant Waydande)

Powai lake, whose scenic shores are now lined with posh housing complexes, is dying. Sewage flowing into the lake has reduced levels of dissolved oxygen (DO) to the lowest since 1961.

Dissolved oxygen is an important measure of water quality, as it is a direct indicator of a water body’s ability to support life. Powai lake is home to carp, eels and other varieties of fish, apart from a small colony of crocodiles, but has already lost many native species.

DO levels have dropped to a minimum of 1.7 milligram per litre (mg/l) at the lake, a water quality study found. As per the Central Pollution Control Board (CPCB), the permissible limit of DO level for propagation of wild life and fisheries is 4 mg/l or more.

Read: Powai lake’s pollution levels five times the safe limit: Survey

PB Salaskar, water quality analyst and secretary from the Naushad Ali Sarovar Samvardhini (NASS), carried out the study and identified the current DO range at Powai to be between 1.7-7.9 mg/l. Between 1961 and 1963, DO levels were in the range of 2.80-7.80 mg/l and 4-12.40 mg/l between 1975-77. Between 1987-89, the range fell between 3.60-12 mg/l and in 1995-96, the range was 3.60-6.31 mg/l. A study in 2016 by Naushad Ali Sarovar Samvardhini, All India Game Fishing Association and the Maharashtra State Angling Association (MSAA) found only 10 native fish species as opposed to 37 species recorded in 1990.

How can Powai be saved?
  • Organise mass awareness training programmes for the locals about the importance of water bodies
  • Create public participation and involve water-related agencies in the maintenance of the lake
  • Involve educational and scientific institutions in periodically monitoring the quality of lake water and in conducting biodiversity surveys
  • Collate the scientific data collected and make it available to everyone, so management strategies can be devised
  • Involve all the stakeholders
  • Construction of a boundary wall after proper survey to prevent encroachments
  • Reducing pollution by improving water quality of the lake through distillation and bio-remediation
  • Installation of flap-gates, construction of proper channels for adequate release of sewage and water-shed management techniques

“The amount of oxygen in the water is an important indicator of overall lake health. DO is the most important parameter which can be used as an index of water quality and pollution,” said Salaskar. “With the drop in DO levels, it is clear that the pollution level at Powai is increasing at an alarming rate. Large fluctuations in DO levels over a short period of time may be the result of an algal bloom, and this is deadly for a water body.”

As opposed to Powai, the study added that unpolluted lakes such as Tulsi and Vihar – which are located inside Sanjay Gandhi National Park, have high transparency, less fluctuations in DO levels and high aquatic diversity.

He added that low levels of oxygen or no oxygen levels can occur when excess organic materials, such as large algal blooms, are decomposed by microorganisms. Salaskar studied the transparency levels of the lake due to the algal bloom and found transparency levels have reduced from 120 cm in 1998 to 17 cm in 2017, which means a drop of 86% in visibility levels. “The only way to stop this is if the civic body conducts a detailed study for the source of water pollutants and sewage entering the lake and then diverts it,” he said.

Read: Temporary ban on boating at Mumbai’s Powai lake

Some organisms, such as catfish and carps, can survive in lower concentration of DO (minimum 2mg/l). “However, it is very difficult for all other species, especially native ones, to survive at the lake. It is estimated that if current pollution levels continue, the entire aquatic diversity of Powai can be wiped out in the next 20 to 25 years,” said Salaskar.

An earlier study from last year saw pollution levels at the 2.1 sq kilometre lake were five times the safety limit. Levels of biochemical oxygen demand (BOD) – level of oxygen that affects the quality of water allowing plants and animals to survive – to be ranging between 27 and 34 mg/L at five locations across the lake, found Salaskar. A BOD reading of more than 3 mg/l is unfit for human consumption, and above 6 mg/l is unsuitable for the survival of fish species, according to CPCB guidelines.

AUTHORITIES SPEAK

“We have been carrying out cleaning operations regularly at Powai. There is a management plan that is in place and it is being followed by our officers. We are also sensitising locals in the area not to dump sewage at the lake,” said a senior official from the civic body’s hydraulics department.

OTHER ISSUES PLAGUING THE ‘GREEN’ LAKE

Crocodile habitats being disturbed due to fishing

In a letter to the forest department, members of Naushad Ali Sarovar Samvardhini (NASS) highlighted last week that extensive fishing at Powai, being done daily, is reducing food for crocodiles and it can force them to look for other options. “Crocodiles at Powai have always known to been harmless unless attacked or disturbed by human beings. But since they have no food, there may be chances of attacks on human beings venturing into the water body,” said a member of NASS. “They currently have no place to bask, no place to lay eggs because of interference by poachers who go on tubes to catch fish.”

The Indian Marsh Crocodile is a highly endangered species, which is a schedule-I species under the Wildlife Protection Act, 1972, that have made Powai lake their natural habitat for many decades.

-Locals unable to get their daily fish catch

Owing to the issue of overfishing at the lake combined with the algal blooms, loss of oxygen, and high level of sewage, the local tribal population living around Powai for decades is not able to catch fish. “Many generations have been surviving on fish from the lake as our staple food. However, the condition has become so poor that we have been forced to search for alternative employment options in other parts of the city,” said Mangesh Inamdar, local resident. “Repeated complaints have gone unheard by the civic body.”