High pollution levels, along with overpopulation of invasive fish species, has led to a 70% decline in native fish species in Powai Lake over the past two decades.
A recent study carried out at the 2.1-sqkm lake by Powai-based non-profit organisations found only 10 native fish species present in the lake, compared to 37 species recorded in 1990.
“The introduction of a number of non-native species such as the African magur, Mozambique tilapia, Arapaima, alligator gar and another 20 such aquarium species have altered the food chain,” said Ali Husaini, president, AIGFA, adding the magur is the most predacious of all other foreign species.
Native species such as the mahaseer, chilva, four species of putnis, barramundi have gone missing, and those such as catla, rohu, mrigal, grass carp, sneakhead or murrels, spiny eel and others continue to survive in the lake.
The survey was carried out by Naushad Ali Sarovar Samvardhini (NASS), All India Game Fishing Association (AIGFA) and the Maharashtra State Angling Association (MSAA). According to MSAA, Powai Lake was created by the British in 1890 by constructing two stone dams on the Powai rivulet.
To increase the count of native species, MSAA introduces 10,000 mahaseer and 10 lakh Indian carps into Powai Lake every few years from Aarey hatcheries and other sources across India. “The fish, however, do not breed owing to water pollution that has led to a loss of oxygen at the lake,” said Husaini. “Other problems such as siltation, low fertility and over fishing are some other reasons that have led to their decline.”
Officials from MSAA said that unidentified persons have been releasing goldfish at the lake, which is predominantly an aquarium fish. “The aquarium hobby in Mumbai is widely pursued by many, but after a while hobbyists either lose interest or cannot manage them properly. Hence, they release their fish into water bodies like the Powai Lake,” said Gordon Rodricks, vice president, MSAA.
He said the MSAA also procures fish from aquaculture farms, and it is difficult to identify each species from a complete batch. “Suppliers sending us fish batches, which have been spawned at local ponds that consist of some invasive fish species, inadvertently find their way to the lake,” said Rodricks.
Researchers said there is a need for mass awareness among anglers to stop the introduction of catfish into water bodies. “The Indian government has already banned introducing catfish into systems where they are absent. Large-scale environmental education drive is necessary to change the mindset of anglers about the illegal introduction,” said PB Salaskar, water quality analyst and secretary, NASS.
HT had reported last Thursday that pollution levels at the 2.1-sqkm 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 27mg/L and 34mg/L at five locations across the lake. A BOD reading of more than 3mg/L is unfit for human consumption, and above 6 mg/L is unsuitable for survival of fish species, according to the Central Pollution Control Board guidelines.
Native fish species still surviving at Powai Lake
Catla catla (Catla) , Labeo rohita (Rohu), Cirrinus mrigala (Mrigal), Labeo calbasu (Calbasu), Osphronrmus goramy (Gouramy) Ctenopharyngodon idella (grass carp), Hypophthalmichthys molitrix (Sliver Carp), Channa argus (Snakehead Fish or Murrels), Cyprinus carpio (Common Carp) and Macrognathus ( Spiny Eel).
The missing species
Mahseer, Puntius, Barramundi, Chilva
The Predacious Catfish
Weighing in at about 10-15kg, the mangur fish or African catfish (Clarias garipinius) is found in many water bodies. They can survive extreme conditions and factors such as - omnivorous diet, ability to survive on land, burrowing capabilities and ability to hide in vegetation - make this species very difficult to control.