The dramatic fishkills in Ulsoor and Devarabisanahalli lakes recently had an obvious reason: asphyxiation with a sudden and considerable fall in the dissolved oxygen (DO) levels.
But an Indian Institute of Science (IISc) study has added another human factor of irresponsibility: introduction of exotic fish species such as African Catfish and Tilapia that are a threat to the native fish, and ammonia toxicity.
Exotic fish species eliminate native biota and affect local biodiversity, concludes the study led by Dr T V Ramachandra. More such fish-kills could be in the offing due to the indiscriminate growing of such species in lakes across Bengaluru.
Killing predator fish
To conserve the native fish, the study has recommended total eradication of the African Catfish, which can attain a size of 59.5 kg and get highly predatory, feeding on other species. The predator fish could be controlled by preventing its access to get its oxygen (since it is an air-breathing fish) from the atmosphere.
How do you do this? “By spreading a net of different mesh size, below 4”-6,” from the surface water area of any water body. This process prevents the fish from coming to the surface to breathe and gets choked,” the study explains.
But the big culprit remains the unregulated inflow of untreated sewage into lakes. Temperatures suddenly go up beyond 34 degrees Celsius, triggering depletion of dissolved oxygen (DO). Higher temperature in sewage-fed lakes also raises the level of ammonia. “Ammonia is toxic to fish at elevated pH and temperature. Fish become more susceptible to viral or bacterial infections during a low DO period and thermal stress (cold or warm temperature shock).”
Most fish can tolerate DO below 2mg/l for short periods, but start dying when the DO drops below 1mg/l.
Dumping of solid waste is another critical factor attributed to fish mortalities. A major fraction (70-75%) of solid waste is organic and the nutrients leach to the water body. The study says only treated sewage should be allowed to get into the lake. There should be no diversion of sewage of one locality to downstream localities. Sewage generated in each locality should be treated locally.
Sewage that goes through a secondary treatment process could be made to pass through constructed wetlands to remove the nutrients. The study notes that this model has been working satisfactorily at Jakkur lake. Water fountains are suggested to improve aeration in the lake.
