Untreated sewage, water stagnation caused fish deaths near Mangaluru

Mangaluru: The mass deaths of fish reported near Raikatte Bridge in Mangaluru was likely caused by untreated sewage inflow, water stagnation, and depletion of dissolved oxygen levels, according to the inspection by the Regional Office, Karnataka State Pollution Control Board (KSPCB),

The Regional Officer of KSPCB, Mangaluru, said that officials had received complaints on February 9 regarding death of fish and blackening of the storm water drain near Raikatte Bridge, which joins the backwaters of the Gurupura river.

During an inspection conducted the same day, officials observed sewage flowing through the storm water drain into the backwaters.

The water in the drain had turned blackish and several dead fish were found in the drain that connects to the river. Preliminary observations indicated that the fish deaths were likely caused by depletion of dissolved oxygen in the water.

Samples from different locations were collected and sent to the Regional Laboratory of KSPCB in Mangaluru for analysis. Officials also confirmed that no industrial effluent discharge was observed from industries in the Baikampady industrial cluster during the inspection. The Department of Fisheries had also  collected fish samples for testing, and the results are awaited.

On February 10, officials noted that construction work was underway for the Kuloor bridge, as well as the bridge connecting Sultan Battery and Tannirbhavi Beach.

The construction of the Sultan Battery–Tannirbhavi bridge, which spanned around 350 metres, involved filling portions of the river with soil.

The construction activity had disrupted the natural flushing of water during high and low tides up to the Maravoor vented dam.

The presence of untreated sewage in the river and limited freshwater flow had created stagnant, cesspool-like conditions in the river. Owing to high summer temperatures, biogeochemical reactions intensify, leading to depletion of dissolved oxygen.

As proper flushing does not occur during high tide and low tide, anoxic conditions have developed resulting in reduced dissolved oxygen levels and fish kill.

The report also noted that residential areas around Raikatte, Kuloor, Baggundi Lake, Kodikere, Kudumburu Colony, and Angaragundi, along with godowns and commercial establishments in the Baikampady industrial area, contributed to sullage.

The area lacks underground drainage facilities and sewage treatment plants. Officials added that related cases are already being heard before the National Green Tribunal (NGT).

Following the inspection, KSPCB issued directions to authorities to take up corrective measures. Mangaluru Smart City Ltd was instructed to install culvert pipes to ensure proper flushing of the backwaters.

The Mangaluru City Corporation was directed to identify and stop sewage entering the Gurupura river through storm water drains and submit an action taken report.

The Karnataka State Coastal Zone Management Authority was asked to submit a report clarifying whether the Smart City bridge construction project was being carried out in compliance with Coastal Regulation Zone (CRZ) conditions.

Based on the direction, Mangaluru Smart City Ltd has installed culvert pipes to restore tidal flushing in the affected area. Officials said the condition of the backwaters and the river arm where the fish kill occurred has improved.