A large number of fish were found floating along the Phalguni backwater stretch downstream of the Maravoor vented dam on Thursday, triggering alarm among residents and prompting an investigation by the Karnataka State Pollution Control Board (KSPCB).
Preliminary findings pointed to suspected discharge of untreated or partially treated sewage into the stretch, sources said. A senior official told TOI that initial inspections suggested raw sewage from the Pachanady Sewage Treatment Plant (STP) could have entered the water body. Teams inspected the STP, and a proposal is already in place for the plant's upgradation.
"We suspect that untreated or partially treated sewage with a high organic load may have been released into the river over the last two to three days. This could have depleted oxygen levels in the water, leading to the fish deaths, possibly during the night," the officer said.
Samples have been collected from multiple points along the affected stretch for laboratory analysis. Officials from the fisheries department and the College of Fisheries have also been asked to collect fish samples to determine the exact cause of death.
Officials said rising temperatures and reduced water levels in the river may have worsened the situation. "When water levels drop, biogeochemical reactions of the organic matter collected over the years can suddenly reduce oxygen content in the water. That could be one of the probable reasons, but that can be confirmed only after test results arrive," a source said.
Residents reported a noticeable change in the colour of the water, while officials warned the situation could worsen if there is no rainfall in the coming days.
Mangaluru City Corporation commissioner has been informed, and civic teams have been directed to inspect the affected area. All industrial discharge points and sewage outlets in the region are now under scrutiny, officials said. The KSPCB is expected to issue a show-cause notice to the corporation commissioner.
