The incident occurred when one of the workers got into the drain at the hatchery premises in Palamaneru mandal, for cleaning it. As he fell unconscious due to asphyxiation, eight other workers got down to rescue him and suffered the same fate, they said, adding the sewer had chemical residue.
Residents of nearby Morum village rushed to the hatchery, broke open the drain's hatch and pulled out the trapped workers. Four of them died on the spot, while three others died while undergoing treatment, the police said.
One worker regained consciousness following treatment and was stated to be stable, while another worker was shifted to the District Headquarters Hospital in Chittoor, they said.
Meanwhile, the police have launched a manhunt for the hatchery manager who is on the run following the deaths.
An investigation into the incident is underway, the police said, adding, according to their initial assessment there was "serious negligence" on part of the hatchery management in ensuring adequate safety measures at the site.