Jal Nigam officials here said they were in the dark about the possible cause of the fish deaths.
The dead fish were first spotted Sunday by morning walkers and others who use the river bank at Tuliyaghat. On hearing the news, fishermen jumped into the river with their nets to gather the fish.
Till late evening officials of the Jal Nigam's Gomti Pollution Unit had no information about the ecological tragedy.
"If true, the matter is serious and a thorough probe would be ordered into the death of the fishes" said Rajendra Kumar, chief engineer of the unit.
Environmentalists say that in the past few days, the water level of the river has gone down considerably, leading to oxygen scarcity.
Officials said they periodically checked the oxygen levels in the river, which invariably record 5-6 milligrams per litre, that is considered safe. Fish die if the oxygen level slips below 3 mg per litre.
There has been some talk here in recent weeks of water being released into the river from the Sharda canal. Also environmentalists report a lot of silt in the Gomti forcing fish to come up, where oxygen levels are low and pollution very high.
IANS learns that the Jal Nigam's work of dredging the river for silt is pending for quite awhile.