The dead fish were found floating in the water body on Saturday and the process of retrieving them has begun, he said.
Preliminary investigation has ruled out poisoning and environmental degradation is suspected to be the cause, the spokesperson said.
“Depletion in oxygen level in the water due to organic overload seems to be the reason behind the mass fish deaths but we can say anything definitely only after a complete investigation,” he said.
Dighalipukhuri or ‘long pond’ in Assamese is around 500 metres long and is located in Ambari area of the state’s largest city.
The pond was dug by Ahom rulers as a canal of the Brahmaputra River and served as a naval yard. The canal was later filled up during British rule.
Dighalipukhuri, which has boating facilities, is a popular tourist spot in the city.