A large number of dead fish were found floating in the waterbody since Saturday night following which the minister had directed officials of his department to visit the site and ascertain the exact reason.
After examination of samples of the dead fish and the Dighalipukhuri or long pond in Assamese, the officials found that depletion of dissolved oxygen in the waterbody coupled with high organic load were the causes behind the mass fish deaths.
''Fish mortality was due to sudden dip in oxygen level and high organic load in the tank. Departmental officers after examination found that the deaths were not due to poisoning, but because of environmental degradation,'' Suklabaidya said.
As a short-term measure, the oxygen level of water has been increased by showering water through pumps and boating, he added.
The dead fish, numbering hundreds, were retrieved since morning and buried at a nearby spot.
The Dighalipukhuri, more than 500 metres long, is located in the Ambari area of the states largest city.
The origin of this dug-out pond is not clear, with some accounts claiming that it dates from the Mahabharata era, while others maintain that it was dug by Ahom rulers.
It was originally connected to the Brahmaputra River, which flows along the length of Guwahati, and had served as a naval yard during the Ahom rule.
The canal was later filled up during British rule.
Dighalipukhuri, which has boating facilities, is a popular tourist spot in the city.