Otago Fish & Game was alerted by concerned anglers to issues at the dam earlier this week.
Anglers described seeing "sluggish" trout sitting in shallow water close to shore and swimming inverted and on their sides.
Fish & Game staff visited the site earlier this week and found about 15 dead rainbow trout and a brook char.
Fish & Game ecologist Jayde Couper said a spot check of the water suggested that despite low temperatures, the oxygen levels in the dam were very low.
Most of the dead trout were retrieved by Fish & Game for analysis.
"The trout appear to have been in very good condition before they died," Mr Couper said.
"We don’t know the cause yet. What seems unusual is that low oxygen levels are often associated with high water temperatures, and in this case the water was cool, at about 12°C."
After having months of dry weather in Central Otago, the dam was observed to be about 30cm below full. The water’s edge was out about 2m in the shallows, Mr Couper said.
Fish & Game has passed on the information to the Otago Regional Council for further investigation.
"In the meantime, we ask that anglers avoid fishing the dam to protect the remaining stressed fish population," Mr Couper said.
Coalpit Dam is a popular fishing and picnic spot in the Naseby Forest. It was built to store water for gold recovery during the mining era and is now stocked with rainbow trout by Fish & Game.
Otago Regional Council compliance manager Tami Sargeant said it might be distressing for the public to see dead fish but asked that they did not handle or remove the fish.
Responders had been on-site at the Coal Pit dam yesterday. They took water samples, and initiated standard responses to these types of events to assess what the cause of the fish deaths might be.
It is likely results from the water testing will not be available until later next week.
