A number of large fish have been seen floating on the surface of a canal but experts admit they are not completely certain of how they died.
Images circulated on social media show around three or four dead bream in the Manchester, Bolton and Bury Canal, near Moses Gate Country Park in Bolton.
Commenters on Facebook blamed the deaths on a number of things including pollution and algae but a spokesman for the Canal and River Trust, the company responsible for the canal, said there is no obvious cause of death.
"We have checked out a report of several dead fish on the Manchester, Bolton and Bury Canal," a Canal and River Trust spokesman said.
"Three or four large bream carcasses are currently floating on the surface of the water.
"There is no sign of pollution or obvious cause or reason for their death."
Although no cause of death could be confirmed, the trust's fisheries manager suggested that it could be down to the warm weather.
A spokesman added: "Sometimes warm weather can affect the amount of oxygen in the water which can leave fish gasping for breath."
The Canal and River Trust confirmed that they will remove the fish carcasses from the water.
