More than 3,000 fish have died in two suspected pollution incidents, the Fisheries Conservation Board has said.
The FCB said it had taken samples from the river
On the River Shimna near Newcastle, County Down, 1,500 fish died.
The FCB Deputy Chief Executive Brian Finn said it was caused by a discharge of sewage thought to be from a Water Service sewage pumping station.
Also in County Down, about 1,500 fish died in the River Lagan at Donaghcloney, caused by a "chemical source".
The FCB said it had taken samples. Mr Finn said that the losses were of concern and that in high temperatures pollution could have a "major effect".
"It may have been a relatively small amount of sewage, I don't particularly know," he said, referring to the Shimna kill.
"The water temperatures in rivers now, in this particularly hot period, are very high, river flows are very low so really any form of pollution from any source is going to have a major effect on fish, ie cause a fish kill."
It is the third chemical spillage to hit the river
Jonathan Kerr, from Iveagh Anglers, said almost £250,000 had recently been spent to enhance the Lagan at Donaghcloney and reintroduce native fish.
"We had put a lot of fish in so it's basically a five to six year plan wiped out," he said.
It is the third chemical spillage to hit the river in the past five years.