The North Western Regional Fisheries Board has defended its controversial decision to object to all major housing developments in Castlebar until a sewage treatment plant in the town is upgraded.
Chief Officer Mr Vincent Roche was speaking after the publication of the board's annual report which highlights a serious decline in water quality in many of the best fishing lakes in the region.
Mr Roche said the board had a statutory responsibility to protect fish stocks.
"We felt we wouldn't be doing our job otherwise and we felt we had to take a stand.
"The more untreated or insufficiently treated sewage that goes in will just add to what is already there," Mr Roche said.
The report says the board's environmental staff found "serious problems" with the Castlebar sewage treatment plant.
"It appeared that untreated or inadequately treated sewage had been discharging to the Castlebar river over a considerable period of time due to overloading at the plant and mechanical failure," it stated.
A report by the Central Fisheries Board found that trout stocks on the river had been "decimated". Angling on Lough Cullin had been wiped out over recent years.
The fisheries board is now examining an Environmental Impact Statement for a proposed upgrading of the Castlebar plant.
But even with a new plant the board is "seriously concerned" about the ability of the Castlebar river to provide sufficient dilution for treated sewage.
Serious concerns are also raised about Lough Conn. After a number of years when there were signs of recovery, latest figures indicate phosphates going into the lake have increased again.
At Lough Arrow in Co Sligo angling was "again disappointing" with poor catches recorded. Algal blooms, a sign of eutrophication, appeared on the lake.