Environment Agency Wales is appealing to all livestock farmers to check silage, slurry and effluent handling facilities and disposal systems to ensure that there are no problems likely to cause pollution.
This follows an incident that occurred during December last year, when serious pollution of the Pont Shan Brook, Narberth, was caused by overflowing farm effluent collection tanks.
As a result, the agency is seeking co-operation from the farming community in the area.
An agency spokesperson said: "Farmers have made tremendous progress in recent years to reduce pollution from farm effluents. However, the agency is concerned that water pollution episodes may represent a potential risk to valuable fish stocks of the Eastern Cleddau and tributaries and also to water supplies.
"We appreciate it's a busy time, but it is important for farmers to spend time making these checks to help prevent a problem and avoid major costs."
People who witness pollution, illegal tipping of waste, poaching, fish in distress or danger to the natural environment can report the incident to the agency's emergency hotline on 0800 807060. The hotline operates 24 hours a day, 365 days a year.