Video filmed by Peter Richards
Natural Resources Wales is investigating a potential pollution incident in the Cleddau Estuary in Haverfordwest.
It says the cause of the discharge near Fortunes Frolic is yet to be established.
Simon Walters, a local farmer, noticed the water looked different when he was checking his cows.
Describing himself as "devastated", Simon told ITV Wales: "This is happening on a regular basis. Salmon stocks have absolutely collapsed in the Western Cleddau. They've been there for around 20 years. Sometimes when you go fishing along the estuary, the fishing rod brings up a sanitary towel. It's a nightmare.
"In Haverfordwest, all the geese that have been here for decades during summertime have gone downstream. The water is less toxic down there. That's happened in the last 2 years."
Simon added: "I took my 3-year-old grandson down to the estuary recently to see some crabs. We were turning over stones to find dead crabs. Something has killed them.
"I can smell the river on times. It's toxic."
In a statement, Nicola Mills, environment team leader for Natural Resources Wales, said: “We were made aware of a potential pollution incident along the Cleddau Estuary, Haverfordwest yesterday and sent out our officers to investigate.
“The nearby outfall and local industrial sites seem to be operating as expected. Our staff have taken water samples for testing but are still looking into the cause of the pollution.”