Sea Trust Wales founder, Cliff Benson, believes many of the fish were catshark.

He said he had reported those found at Saundersfoot to Natural Resources Wales (NRW) after seeing an image posted on social media.

“It could have been from a trawler,” he said.

NRW said the dead fish at Pembrey Beach had not been reported to them, but that the fish appeared to be discarded or part of a lost catch.

An officer had visited Saundersfoot beach, NRW said, but found no trace of the animals as they had been washed back into the sea.

A spokesperson said: “We investigated reports of dead fish at Saundersfoot and found no evidence of a pollution incident.

“The fish appear to be dogfish, and this is likely linked to fishing by-catch being discarded at sea.

“Tides have since cleared the majority from the shoreline and no wider environmental impacts were identified.

“We have informed the local authority and relevant partners.”

Carmarthenshire and Pembrokeshire councils have been approached for comment.