Scallop fishermen in Cornwall are angry at plans to ban them from dredging in the Fal Estuary.
Scientists say the maerl dates back 4,000 years
Environmentalists say the ban is necessary to protect a rare type of seaweed.
But the fishermen deny they are causing any damage.
From Tuesday scallopers will be banned from using a fishery in the Carrick Roads, the waters around Falmouth.
Stephen Long has been a fishermen for 33 years and in the last few years he has concentrated his efforts on scallop dredging.
"It's the only free fishery left, everything else has pressure stocks and you can only catch so much at certain times of the year," said Mr Long.
But environmentalists say that scallop dredging is destroying a site of ecological importance, a bed of coral like seaweed called maerl.
"Scallop dredge is a very heavy piece of fishing gear which drags along the seabed with teeth which dig in to catch the scallops," said Roger Covey of English Nature.
"It's similar to dragging a plough across a field it turns over the seabed and causes immense amounts of damage."
Scientists at Plymouth University have carbon-dated the maerl and say it dates back 4,000 years.
"People have to remember Falmouth has been a commercial port for the last 300 years, thousands of ships have been dropping anchor and laying chains, they have done more damage to the maerl than we ever have done," said Stephen Long .
The conservationists say the ban ends confusion over a century-old bye-law which banned trawling in the Fal, but had not included dredging.
The fishermen argue it is unnecessary as they would only need to dredge in the Fal when the weather was poor and forced them inshore.
They say their ability to make an living in the winter months will be affected.