The Irish government should give up its claim on Lough Foyle, the Democratic Unionist Party (DUP) has said.
The party said Dublin needed to "respect" the fact that the disputed lough is in UK territory.
Sinn Féin has dismissed the claim as "nonsense" and accused the DUP of being "provocative".
Meanwhile, fishermen who have taken advantage of the dispute to set up oyster farms without a licence say they are not breaking the law.
"We are fishing here legally because there are no laws to prevent us," said William Lynch, who farms about eight million oysters on the lough. "It might be different in other parts of Ireland but we are talking about the Foyle which is unique."
'Do things properly'
Mr Lynch is one of dozens of unregulated oyster farmers operating on Lough Foyle.
The dispute between London and Dublin over ownership means they are free to operate without a licence.
The Loughs Agency - the cross-border body set up to manage Lough Foyle - has no powers of enforcement while ownership is disputed.
"I want to do things properly," he said.
"We have to look after what I sell and we have to safeguard the general public so they know exactly where my oysters are coming from."
"Even though there is no legislation and no obligation for me to do it, I do work with the authorities and I carry out all the best practice of both sides.
"I do the testing that anyone on any full licensed site would do. "
In the past four years the numbers of oyster trestles in the lough have increased dramatically from 2,000 in 2014 to 30,000 in 2018.
Sinn Féin senator Padraig McLaughlin said it was like the "rush to the Clondyke".
"This is a Wild West operation here" he said.
"It's a bit like no man's land where the British government claims jurisdiction over the entire Lough Foyle quite arrogantly and the Irish government disputes this," he said.
"And despite the fact we have a Good Friday Agreement, over 20 years they still haven't sat down and worked out some sensible agreement how we manage the immense resource of this lough."
But the DUP has blamed Dublin for the stalemate.
The party's Foyle MLA Gary Middleton said it was "time Dublin respected the UK waters".
"Dublin needs to take a step back because the UK government has always taken the stance that its waters run right up to the high-tide mark on the Donegal shore and that should be the way its maintained," he said.
But Senator McLaughlin dismissed that claim as "absolute nonsense".
"This is the Inishowen Peninsula on the shores of Lough Foyle, we all have to be sensible about this and we need to share this resource for all of our people," he said.
"Under the Good Friday Agreement we have the Lough Agencys to manage Lough Foyle and Carlingford but they haven't been given the sufficient powers.
"The DUP have to be sensible and not provocative."