A court has ruled that an 40-year-old agreement allowing cross border access to inshore waters is unlawful.
A decision from the Irish Supreme Court could have "catastrophic consequences" for a section of the shellfish industry, it has been claimed.
The court has ruled that a 40-year-old agreement allowing cross-border access to inshore waters is unlawful.
Five Irish fishermen took the case claiming too much of their seed mussel was being taken by NI-based boats.
They also claimed some were predominately owned by European interests.
They said the agreement was being exploited.
Industry representatives say the decision could have "catastrophic consequences" for the industry.
The ruling means boats which collect seed mussel in Dublin Bay for maturing in Northern Ireland waters could be denied permission to fish there.
Warrenpoint fisherman Brian Cunningham said it could affect about 10 companies.
He said it could have "catastrophic consequences" for the industry.
The court ruled that seed mussel was a national resource and rules around its use should have been based on a law passed in the Dáil (Irish parliament).
The agreement was made after an exchange of letters between civil servants on both sides of the border in the 1960s.
It gave fishermen from both jurisdictions access to each other's waters out to a six mile limit.
Boats which collect seed mussel in Dublin Bay for maturing in Northern Ireland waters could be denied permission to fish there.
The industry has concerns that it might also have implications for the prawn fleet and is seeking clarification.
A spokesman for the DAERA said: "The minister is aware of the judgement by the Republic of Ireland's Supreme Court, following an appeal taken against a High Court ruling concerning the Republic's sea-fisheries relationship with Northern Ireland.
"The department and its legal advisors are currently giving full consideration to the judgement, its contents and findings, as it considers its next steps.
"The Supreme Court issued an unapproved judgment in this particular case on 27 October [Thursday]. It did not make any order. At this time, there is no change to current arrangements. The department understands that the Supreme Court will set a date to consider the form of order it will make."
