Fishing boats are "not able to land any fish" in France currently, the Guernsey Fishermen's Association said.
Barry Paint, association president, said since Brexit one member had been told he was "on no account can he go to France to unload his fish".
Jersey fishermen have reported issues with paperwork after a boat was turned away over a missing signature before being able to land a day later.
One fisherman said he'd be "out of work completely" if disruption continues.
Mr Paint said he hoped the French would come to an agreement over landing.
He explained the industry exports 80% of its catch via France and there is not a large enough local market to compensate.
Mr Paint said: "It appears that a market has been closed to us for the time being.
"So what we've got to to is look for somewhere else to sell, which we're in the process of doing."
'Can't keep on'
Vice president of the association Dougal Lane, said the French authorities actions had effectively removed their market.
He explained they had informed him of ports available for use, but there had been contradictions in what he had been told.
Mr Lane said those options were also not practical in winter with disruption at the port of Cherbourg and the distance to Granville being too great for a small boat to sail to in rough conditions.
He added: "If it continues I'll be out of work completely. At the moment I'm doing a few jobs on the boat to tide me over but it can't keep on."