French fishermen maintained a blockade of Channel ports today, disrupting ferry traffic to Britain, to demand an increase to limits on cod catches.

Fishing boats blocked the entry to the ports of Calais, Dunkirk and Boulogne and unions said they could extend their protests against European Union limits aimed at protecting dwindling cod stocks in the North Sea.

"We're keeping up our blockade of the ports," Stephane Pinto, a representative of the CFDT union told Reuters.

Fishermen were forced to throw away fish caught in their nets because to take them ashore would breach quotas, he said.

"There's fish, we're catching it, the resources exist and this quota policy forces us to throw back into the sea our reason for working," he said.

Blockading ports is a time-honoured tactic by French fishermen and there were several episodes last year to demand help with spiralling fuel prices at a time when crude oil cost more than $100 a barrel.

French Agriculture Minister Michel Barnier has offered to meet industry representatives to discuss financial support for fishing boats in difficulty but the government gave no suggestion today that the quota limits would change.

"It's an annual negotiation that takes place at the start of the year," government spokesman Luc Chatel told France 2 television, saying that in the last negotiations, cod quotas in the eastern part of the Channel had been increased 30 per cent.

"I'd also just point out that if there are quotas it's also to preserve fish stocks, to protect fishing in future years," he said. "We have to manage the resource over time to protect our country's fishing industry."

For years, scientists have said cod is so badly overfished in EU waters that it runs the risk of extinction and the European Commission ruled out any increase in quotas that are agreed in annual meetings in December.

"There isn't any room for manouevre on the quotas," a Commission spokeswoman said. "Member states decide on quotas at the December fisheries council. The decision is based on scientific opinions."

"The solution is not to increase the quota, which would only endanger fishing stocks more and ultimately lead to the ruin of the fishing fleet. France is only doing its duty, which is the duty of every member state."

President Nicolas Sarkozy said last year that France would seek an overhaul of the quota system but fishermen said the limits had become more severe.

A spokesman for P&O Ferries told BBC radio the effect on its services was "pretty terrible".

"Nothing is running this morning," he said. "They (the fishing vessels) string themselves across the entrance to the port and that's it, we can't go anywhere."