The new lockdown may cause fish prices to plummet as the hospitality industry closes its doors and European markets dwindle, seafood producers fear.
Prices dropped in March when the first coronavirus lockdown caused worldwide demand for seafood to fall drastically.
Jim Portus, from the South Western Fish Producers' Association, said there was "a feeling of dread that it may be as bad or worse than the first lockdown".
The government said it was continuing "to monitor the situation".
"This is a very uncertain time for the fishing industry," said Mr Portus, who has called on the Department of Environment, Food and Rural Affairs (Defra) to make a case to the government for financial support.
"If grants are going to be made available this time then it should be for all sizes of vessels and it should be for all the fish processing businesses," he continued.
"They are all going to suffer equally."
Mr Portus said Italy, France and Spain were major buyers of British shellfish so it was "not just the shutdown of the UK that will have an impact", further coronavirus restrictions across the continent will also cause problems.
Ian Jepson, of Three Jays Shellfish in Newquay, began selling his catch direct to local customers during the first lockdown after his two main merchants, which supply shellfish to supermarkets, shut down.
Mr Jepson said: "We are hoping to keep on going as normal but for the boats catching the wet fish with the European markets closing down we are all thinking it is going to be a bit harder."
Tristan Northway, skipper of Brixham's trawler Adela, said he turned his business on its head in March and now sells his fish direct to the public.
He said: "It took off but this time I think is going to be completely different" as people do not have the money - because of job losses and Christmas coming up - to spend on fish.
Rob Wing, chair of the Newlyn Harbour Commissioners who runs the Cornish Fishmonger, said the price of some types of fish would depend on the British public, who he called on to buy seafood.
He said fishermen were "very adversely affected" during the last lockdown.
"Having their markets taken away from them will cause untold hardship," he said.
"I do call on the government to look specifically at the fishing industry and see how it can support our fishermen in their time of need."
A spokesman from Defra said: "Our Fisheries Response Fund provided £10m for the sector to help fishermen cover costs and we encourage them to take up the other financial support currently available to businesses."