Marine law enforcement officers swooped on illegal shark fin operators in Cape Town harbour on Monday, seizing thousands of fins in what they say is probably the city's biggest haul of illegal shark fin yet.

The fins were confiscated from four different premises in the harbour.

In a separate incident on Monday, officers from Marine and Coastal Management seized a South African fishing trawler, the African Queen, which had more than 1 000 rock lobster tails hidden in the hold under a cargo of fish.

The master of the trawler, owned by Bay-King Fishing, was arrested. The vessel was fishing for Bay View Fishing.

By late Monday afternoon no arrests had been made in the shark fin incident.

Marcel Kroese, head of Marine and Coastal Management's law enforcement section, told the Cape Times on Monday they had bagged the fins but had not yet weighed them so they could not say exactly how big the haul was.

The first batch was seized outside the premises of Blue Continental Products, a local company.

"This could be the biggest amount of shark fin we've ever seized," Kroese said. "We're doing three other premises in the harbour right now. There are thousands of fins, some really small ones, probably from juveniles, and some really large ones."

The fins had been laid out on tarpaulins to dry in the sun.

Shark fins are often hacked off at sea and the shark is thrown back into the water while it is still alive. The fins command high prices, particularly in Asia, where shark fin soup is a delicacy.

"The implication of this lot for the shark population is severe. We are very concerned about the over-exploitation of sharks globally and we don't want South Africa to be part of an illegal trade in sharks," Kroese said.

He said possible charges to be laid against the companies include operating a fish processing plant and importing shark fin without permits.

Kroese said Marine and Coastal Management also arrested a fishing trawler in the harbour on Monday in a joint operation with the harbour police, who had received information about the cargo on board.

"We found West Coast rock lobster tails packed in bags hidden on ice under the other fish. The vessel may have been fishing legally for hake, but a vessel cannot land any other fish at the same time.

"We want to know how the vessel came to have rock lobster on board when the rock lobster season is closed. It is also illegal for a vessel to be in possession of the West Coast lobster tails only, instead of the whole lobster. Having tails only is a poacher's favourite trick because it makes it far more difficult to identify the lobster and to determine the size," Kroese said.

Kroese could not say what charges would be laid until his investigation had been completed, but said they were likely to include being in possession of rock lobster without a permit, being in possession of lobster out of season and being in possession of lobster not in a whole state.