Environmental activist group Sea Shepherd said Friday it has been chasing a "poaching" ship for three weeks amid heavy ice flows in an attempt to stop the crew from illegal fishing in the Southern Ocean.

Peter Hammarstedt, the captain of Sea Shepherd's lead ship, Bob Barker, said his crew has been pursuing the Nigerian-flagged boat Thunder for 22 days, in what the group said is the world's longest sea chase of an alleged poaching vessel.

"When we found them, they were actively fishing," Hammarstedt told AFP from the Bob Barker, which on Friday was about 900 nautical miles south-east of South Africa.

The chase started 2,300 nautical miles from South Africa -- or about 80 nautical miles outside of Australian Antarctic waters -- in a fishing area regulated by the Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR), a multi-national body.

"I radioed them and told them they were committing a crime... it's been 22 days since then and they've taken us through a gauntlet of heavy ice and heavy seas," he said.

"Certainly we are prepared to chase these poachers to the ends of the Earth and back if we have to."

Hammarstedt said Thunder's crew had tried to shake off their pursuers by sailing through waters with moderate and heavy ice flows.

At one stage, the ice had become so heavy the captain said he had to use the Bob Barker as a "500-tonne steel snow plough to get through".

Thunder, on a list of boats deemed to have engaged in illegal, unreported, or unregulated fishing activities by CCAMLR, is suspected of illegal fishing for Patagonian toothfish and other rare species in the Antarctic.

Two gillnets left behind by Thunder were retrieved with more than 700 Patagonian toothfish and other marine life dead in the mesh, Sea Shepherd said.

Toothfish are sold as Chilean sea bass which is popular in high-end restaurants. It sells primarily in the United States, Europe and Japan, though there is also a growing market in China.