Environmental activist group Sea Shepherd said Thursday it confronted a ship known for poaching Patagonian toothfish and other rare species in the Antarctic, part of its efforts to target illegal fishing in the Southern Ocean.

Though the Nigerian-flagged boat, the Thunder, managed to speed away, Sea Shepherd said it remained in pursuit, threatening "to directly intervene in order to obstruct their continued illegal activities" if they did not report to Australian authorities.

Sea Shepherd did not elaborate on what it meant by intervening, but during a decade of harassment that successfully saw off Japanese whalers the group used all manner of obstructions, including destroying fishing nets and even boarding boats.

Its lead ship, the Bob Barker, left Australia on December 3 and the group intercepted the Thunder in a fishing area regulated by the Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR), a multi-national body.

Bob Barker skipper Peter Hammarstedt said he had notified the Thunder captain and his crew "that they have been placed under citizen's arrest".

"They must cease their illegal fishing activities immediately and report to the Australian authorities," he added.

"Should they ignore this order, I have notified the Thunder that Sea Shepherd has no choice but to directly intervene in order to obstruct their continued illegal activities."

The Thunder, on a list of boats deemed to have engaged in illegal, unreported or unregulated fishing activities by CCAMLR, was last seen steaming off to the west with the Bob Barker in pursuit.

Hammarstedt said he had notified CCAMLR authorities, the Australian Federal Police and Australian Fisheries Management Authority.

Toothfish live in Antarctic waters at depths of 300 to 2,500 metres (1,000 to 8,200 feet) and are long-lived species, which means they are vulnerable to over-fishing due to their slowness to mature.

Sea Shepherd said increased surveillance and patrolling of waters by authorities in Australia and New Zealand had improved the toothfish situation in some areas.

But illegal fishing by poachers was continuing in what the group calls the "shadowlands" of the Southern Ocean, which are extremely remote and outside national jurisdictions.

At least six illegal vessels are known to operate in the area close to Antarctica, the group 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.