Conservation group Sea Shepherd hopes those responsible for illegal toothfish poaching are prosecuted.

Notorious poaching vessels the Songhua and Yongding have been boarded by local authorities in Cabo Verde, off the northwest coast of Africa.

Earlier in the year the New Zealand Navy intercepted the two vessels which had been operating in the Southern Ocean.

Sea Shepherd New Zealand director Michael Lawry believes it's the officers in charge of these vessels who are responsible for decimating important marine ecosystems.

"We hope the officers will be detained and prosecuted and that the crew will be let go," he said. "Often the crew on board these illegal vessels are virtually slave labour."

Lawry said it was a remarkable coincidence, as Captain Peter Hammarstedt happened to be in Cabo Verde and spotted the ships.

"The military police boarded the vessels and detained them and it looks like the last of them have been detained," he said. 

The vessels have changed their names to Kadei and Luampa, and been illegally re-flagged to Sierra Leone.