The two ships were captured three months ago in what the Australian navy says was a daring and extremely hazardous operation. With the help of French and New Zealand maritime authorities, the navy frigate Anzac chased and caught the ships near Macdonald Island, about 4,000 kilometres south-west of Perth.

In strong winds and high seas, a navy helicopter put a boarding party aboard each ship and escorted them to western Australia. Charges against the captains allege they were catching Patagonian tooth fish, one of the largest species of Antarctic fish.

Choice cuts of Patagonian tooth fish fetch up to $17 a kilogram in the United States. If found guilty, the owners and captains face fines of more than $200,000, as well as forfeiture of the ships.

Both vessels are Spanish-owned, but operate through a company in South Africa. One ship, called Salvora, was allowed to sail to Uruguay after the owners paid a bond of more than $1m, and installed a satellite system to let Australian authorities monitor her whereabouts.