Maritime authorities in New Caledonia say 70 so-called "blue boats" from Vietnam have been identified fishing illegally for beche-de-mer in the past nine months.
The commander of the French navy in the territory, Jean-Louis Fournier, said 55 vessels had been warned and five had been detained.
Five captains were now serving eight to 10 months in a New Caledonian jail for illegal fishing and 30 fishermen have been flown home.
The commander said those on board the "blue boats" had no identification papers and no money but were determined to dive for beche-de-mer as part of well-organised fleets.
He said the boats took weeks to get from Vietnam to New Caledonia's waters, where they had engaged in fights with local fishermen.
Beche-de-mer is dried and sells for more than $US2000 a kilogram as it is sought after in China for soup.