There’s a lot of ocean out there, and boats engaging in illegal fishing or human trafficking have good reason to hide.

But even the stealthiest vessels — the ones that turn off their transponders — aren’t completely invisible: Albatrosses, outfitted with radar detectors, can spot them, new research has shown. And a lot of ships may be trying to disappear. Roughly a third of vessels in the Southern Indian Ocean were not broadcasting their whereabouts, the bird patrol revealed.

Albatrosses are ideal sentinels of the open ocean, said Henri Weimerskirch, a marine ecologist at a French National Center for Scientific Research in Chizé, France, and the lead author of the new study published on Monday in Proceedings of the National Academy of Sciences. “They are large birds, they travel over huge distances and they are very attracted by fishing vessels.”

Dr. Weimerskirch and his colleagues visited albatross breeding colonies on the Amsterdam, Crozet and Kerguelen Islands, French outposts in the Southern Indian Ocean. The team attached roughly two-ounce data loggers to 169 adult and juvenile birds. The equipment consisted of a GPS antenna, a radar detector and an antenna for transmitting data to a constellation of satellites.