Japan's coast guard says it has arrested an anti-whaling activist from New Zealand for illegally boarding a whaling ship.

Coast guard spokesman Tomonobu Higashi says Peter Bethune, a member of the U.S.-based Sea Shepherd activist group, was arrested Friday.

Bethune jumped aboard the whaling vessel from a Jet Ski on Feb. 15 in the Antarctic, where Japan was conducting its annual whale hunt. Since then, Bethune has been in custody on the Japanese whaling ship.

The vessel — the Shonan Maru 2 — returned to a Tokyo port Friday.

The boarding was the latest escalation of a campaign by Sea Shepherd to hamper Japanese whaling activities.

SLIDESHOW: Whale Wars Heat Up.