A Sea Shepherd activist pleaded guilty Thursday at the start of his Tokyo trial to trespassing onto a Japanese research whaling vessel in the Antarctic Ocean in February but denied trying to hurt a crew member.

New Zealander Peter Bethune, 45, told the Tokyo District Court that he did not intend to harm the 24-year-old crew member aboard the Shonan Maru No. 2 when he fired a bottle containing butyric acid, or rancid butter, at the Japanese vessel.

"I did not mean to injure anyone," Bethune, a member of the U.S.-based Sea Shepherd Conservation Society, said when asked to enter his plea as the trial opened.