Tokyo, Japan (CNN) -- Anti-whaling activist Peter Bethune testified in a trial Monday that he had no intention of hurting anyone when he protested Japan's whale hunt.
The New Zealand activist from the Sea Shepherd Conservation Society, standing trial in Tokyo District Court, said that he believed bottles of butyric acid he threw at the Shonan Maru 2 whaling ship were non-toxic and would not harm anyone.
Bethune faces five charges, including assault against whalers and trespassing into a whaling vessel.
He pleaded guilty last week to all charges but assault. If convicted, Bethune faces a maximum of 15 years behind bars.
At Monday's hearing he tearfully described the January collision between the Shonan Maru 2 and the Sea Shepherd's multi-million-dollar speedboat, the Ady Gil. The crash sunk the Ady Gil, which Bethune captained.
Weeks later, Bethune jumped aboard the Shonan Maru 2 and attempted to make a citizen's arrest of the captain. He was arrested and brought back to Japan to face criminal charges.
In court last week, Bethune admitted to all the charges, except for assault.
"I admit that I boarded the Shonan Maru, but I believe that I have good reason to do so," he said. "I admit that I fired the butyric acid, but there were additional circumstances that we will discuss in court."
Prosecutors say the butyric acid burned two crew members of the Japanese whaling fleet, but Sea Shepherd calls it a harmless, albeit rancid, liquid. Butyric acid is found in rancid butter and vomit.
Bethune's case is the first time a Sea Shepherd activist has been tried in a Japanese criminal court in the group's long-running battle with Japan's whalers in the icy waters of the Antarctic.
Japan annually hunts whales in the Antarctic, despite a worldwide moratorium on whaling, under the loophole that a country may legally do so if its purpose is scientific research.
Sea Shepherd has claimed the science argument is a sham, noting that the whale meat then gets sold in Japanese markets and served in restaurants.
A verdict is expected June 10.
CNN's Yoko Wakatsuki and Junko Ogura contributed to this report.