( CNN ) -- Anti-whaling activist Peter Bethune was given a two-year suspended prison sentence and five years ' probation Wednesday by a Tokyo district court judge for his role in boarding a Japanese whaling ship . Bethune was found guilty on five charges , ranging from assault against whalers to trespassing into a whaling vessel . Bethune had previously pleaded guilty to all charges but assault . He could have received up to 15 years behind bars on charges . Bethune testified during his trial in May 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 said that he believed the bottles of butyric acid he threw at the Shonan Maru 2 whaling ship were non-toxic and would not harm anyone . Prosecutors said the butyric acid burned two crew members of the Japanese whaling fleet , but Sea Shepherd called it a harmless , albeit rancid , liquid . Butyric acid is found in rancid butter and vomit . At the May 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 . `` 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 . '' 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 . `` It 's encouraged us . It 's certainly motivated us , and we 're going back to the Southern Ocean with far more support than ever , '' said Paul Watson , Sea Shepherd founder . `` I hope that we 'll be much more effective because of it . '' `` Pete Bethune is a hero in New Zealand , '' Watson added . `` He 's a hero worldwide to people who want to see the end of whaling . '' 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 . `` They 're targeting endangered whales in an established international whale sanctuary in violation of the Antarctic treaty , '' Watson said . `` They 're criminals . '' CNN 's Junko Ogura contributed to this report .
Anti-whaling activist receives suspended sentence and probation . The court ruled that the prosecution had failed to prove its case against him. Bethune admits he threw butyric acid at Japanese whaling ship . Activists were protesting Japan 's whale-hunting practices .