A Sea Shepherd helicopter spotted the vessel yesterday, with two harpoon ships nearby.
"They're hit with an explosive harpoon that goes straight into their body, hooks come out and shrapnel is sent through their body, it's a terrible bloody death … these whales can take up to 30 or 40 minutes to die," Jeff Hansen from Sea Shepherd said.
The marine conservation group said the Japanese crew quickly covered the whale with a tarpaulin.
