Investigators looking into a collision between a Japanese whaler and a protest boat on the high seas off Antarctica earlier this year said Thursday the captains of both vessels were to blame.

The Sea Shepherd Conservation Society had accused the Japanese ship of deliberately ramming its vessel, the Ady Gil, on Jan. 6, slicing the bow off the vessel and eventually causing it to sink. The whalers denied it, saying the Ady Gil's captain deliberately put his vessel in the ship's path.

The clash was the most serious in the several years that Sea Shepherd has sent vessels into far southern waters to try to harass the Japanese fleet into ceasing its annual whale cull.

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The Ady Gil is shown in the photo before it collided with a Japanese whaling ship and eventually sank. ((Institute of Cetacean Research/Associated Press))

Government safety agency Maritime New Zealand said in a report released Thursday the captains of both the Ady Gil and the whaler, the Shonan Maru No. 2, "were responsible for either contributing to, or failing to respond to the 'close quarters' situation that led to the collision."

It "found no evidence that either vessel master had deliberately caused the collision," but said there was "a failure by both masters and the crew(s) … to appreciate and react appropriately to avoid colliding."

Sea Shepherd's high-tech anti-whaling speed boat, the Ady Gil, captained by New Zealander Peter Betune, had 3.5 metres sheared off its bow in the collision in frigid waters about 290 kilometres north of Antarctica. The boat's six crew members were rescued and the hulk later sank while under tow.

The Shonan Maru No. 2 suffered no apparent damage.

Both sides blamed the other for the collision, which occurred as the Ady Gil harassed the Japanese whaling fleet.

Maritime New Zealand director Catherine Taylor said the Japanese skipper initially had responsibility for keeping his vessel clear of the Ady Gil as the overtaking vessel.

"He had ample opportunity to avoid the close quarters situation that subsequently developed, but failed to do so," she said. He also "failed to take positive and ample action to avoid colliding with Ady Gil."

However, once the vessels were at close quarters, the master of the Ady Gil "failed to respond by taking appropriate evasive action — choosing instead to maintain his course and speed, which allowed the close quarters situation to develop into a collision risk," Taylor said.

The protest ship also was found to have failed to maintain a proper lookout.