Rare video footage shows a giant octopus attacking a small submarine off the west coast of Vancouver Island.

Salmon researchers working on the Brooks Peninsula were shocked last November when an octopus attacked their expensive and sensitive equipment.

The giant Pacific octopus weighs about 45 kilograms, powerful enough to damage Mike Wood's remote-controlled submarine.

(courtesy SubOceanic Sciences Canada Ltd )

Wood's first reaction was to panic, knowing the marine creature can exert a powerful bite.

"I go full reverse and blast him with all these seabed particles," said Wood, describing the attack shown in the video. "Finally, he lets go and disappears off into the gloom.

"It was desperation. It's a $200,000 machine, and it's not insured," said Wood, who runs SubOceanic Sciences Canada in Duncan, B.C.

Mini-sub survived the attack.

The rare footage, which has just been released, is believed to be the first documented attack of an octopus on a sub.

"It was only afterwards when I replayed the video and I thought, 'Oh, yeah, that's pretty neat.' But at the time, it was just scary."

No one knows what caused the octopus to attack. It may have been curious, looking for a meal or a girlfriend, said Jim Cosgrove of the Royal B.C. Museum.

"It's certainly a mature male from what I can see in the video," said Cosgrove. "Old octopuses become what we call senescent, or senile, reaching the end of their life. And sometimes their actions are very inappropriate."

Such large, powerful animals deserve respect, Cosgrove said.

The octopus left unscathed. The submarine's only defence was its thrusters, but the machine survived the attack.