Watch: Horrific moment orca pod kills grey whale and calf

Five orca hunted and killed a grey whale calf off the coast of Monterey, California during a four-hour attack caught on video.

Footage shows the desperate mother battling for hours to save her calf as the orca repeatedly rams it, attempting to separate the pair during the ambush attack which briefly turns the sea red.

As a boatload of whale watchers looked on, the orca approached in a zig-zag pattern before striking.

The attack lasted for four hours, turning the sea red. Photo / Monterey Bay Whale Watch/gowhales.com

"It really was a showdown in the sea. Though it can be sad to think of the grey whales becoming prey, we must remember that the killer whales have to eat in order to survive.

'Transient killer whales, the type that we typically see here in Monterey, have a diet of other mammals, making springtime in Monterey Bay a haven for these tactically sound hunters,' Monterey Bay Whale Watch posted on Facebook.

"This experience was a testament to the interconnectedness of the different creatures of the ocean, and the circle of life."

The mother bravely battled to save her calf. Photo / Monterey Bay Whale Watch/gowhales.com

Nancy Black, marine biologist and owner of Monterey Bay Whale Watch, told KRON4 that the area the mother and calf were passing through is well known as a danger zone for migrating whales.

Black explains that the grey whales were passing through Monterey Bay on their way to Alaska when they passed over the dangerous Monterey Submarine Canyon.

Entering deep water, the whales are vulnerable to attack and the waiting orca targeted the pair as a whale-watching boat looked on.

A marine biologist described the events as a "harsh fact of nature". Photo / Monterey Bay Whale Watch/gowhales.com

Tourists who witnessed the drama play out took to Facebook after the experience, with one man writing: "This was an amazing thing to see and witness. Definitely an experience of a lifetime both good and upsetting."

Seeing the harsh reality of the animal kingdom up close impacted another witness, saying "you had to be there" and that she was an "animal lover/fanatic .. .but it was different to watch it in real time. Sad for sure"