Man filmed holding baby dolphin underwater as Japan's hunting season exposed in The Cove resumes

A dolphin being held underwater. Photo / Dolphin Project

WARNING: Graphic content

Japan's annual dolphin slaughter season has taken place, once again sparking outrage around the world.

The annual event attracted international attention and condemnation following the release of Oscar-winning documentary The Cove in 2009, which shocked viewers with footage of the waters turning blood red as dolphins are rounded up and killed.

This year, onlookers have been confronted with more shocking images, including a clip of a man holding a baby dolphin underwater before dragging it away to be killed under a tarp.

The incident took place in September and footage has been shared online this month.

The dolphin is believed to only be around three months old.

Every autumn and winter, hunters in a Japanese fishing village corral thousands of dolphins into a tiny, isolated cove and kill them for meat and fertiliser. Photo / Sea Shepherd Conservation Society

Life Investigation Agency (LIA) director Ren Yabuki watched the Risso's dolphin calf die, and told 9NEWS it was one of 32 juvenile dolphins killed as part of the slaughter in the last two years.

"There was no reason the babies are killed more than the bigger dolphins," he said.

Yabuki said dolphin pods would often try to protect the smaller calves, sometimes leading to the death of entire pods.

He believes the juvenile dolphin was killed with a metal rod used to sever its spine, a process known as "pithing", though he couldn't say for sure as the process was performed under a tarp.

The Japanese government argues dolphins killed during the annual hunting season are slaughtered in a humane manner, but the non-profit Dolphin Project cove monitor Heather Hill told 9news.com.au that was unlikely.

"The Japanese government claims this causes an instant death and argues it to be humane, although extremely precise accuracy would be required to ensure a quick death," Hill said.

"This cannot be assured in such conditions."

The government will permit the slaughter of up to 1749 dolphins throughout the season.

Other dolphins are captured for live trade.