Baby dolphins are being abandoned to die alone as their families are rounded up and their parents slaughtered in an annual multi-million-pound hunt in Japan.

Pods of the mammals are being captured off the coast in the six-month marine hunting season at Taiji, where the adults are either butchered for meat or sold on to aquariums around the world.

Juveniles are being included in catches but thrown back into the sea where they stand almost no chance of survival without their parents to feed, protect and teach them life skills, witnesses say.

Volunteers monitoring this year’s hunt say that sacrificing young ones is just one of a host of cruel practices they have seen since the season began at the start of last month.

They are now keeping a close eye on one captive caught 12 ago that they say is showing signs of depression.

The Japanese hunters devise annual quotas of numbers they may catch. The Sea Shepherd Cove Guardians group says the 2018-19 quota is 2,040, although most years fewer than 1,000 animals are usually taken. Critics say the cap is set high to make the real number killed appear low.

Nikki Botha, from Sea Shepherd, who is often based at the port monitoring the hunt, told The Independent the world would be horrified if people knew how babies are regularly thrown back into the sea just to die.

“These young animals are separated from their mothers and pod mates as hunters don’t want to waste their self-imposed quota on such a small amount of ‘product’,” she said.

“The chances of survival of these young animals are extremely slim without the protection of their pod and their mothers’ guidance in survival.

“The juveniles are hidden under tarpaulins on a skiff [boat], flukes [tails] furiously slapping in panic and confusion.”

She said she had seen hunters laughing at the animals’ distress.

Once the cove went silent and the tarps were fully drawn back, our team got a glimpse of bright red blood in the shallows Dolphin Project volunteer

In the wild, dolphin calves consume their mothers’ milk for about the first two years of life, then stay with them for up to six more years.

Campaigners say that even if a youngster was able to survive physically being orphaned, seeing its family slaughtered would leave it highly traumatised.

The port of Taiji has become a byword among campaigners for the “barbarity” of devastating dolphin families and turning the ocean red with blood.

Sea Shepherd has this season counted four captives, four juveniles returned to the sea and 18 adults slaughtered, across four hunt drives.

Last month, the Ric O’Barry’s Dolphin Project group, which also has volunteers in place, reported how a pod of Risso’s dolphins was driven into the cove “where they met a heartbreaking fate”.

“Among them were two traumatised juveniles, which were taken from their mothers’ sides and dumped back out at sea,” the group said.

“The remaining ones were dragged under the cove’s tarps and slaughtered. A skiff went under the tarps and came out dragging the carcasses of the five remaining Risso’s, under a grey cover.

A dolphin being held in a pen at Taiji harbour (Ric O’Barry’s Dolphin Project) (Ric O'Barry's Dolphin Project)

“Once the cove went silent and the tarps were fully drawn back, our team got a glimpse of bright red blood in the shallows.”

Dolphin drive hunts are carried out by fleets of boats that chase pods on migration routes by banging on metal poles placed in the sea to create a wall of sound that drives the mammals towards the shore.

Dolphins depend on echolocation – using sound waves that bounce off objects – to navigate under water, so are susceptible to the technique.

Once cornered, they are killed for meat – despite warnings that their flesh is contaminated with mercury – or held in pens and sold to dolphinariums where they spend their lives in pools that are condemned for being just a fraction of the size of the oceans they would use in the wild.

Hunters receive about $500 (£380) for a dead dolphin sold for meat, but the Taiji Whale Museum, which buys them live from hunters and holds them in pens at the harbour, receives more than $150,000 from aquariums for one that has been trained, according to Ric O’Barry’s Dolphin Project. The museum subsidises the drive hunts for that reason.

Last week monitors photographed a trainer inserting a tube into a “seemingly unwilling” dolphin. “Captured dolphins are conditioned to accept these unpleasant sensations on a regular basis to prepare them for procedures such as force feeding and blood draws,” a volunteer said.

The group also reported how three Risso’s dolphins held in Taiji’s harbour were reluctant to eat after being caught. After a few days they started to accept squid thrown in the water by trainers.

But the Cove Guardians reported that one in particular was in trouble. Niji was completely uninterested in food and her behaviour had changed.

Niji was uninterested in food and is clearly depressed Cove Guardians

“In previous days, her surface time and location were steady, predictable and repetitive. This morning her surfacing pattern was erratic – anywhere from 7 to 34 seconds.

“This afternoon, however, we noticed something worrying. Each time Niji surfaced, she was lopsided, favouring her right side. This is not a good sign. It is clear that she is depressed and struggling to adjust to her new circumstances.

“Needless to say we are very, very worried about her.”

Niji was not fed at all today, they said.

“They [the hunters] were fixing the corner of the pen next to her, banging on steel and the sound was reminiscent of the same banging on the steel poles when her pod was hunted.”

On another occasion, witnesses saw a pod of eight driven into the cove where five adults were slaughtered, a juvenile was taken into captivity and two others were dumped back at sea.

Three weeks ago four dolphins owned by the Taiji Whale Museum and held in harbour pens died in the typhoon that swept Japan.

Dying to make us happy: The bloody truth behind the dolphinarium Show all 5 1 /5 Dying to make us happy: The bloody truth behind the dolphinarium Dying to make us happy: The bloody truth behind the dolphinarium 249404.bin Atlantic bottlenose dolphins perform for the crowds in Las Vegas Getty Images Dying to make us happy: The bloody truth behind the dolphinarium 249405.bin Making a splash: A scene for the 1960s American TV series Flipper Getty Images Dying to make us happy: The bloody truth behind the dolphinarium 249406.bin The sea turns blood-red during the dolphin slaughter in Taiji, as seen in The Cove AP Dying to make us happy: The bloody truth behind the dolphinarium 249407.bin Ric O'Barry has now dedicated his life to stopping the capture of dolphins Louie Psihoyos Dying to make us happy: The bloody truth behind the dolphinarium 249408.bin A dolphin-shaped 'blimp' was launched to get aerial footage of the dolphins' plight in Taiji, but didn't manage to capture any film

At least one died of respiratory failure, and it’s feared others were injured.

Anna Oliver, of Sea Shepherd UK, said they might have been hit by flying debris or died of stress or dehydration if they were not fed during the storm.

Taiji’s mayor has said in the long term he wants to convert a bay into a massive dolphin pool as a “research centre”, suggesting he has no intention of heeding calls for the hunting to end.

Annual dolphin catches have declined, in part because of growing awareness of international distaste for the hunt and alarm over mercury poisoning, and in part because of 2009 documentary The Cove, in which Ric O’Barry revealed the practices used at Taiji.