For more than a decade scientists and environmentalists have been warning of a mass slaughter of dolphins around Peru.
Hundreds of fishing boats have been accused of killing the animals and using their fatty bodies as bait to catch sharks.
The killings, as many as two or three per hunt, by hundreds of fishing boats around the country, add up each year to make it the biggest illegal slaughter of dolphins in the world, according to environmentalists.
ITV News has obtained footage that shows fishermen harpooning dolphins for bait. It's the first time this secret hunting has been filmed.
Science Editor Lawrence McGinty's report contains images you may find disturbing
The footage, shot undercover by British journalist Jim Wickens, shows the hunting taking place, hundreds of miles off the coast of Peru.
The dolphin is harpooned, brought on to the boat, then cut up and used as bait for shark fishing.
The carcass of the dolphin is then tossed over the side of the boat, and can often wash up on beaches in shores around Peru.
Though illegal under Peruvian law, the slaughter is an "open secret" within fishing communities, according to marine conservation group Mundo Azul.
Previously there has been no evidence to support the claims made by environment groups, as fishing communities and officials denied it was going on.
Working undercover with Mundo Azul, British journalist Jim Wickens spent a week on board a shark fishing boat 100km off the coast of Peru, enduring rough seas and a near-death shipwreck incident in order to film the hunt.
It is estimated that 500 boats are involved in shark fishing, which means as many as 10,000 dolphins could be hunted each year and used as bait. Journalist Jim Wickens, who filmed the footage, said:
With hundreds of boats operating in this way every day in the Pacific, the death count for the this threatened species of dolphin is truly staggering.
The practice of hunting is increasingly common as dolphin meat is a cheap alternative to other forms of bait.
Using dolphins in this way allows small scale fishermen, often poorly paid for their arduous work, to save money needed for fuel and other necessities.
The meat is also traded in exchange for petrol or fishing lines, according to a fisherman who previously took part in the hunt. Jose, not his real name, said a culture of secrecy surrounded the practice.
A lot of people know what is happening, but they don't want to see it. They don't want to admit that yes, they hunt dolphins in this way.
Alison Wood from Whale and Dolphin Conservation said the practice was completely unnecessary, as well as inhumane.
You can use fish guts, and all sorts of things to bait your hook, you don't need to use a sentient, intelligent animal like a dolphin. It is an awful waste. We are talking about a highly intelligent, social, feeling animal, and to see them hunted like this, is a terrible waste.