Slaughtering dolphins at sea is a cheaper alternative to other forms of bait for sharks, and the method of killing, harpooning, means that the dolphin can bleed for up to 15 minutes. The blood of the dolphin, then attracts sharks.
The illegal slaughter of dolphins for use as bait to hunt sharks is "an open secret" within fishing communities across Peru, according to marine conservation group Mundo Azul.
Working undercover with the group, 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.
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, 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.
Two British women who pleaded guilty to drug charges in Peru could face a six-month investigation if their pleas are not accepted, the Peruvian prosecutor in charge of their case said.
Juan Mendoza made the comments after Michaella McCollum and Melissa Reid, both 20, made a second appearance before a judge at Sarita Colonia del Callao jail, in Callao, near Lima.
He did not give details of the private hearing but suggested the women's confessions had not satisfied prosecutors.
Last week, McCollum and Reid pleaded guilty to drug smuggling and had hoped their admission would be enough to secure a shorter sentence.
But prosecutors have demanded more information before accepting their admissions of guilt which the women hope will bring their jail time down to six years and eight months.
Two British women who pleaded guilty to charges of drug trafficking in Peru were taken to court for further proceedings in their case.
Melissa Reid and Michaella McCollum Connolly were arrested at Peru's airport in mid-August and pleaded guilty to charges of drug trafficking earlier last week
Michaella McCollum and Melissa Reid are in brilliant health in a modern well-equipped prison, an Irish priest said. Last week the pair pleaded guilty to attempting to smuggle cocaine out of the country.
Father Foley visited the pair last Saturday and said he found them "in great form," despite his continued belief that they are victims of drug gangs, and forced into smuggling. He said:
They weren't in a cell. They were out in a wide open space sitting at a table with a parasol, they were talking and drinking coffee.
As well as that they had telephone communication and they could use it for calling home. I thought they were in great form actually.
My firm belief is that they were conned, they were backed into a compromising situation and their handlers worked on that and got them to go to customs with drugs.
What very likely happened was that people coming behind them in the queue in the airport were the ones who walked through with the drugs.
The guilty pleas made by Michaella McCollum and Melissa Reid have not been accepted because the British women need to provide more information on who supplied the drugs they were found with at Lima airport, prosecutors said.
The two drug mules' guilty pleas have not been fully accepted, as far as the prosecutor is concerned, until they give more details.
They will be asked to give another statement before the judge explaining where the drugs came from, who supplied them and why they said they had been forced to carry them by an armed gang.
The guilty pleas made by Melissa Reid and Michaella McCollum, arrested with almost 12 kilos of cocaine in their luggage in Peru, will not be accepted, the prosecution said.
Yesterday the pair pleaded guilty to drug trafficking charges in front of a judge. However anti-narcotics prosecutor Juan Rosas said that as the new plea only claimed partial guilty for their actions, it would not be accepted by the prosecution.
Mr Rosas said the women would have to release a new statement, admitting they made up the story about being kidnapped and threatened. He said:
"A new statement is required by the women and they will have to admit not only that they came here to traffic drugs but also that they did not tell the truth with respect to them being kidnapped.
"They have to make a full admission of responsibility for all of the charges brought by the prosecution."
An earthquake has shaken buildings in Peru's capital Lima, Reuters reports, citing witnesses.
The lawyer representing Michaella McCollum has said he is unable to disclose the details of the hearing that took place on Tuesday.
Peter Madden said McCollum accepted the charges and a sentencing hearing will take place next Tuesday 1 October.
Madden said: "I intend to travel to Lima next week to attend the sentencing hearing and after the sentence is handed down by the judge, it will be possible make further comment."