Kenya Wildlife Service (KWS) transported critically endangered animals from Nairobi to Tsavo East National Park in the country’s southeast last month, to boost the population of the beasts there, it said.
The organisation, which has carried out numerous successful moves in the past, has opened an investigation into their deaths.
It has not yet been established how the rhinos died, but preliminary investigations point to salt poisoning as the animals tried to adapt to saltier water in their new home, the ministry of tourism and wildlife said.
The eight were among 14 black rhinos, eight from Nairobi National Park and six from Lake Nakuru National Park, which were transported in June in an operation announced by Kenyan Tourism and Wildlife Minister Najib Balala, who has yet to comment on the outcome.
The relocation of endangered animals, known as translocation, involves putting them to sleep for the journey and then reviving them.
The process carries risks but the loss of half of them is highly unusual.
Paula Kahumba, of WildlifeDirect, called it “a complete disaster” and said officials must explain what went wrong.
“Rhinos have died, we have to say it openly when it happens, not a week later or a month later,” she said.
“Something must have gone wrong, and we want to know what it is,” she added.
The World Wildlife Fund said in a statement: “Translocating wild animals of this size is extremely challenging and not without risk, but black rhinos are under enormous threat so efforts to try and better protect them, such as translocation, are crucial for future generations.”
Save the Rhino estimates there are fewer than 5,500 black rhinos in the world and around 750 of them are thought to be in Kenya.
Conservationists have been working hard to protect the black rhino from poachers targeting the animal for its horn.
According to KWS figures, nine rhinos were killed in Kenya last year.
In May, three more were shot dead inside a specially protected sanctuary in northern Kenya and had their horns removed.
In March, the last male northern white rhino on the planet, an older bull named Sudan, was put down by Kenyan vets after becoming ill.