For the last few years there has been a torrent of stories of captive orcas suffering severe health problems, and in some cases attacking and even killing their trainers.

Many of these stories have focused on an orca called Tilikum, who lives at SeaWorld Orlando in Florida. Tilikum has been involved in three deaths during his time in captivity.

SeaWorld has now announced that Tilikum's health appears to be deteriorating, possibly due to a bacterial infection in his lungs.

In response, conservation groups are once again calling for an end to the practice of keeping orcas, and other large marine mammals, in captivity. Are they right?”

In front of a crowd of visitors, Tilikum dragged his trainer Dawn Brancheau under the water and killed her. He had previously been part of a group of three orcas that drowned a trainer in 1991, and in 1999 he apparently drowned a man who was trespassing in the park.

The 2010 killing initially sparked headlines around the world expressing shock, and calling for Tilikum to be put down.

But others, especially marine mammal scientists, were not only sympathetic to Tilikum: they blamed his keepers. The 2013 documentary Blackfish argued that his violent outbursts were directly brought on by the stressful conditions of his captivity.

In line with this, several decades of observation show that orcas are not naturally violent towards humans. There are no recorded cases of a wild orca killing a human.

"In captivity, we force this artificial proximity to human beings, so the orcas do act out once in a while and kill you," says marine mammal scientist Naomi Rose of the Animal Welfare Institute in Washington, DC. "They are too large to be kept in captivity."