Something disquieting happened at SeaWorld marine parks this year. Numbers attending the group’s popular US centres between January and March dropped, from 3.5 million in 2013 to 3.05 million this year, a decline of 13%.

Nor is it hard to guess the cause, say wildlife campaigners. They see a clear link between the attendance slide and the release last year of the documentary Blackfish, which told the story of SeaWorld trainer Dawn Brancheau who was killed by Tilikum, a bull orca. The killer whale, it was also revealed, had been involved in the deaths of other individuals while in captivity.

Blackfish focuses on the distress experienced by killer whales who are depicted as complex, highly intelligent creatures which are taken from their families, kept in small pools and given psychotropic drugs to calm them and help them perform tricks that include balancing human trainers on their snouts, rotating in the water to pop music, waving their flippers and tails, and floating on their backs. The film triggered widespread public outrage against marine parks in general and a petition, signed by 1.2 million people, was handed into the California state assembly calling for a ban on killer whale shows. Earlier this month, a bill legalising the ban was put on hold for the next 12 months. Campaigners are still hopeful it will be enacted next year.

Read more Deaths, drugs, distress: why marine parks are losing their attraction