A killer whale has died “unexpectedly” and “suddenly” at SeaWorld San Diego, the aquarium and theme park said on Friday.

The six-year-old female orca named Amaya died on Thursday and the cause of death will not be known until results of a post-mortem are complete, which could take weeks, SeaWorld’s statement said.

“Amaya began showing signs of illness on 18 August and animal care specialists and veterinarians began treating her immediately. Despite her care team’s efforts, Amaya’s condition continued to decline rapidly. Her death was sudden and unexpected,” it said.

SeaWorld said the specialists who cared for Amaya were heartbroken.

“This is a very difficult time for those who knew and loved Amaya. She inspired millions of guests to appreciate and learn more about this amazing species,” the statement added.

Seaworld has been beset by criticism from animal rights activists over keeping Orcas in captivity.

There are three SeaWorld parks in the US; in California, Florida and Texas.

Public perception of its parks dived following the 2013 documentary Blackfish, which shone a light on the lives of killer whales in captivity.

Attendance dipped following the release of Blackfish, which focused on a captive orca named Tilikum who was involved in the deaths of three people, including a SeaWorld Orlando trainer.

He was captured in Iceland in 1983 and was eventually transferred to the Florida park in 1992, where he stayed until he died at the age of 35 in 2017.

SeaWorld described Blackfish as “inaccurate and misleading” at the time of its release, and the parks have since recovered in attendance figures thanks to new rides and animal exhibits.

In 2016, SeaWorld also announced it would end its in-park orca breeding programme and eventually phase out theatrical whale shows that have been a staple of its parks for years.

According to animal rights group PETA, Tilikum is one of more than 40 orcas to have died at SeaWorld parks from causes including severe trauma, intestinal gangrene and chronic cardiovascular failure.

Dozens of dolphins have also died at the parks, PETA says.

In 2019 Virgin Holidays followed the example of Thomas Cook and said it would stop selling and promoting attractions involving “captive cetaceans, such as whales and dolphins”, with SeaWorld parks in San Diego and Florida among those affected.

SeaWorld Abu Dhabi is scheduled to open in 2022.

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