SAN ANTONIO (Reuters) - An 18-year-old killer whale has died at the SeaWorld park in San Antonio, the park said on Tuesday (Dec 22), raising concern among animal groups that have criticised the company for years over its treatment of captive marine mammals.

The female whale, named Unna, was the third whale to die at the Texas park in six months.

SeaWorld said in a statement that Unna died on Monday from a "resistant strain of a fungus called Candida."

The killer whale, or orca, had been treated by experts from around the country, it added. "While there were some indications that the treatment was having a positive effect, Unna had remained in serious condition and under 24/7 care," SeaWorld said, adding that a necropsy will be conducted.

SeaWorld has faced heated criticism and declining revenues since the release of the 2013 documentary film "Blackfish,"which depicted the captivity and public exhibition of killer whales as inherently cruel.

The film, which SeaWorld has criticised as inaccurate and misleading, also explored the circumstances leading to the 2010 death of a top SeaWorld trainer, who was pulled underwater and drowned by an orca she had performed with in Florida.

SeaWorld in San Diego recently announced plans to suspend its live whale shows. But the other Sea World parks, including the one in San Antonio, say they will continue, contending the orcas are well treated.

Critics contend it is inhumane to confine the intelligent and wide-ranging orcas at sea parks, where they perform in circus-like shows.