So many animals have perished at Indonesia's biggest zoo that wildlife activists call it the "zoo of death" and are demanding an overhaul of its management.

Activists say many of the more than 2,200 animals at the zoo in the city of Surabaya are crowded into cages and enclosures far too small for them, and they also face a shortage of proper feed.

One of the latest losses was a rare Sumatran tiger that died unexpectedly last month.

"They need to make an effort to ease the overpopulation of animals," Petrus Riski of the Indonesia Wildlife Communication Forum said at the zoo as keepers carried a crate of fish into a congested pen of pelicans.

"It can be done by sending them to other conservation institutes."

Zoo keepers attribute most of the deaths to natural causes, and said the tiger's death was still unexplained. But activists point to a string of unusual incidents that undermine their confidence in the zoo, which was founded in 1916.

An 18-month African lion was found hanging dead in its cage in 2014 and a dead giraffe was found with about 18 kg of plastic in its stomach - rubbish thrown into its cage by visitors.

About 45 Komodo dragons, a large species of lizards only found in eastern Indonesia, died in battles they fought against each other in their overcrowded cage.

The zoo's director blamed bureaucratic hurdles hampering efforts to improve conditions.

"We've been trying to resolve these issues one by one," said director Aschta Boestani Tajudin.

"I hope in three to four months from now we can finally solve the problem."

But critics are not convinced. They say poor staff training and outdated facilities are to blame for the zoo's woeful record.

"They need more support and funds to really fix things," said Tony Sumampau, secretary-general of the Indonesian zoo and aquarium association.