Bat experts have slammed the recent filming of a Sylvester Stallone movie in a Bulgarian cave, saying thousands of the mammals had suffered from the Hollywood treatment. \"Several thousand bats are missing from the Devetashka cave since our latest count in January when the visible colonies numbered 33,800 bats,\" Boyan Petrov of the Museum of Natural History said Thursday. A rough count after the filming of the star-studded Hollywood action blockbuster \"The Expendables 2\" in November found only about 8,500 bats. \"Never before have we counted less than 10,000 bats in wintertime,\" Petrov said, adding however that he hoped the rest had just been chased away to adjacent caves. The environment ministry, which authorised the shooting, said this week in a statement that it had found no problem with the bat colonies. But the experts insisted filming inside the cave should never have been allowed. \"As a protected nature territory and important bat habitat, the cave is only open for visits by tourists and researchers and totally sealed during the breeding period in June and July,\" said Dobromir Dobrinov of the Green Balkans nature conservation group. Expert Antonia Hubancheva, who visited the cave after the shooting, said the damage was \"unquestionable.\" She said the animals were subjected to stress, loud noise from heavy machinery and construction works, bright projector lights and crowds of people which kept them awake during a period when they should have been hibernating. \"The vandalism which was allowed to happen is intolerable,\" bat researcher Vladimir Beshkov fumed. \"It is scientifically proven that all bats disturbed during the winter period drop off when spring comes. And these bats were badly harmed -- they were scattered, scared, chased, woken up!\" he said. The Devetashka Cave, near Lovech in northern Bulgaria, is one of the top three most important bat habitats in Europe, the experts pointed out. \"It is home to 15 protected species of bats, or half of all bats found in Bulgaria,\" Petrov said. Researchers will perform a new count of the colonies next spring to assess the damage. Filming in Bulgaria for \"The Expendables 2,\" starring Stallone, Arnold Schwarzenegger, Bruce Willis and Jean-Claude Van Damme, was already marred by an accident in which one stuntman died and another was injured during a lake scene involving an explosion. The movie is due for release in August 2012.