The number of tigers, Panthera tigris sumatrae, in the park has declined from 300 to 165 in the last decade, Dian Rusdianto of Indonesia's TNKS tiger conservancy said.
Poaching remains a major problem in the region. TNKS officers recently confiscated 120 tiger traps hunters had set in the park, China's state-run Xinhua News Agency reported Thursday.
Deforestation is also having an impact on the tigers, Rusdianto said.
"The diminishing population isn't only caused by hunting and poaching, but also forest destruction by corporations which have damaged the park," she said.
The forest damage not only threatens the habitat of the Sumatran tiger but also the tigers' prey on such as deer, tapir and warthog, she said.
"The ones that need to be protected are animals which the tigers prey on because they can survive in any conditions as long as they have food all the time," Dian said.
The Sumatran tiger is the last tiger species in Indonesia; two other species of the big cats, the Bali and Java tigers, have been extinct since the 1950s and 1960s, respectively.