The population of the famous Royal Bengal tiger is decreasing as a result of dwindling prey animals, particularly deer, in India's tiger reserves, as shown by studies of environmental groups.

After the treatment of a Bengal tiger at the Alipore Zoological Gardens for starvation, a report by the Wildlife Institute of India (WII) has put the prey base inside the Sunderbans Tiger Reserve (STR) in India's eastern state of West Bengal under scrutiny.

The progress report of the study for 2011, titled 'Population Estimation And The Ecology Of Tigers In Sunderbans Tiger Reserve(STR)', was done jointly by WII, National Tiger Conservation Authority(NTCA) and the STR.

It has found a very low prey density in Sajnekhali, in the mangroves, and the west range of STR.

WII's scientist Y. V. Jhala said a total of 187 kilometers of boat transects was carried out between February and May to get an idea on the prey density based on the visual detection of animals. "Each boat transect was repeated for a minimum of three times and a maximum of six times. Prey density along the creeks surveyed in Sajnekhali and west of STR is comparatively low with only 13.3 chitals (deer) per square kilometer," Jhala said. "Taking into account the entire 2,500 square kilometers of STR area, the number of deer, considered the major prey for tigers, will be only a little over 30,000. This prey density is very low considering the huge area of Sunderbans," he said.

Earlier a study by scientists Ullas Karanth and George Schaller had said that there should always be a natural balance between the prey and number of tigers. A total of 500 deer could provide sufficient food base to only one tiger, keeping in mind that the breeding rate of deer was always balanced by the killing rate of tigers. Going by that figure, the poor density of deer, considered a hoofed prey for the big cats, could only support a population of 60 to 65 tigers in the Sunderbans, the study said.

The WII's alert that herbivores cannot take salinity after a point of time only points to the dwindling prey base and eventually a likely fall in tigers' number. Eminent conservationist and scientist Ullas Karanth also agreed."Broadly, the Sunderbans is an inherently low prey density, poor quality tiger habitat, as my first ever camera trap study in 1998 showed clearly.

This fact is now being reinforced by more detailed behavioral studies being done by WII. If wild tigers get sick or injured naturally (not through human interventions),it is not necessary to rescue and treat them," Karanth said.

"Only long term population dynamic studies of the kind we are doing in Karnataka will tell us if the population in Sunderbans' is stable, increasing or declining...This should be the priority, if rational management decisions are to be made," he said.

A member of West Bengal wildlife advisory board, Biswajit Roy Chowdhury, said that after hurricane Aila hit the area, the prey base in the mangroves has definitely dwindled. "The starvation in this Sunderbans tiger only points to lack of food inside the forests," he said.

Another member of the board, Joydip Kundu, said: "If the study is to be believed, there is immediate need to increase the prey base by shifting additional prey animals from other sanctuaries to Sunderbans."

The 187 kilometers of WII's study area inside Sunderbans Tiger Reserve recorded very low density of deer and almost no sighting of wild pigs. Experts called for detailed study to check whether any disease outbreak is behind repeated tiger straying in the Sunderbans.

In 2009-2010, there were reports of frequent straying of Siberian tigers into Russian villages and towns. It was later found the big cats were infected with canine distemper.