Keeping track

Shirish Joshi makes out a case for using the camera trapping method over other alternatives to count the tigers

The pugmark method to count tiger population has limitations
The pugmark method to count tiger population has limitations

The Central Bureau of Investigation (CBI) has reported that there are no tigers in the Sariska Tiger Reserve. The seam reveals that the tiger census is a lie. It is a lie because the method it is based on i.e. counting by tracking pugmarks is inadequate.

How do they count tigers? For the past 30 years, India’s wildlife officials have used only the pugmark method to monitor tiger populations. Invented in 1966 by an Indian forester S.R. Choudhury, the method requires thousands of people to fan out across jungles for a one to two — week period every year, searching for tiger tracks.

Once located, plaster casts (or tracings) are taken of the left hind paw. Pugmarks so collected are then compared to identify individual tigers.

Cross-comparisons among census blocks and reserves refine the identifications. A reliable estimate of tiger numbers in India is made from the data.

According to Dr K. Ullas Karanth, an authority on tigers, the discrimination ability of the pugmark method completely breaks down when data from different grounds is pooled in.

In a test case, he took 32 tiger pugmark tracings from two different areas of four captive tigers in the Mysore zoo.

When shown to India’s tiger census experts in a blind test, none of them was able to segregate the individuals. The estimates of the total number of animals ranged from eight to 23 individual tigers.

This test categorically demonstrated that the pugmark method does not work and over-estimated the number.

Another method involves tranquilising the tiger first by an injection, often from the back of an elephant. After sedation, a radio-collar is fitted. The signals emitted from the collar are picked up through a hand-held antenna, which helps locate the tiger. The normal life of the antenna’s battery is three years. This is the using best method to track tigers throughout the year.

The third method involves photographing the tigers. Infrared cameras are placed around waterholes. Presence of any animal in the area triggers off the camera.

On the basis on the photographs, the tigers’ stripes are analysed. As the tigers’ have different patterns of stripes, like human fingerprints, each one is given a particular number tag.

The photographic records of individual tigers are kept over a period of time. Their presence is checked periodically, if an animal is found missing, it indicates a loss.

The greatest advantages of camera trapping could be its forensic potential. It can help trace the origin of skins and fix the place from where the tigers were poached.

As of now, tiger reserve authorities everywhere shrug off the responsibility for its skins recovered from the illegal wild life market. That will become difficult with Camera trapping.

Dr Karanth favours the adoption of the camera trapping method instead of the pugmark method to estimate the population of tigers in the country.

Dr K. Ullas Kasanth and N Samba Kumar from the Wildlife Conservation Society, Bangalore, along with William A Link, Willam E Hines and James D Nichols from the US Geological Survey have developed a mathematical model that predicts populations of predators such as tigers on the basis of the abundance of their prey.

The model was tested using data from a field study that estimated tigers and their prey densities in 11 ecologically divers sites in India. This model can be used to verify field study results.