Decoded: Sundarbans crocodiles’ salinity tolerance, basking temp & creek preference
Times of India | 11 July 2024
Kolkata: A recent survey has revealed the saltwater crocodiles’ habitat preference in the Indian Sundarbans. The comprehensive crocodile census report in the Indian Sundarbans, released recently, said saltwater crocodiles here exhibit a preference for creeks with width less than 170 metres.
Salinity tolerance for the crocodiles also fluctuate between 14 and 26 parts per thousands (PPT) — grams of salt per kilogram of seawater.The Indian Sundarbans is now home to 204-234 saltwater crocodiles. These are merely nuggets of information gathered on saltwater crocodiles during the estimation survey earlier this year.
It also furnished data on their preference for ambient temperature during basking. Spanning over three days in Jan this year, the study says that crocodiles here prefer an ambient temperature of 20 to 28°C for basking.
“Maximum sightings happened when the ambient temperature was 22.7 degrees Celsius. With the water surface temperature (WST) lingering around 20°C and 22°C throughout the exercise, the crocodiles emerged in maximum numbers when the ambient temperature was 1-3 degrees Celsius above the WST,” said Rajendra Jakhar, field director of Sundarbans Tiger Reserve (STR).
While the crocodiles here are tolerant to a wide range of salinity extending from 10 PPT to 28 PPT, a closer analysis divulged that the preferred range is between 14 and 26 PPT. The study also warned that rising salinity may reduce their habitat in the Sundarbans.
Asked about how the salinity figures were derived, Jakhar said the teams involved collected water samples from areas, particularly those zones where sightings occurred, and measured salinity levels with a device called a refractometer.
Sightings became nearly negligible in creeks and rivers having a width of over 250 metres. “They prefer creeks wide from 10 metres to 170 metres during high tide,” the study said. The forest department has decided to continue the exercise for three more years. “Every year, the exercise will span over three months — Nov, Dec, and Jan,” added Jakhar. The last exercise in 2012 had pegged the number at 140.
“I always felt the Sundarbans crocodile population is stable and this data backs it up. This initiative should be carried out more frequently. There is a need to study impact of global warming on their population and male-female ratio. A crocodile embryo’s sex is dependent on the ambient temperature at which it gets incubated, and that determines the sex of the hatchling. Due to global warming, number of female hatchlings may overshadow male hatchlings. This may jeopardise the stable population in future,” said herpetologist Anirban Chaudhuri, who has worked with the Sundarbans saltwater crocodile breeding program for over a decade and was part of the survey team.