Sunday 14 October 2012

Rigorous gharial population estimation in the Chambal: implications for conservation and management of a globally threatened crocodilian– via Herp Digest


Journal of Applied Ecology
Volume 49, Issue 5, pages 1046–1054, October 2012
Tarun Nair1,2,*,
John B. Thorbjarnarson3,†,
Patrick Aust4,
Jagdish Krishnaswamy1,5
Article first published online: 13 AUG 2012
How to Cite
Nair, T., Thorbjarnarson, J. B., Aust, P., Krishnaswamy, J. (2012), Rigorous gharial population estimation in the Chambal: implications for conservation and management of a globally threatened crocodilian. Journal of Applied Ecology, 49: 1046–1054. doi: 10.1111/j.1365-2664.2012.02189.x
Author Information
1Post-graduate Program in Wildlife Biology and Conservation, Wildlife Conservation Society—India Program, National Centre for Biological Sciences, Bangalore, India
2Centre for Wildlife Studies, Bangalore, India
3Wildlife Conservation Society, Gainsville, FL, USA
4Centre for Herpetology, Madras Crocodile Bank Trust, Mamallapuram, Tamil Nadu, India
5Centre for Biodiversity and Conservation, Ashoka Trust for Research in Ecology and the Environment (ATREE), Bangalore, India
†In memoriam.
*Correspondence author. E-mail: tarunnair1982@gmail.com
Summary
India's Chambal River hosts the largest population of the critically endangered gharial. Boat-based daylight surveys to date only provide indices of relative abundance, without measures of survey bias or error. No attempt to quantify detection probabilities in these surveys has yet been made, and thus, absolute density estimates of this population remain unknown.
We surveyed 75 km of the River Chambal and photographed individual gharials for capture–recapture analysis. The total sampling effort yielded 400 captures. Population closure was supported (z = −1·48, P = 0·069), and closed-population models were used to estimate abundances.
Models were selected using the Akaike Information Criterion (AIC) index of model fit. The best model estimated 231 ± 32 adult, 83 ± 23 subadult and 89 ± 19 juvenile gharials (Mean ± SE), respectively, while the model-averaged estimate was 220 ± 28 adult, 76 ± 16 subadults and 93 ± 16 juvenile gharials, respectively.
The best model estimated absolute densities of 3·08 ± 0·43, 1·11 ± 0·3 and 1·19 ± 0·25 adult, subadult and juvenile gharials km−1, respectively, while the model-averaged estimate was 2·93 ± 0·37, 1·01 ± 0·21 and 1·24 ± 0·21 adult, subadult and juvenile gharials km−1, respectively, compared with relative densities of 0·94, 0·45 and 0·30 adult, subadult and juvenile gharials km−1, respectively, from boat-based daylight surveys. On the basis of our best model, we suggest a detection probability based correction factor of 3·27, 2·47 and 3·97 to boat-based daylight survey estimates of adult, subadult and juvenile gharials, respectively.
Synthesis and applications. Used within the framework of capture–recapture analysis, photoidentification provides a reliable and noninvasive method of estimating population size and structure in crocodilians. We also opine that without determining the current status of gharials, highly intensive strategies, such as the egg-collection and rear-and-release programmes being implemented currently, initiated on the basis of underestimates of population sizes, are unwarranted and divert valuable conservation resources away from field-based protection measures, which are essential in the face of threats like hydrologic diversions, sand mining, fishing and bankside cultivation.

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