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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