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A sampling design and model for estimating abundance of Nile crocodiles while accounting for heterogeneity of detectability of multiple observers

Journal of Wildlife Management

By:
, , , , , and
DOI: 10.1002/jwmg.348

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Abstract

As part of the development of a management program for Nile crocodiles in Lake Nasser, Egypt, we used a dependent double-observer sampling protocol with multiple observers to compute estimates of population size. To analyze the data, we developed a hierarchical model that allowed us to assess variation in detection probabilities among observers and survey dates, as well as account for variation in crocodile abundance among sites and habitats. We conducted surveys from July 2008-June 2009 in 15 areas of Lake Nasser that were representative of 3 main habitat categories. During these surveys, we sampled 1,086 km of lake shore wherein we detected 386 crocodiles. Analysis of the data revealed significant variability in both inter- and intra-observer detection probabilities. Our raw encounter rate was 0.355 crocodiles/km. When we accounted for observer effects and habitat, we estimated a surface population abundance of 2,581 (2,239-2,987, 95% credible intervals) crocodiles in Lake Nasser. Our results underscore the importance of well-trained, experienced monitoring personnel in order to decrease heterogeneity in intra-observer detection probability and to better detect changes in the population based on survey indices. This study will assist the Egyptian government establish a monitoring program as an integral part of future crocodile harvest activities in Lake Nasser

Additional Publication Details

Publication type:
Article
Publication Subtype:
Journal Article
Title:
A sampling design and model for estimating abundance of Nile crocodiles while accounting for heterogeneity of detectability of multiple observers
Series title:
Journal of Wildlife Management
DOI:
10.1002/jwmg.348
Volume
76
Issue:
5
Year Published:
2012
Language:
English
Publisher:
Wiley
Publisher location:
Hoboken, NJ
Contributing office(s):
Southeast Ecological Science Center
Description:
9 p.
Larger Work Type:
Article
Larger Work Subtype:
Journal Article
Larger Work Title:
Journal of Wildlife Management
First page:
966
Last page:
975
Country:
Egypt
Other Geospatial:
Lake Nasser