Instantaneous Sampling: An Approach to Estimating Population Size Using Imagery of Unmarked Animals
B.J. Adams, Kanalu Sproat, Randy T. Larsen, Brock R. McMillan
Introduction
Accurate estimations of population size can help inform wildlife management decisions.
Traditional approaches to estimating population size can be expensive, risky, and invasive.
Remote cameras have become a popular method of estimating population size via methods such as Capture-Recapture (CR) which involves placement of anthropogenic marks such as ear tags to re-identify individuals.
Relying on resightability of marked animals can potentially introduce bias in estimations (ie. animals improperly postured to clearly display marks).
Our objective is to estimate population size with a developing methodology known as Instantaneous Sampling (IS) and compare it with estimates from a traditional method (CR).
Accurate estimations of population size can help inform wildlife management decisions.
Traditional approaches to estimating population size can be expensive, risky, and invasive.
Remote cameras have become a popular method of estimating population size via methods such as Capture-Recapture (CR) which involves placement of anthropogenic marks such as ear tags to re-identify individuals.
Relying on resightability of marked animals can potentially introduce bias in estimations (ie. animals improperly postured to clearly display marks).
Our objective is to estimate population size with a developing methodology known as Instantaneous Sampling (IS) and compare it with estimates from a traditional method (CR).
Statistical Concepts
Instanateous Sampling calculates population density (D̄) within the cameras' viewable area (aij) by taking the mean count (nij) of individuals at location i = 1, 2,... M and occasion j = 1, 2,... J.
Viewable area (aij) is calculated as a sector of a circle determined by the camera's lens angle (ϴij) and view distance (rij):
Instanateous Sampling calculates population density (D̄) within the cameras' viewable area (aij) by taking the mean count (nij) of individuals at location i = 1, 2,... M and occasion j = 1, 2,... J.
Viewable area (aij) is calculated as a sector of a circle determined by the camera's lens angle (ϴij) and view distance (rij):
Methods
We marked 120 feral sheep (O. gmelini musimon and O. aries) during September 2016 - October 2017 to enable resighting for CR.
When sheep with ear tags are found in camera imagery, they are considered "recaptured."