A 40-page 2017 study discussing how many radio collars would be necessary to track particular herds in the Northwest Territories.
Abstract
This report addresses two complementary uses of radio collars to monitor dynamics in barrenground caribou populations in the Northwest Territories. First, I employed power analysis to evaluate the number of radio collars required to detect a change in adult female survival over time. Second, I used computer simulations to evaluate how the number of radio collars in a herd affects the probability of detecting specific proportions of a herd in aerial surveys that depend on locating radio collared animals and counting the animals in the groups to which they belong. Independent simulations were conducted for the Bluenose-East, Bluenose-West, Cape Bathurst, and Upper Tuktoyaktuk Peninsula populations. The simulations relied on 2006 survey data for group size information but I also created four artificial herds by combining the population size from one herd with the number of groups and proportional distribution of animals among groups from a different herd. In this way I was able to examine what sample size requirements might be for Bluenose-East, Bluenose-West, and Cape Bathurst herds if the animals were distributed differently among groups. For comparison, I also simulated surveys for three Alaska caribou populations based on survey data from the 1980s. To detect moderate changes in annual adult female survival (e.g. 6-7% per year, persistent for three or more years) would require 80 or more radio collars in a herd and the monitoring would need to be conducted in two or more discrete time periods. The number of radio collared animals required is the same for all herds. From the simulations I provide the numbers of radio collars that would be required to have more than 80% probability of detecting at least 90% of each herd. Figures and tabled information provide an indication of the marginal value of each additional radio collar deployed in each herd. For each sample size, I have also provided the probabilities of missing groups comprising either 10% or 5% of the total population. The number of radio collars required for a given level of probability of detecting a given proportion of a population is different for each population. Group size at the time of the survey will play a key part in survey success and is an important determinant of the number of radio collars provided. Including an assumed 6.4% observations in addition to animals in groups containing a radio collared animal, the simulations based on 2006 survey data for each herd support the following numbers of radio collars for each herd: Bluenose-East = 38, Bluenose-West = 81, Cape Bathurst = 35, and Upper Tuktoyaktuk Peninsula = 21. The marginal value of each radio collar is particularly low for the Cape Bathurst herd and reasonably low for the Bluenose-West herd, iv suggesting that lower sample sizes could be justified without much risk of underestimating herd size. By also examining the results from artificial herd simulations and considering the marginal values associated with different numbers of radio collars my recommended sample sizes for each herd are: Bluenose-East 40-60 radio collars, Bluenose-West 60 radio collars, Cape Bathurst 30 radio collars, and Upper Tuktoyaktuk Peninsula 30 radio collars. My recommendations are without full consideration of budget limitations and other factors that affect management decision making.
Related Herds: Barren-ground
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