13.4.4Measuring harvest rate
We focus attention first on estimation of legal harvest rate. In cases where it is
possible to estimate both harvest (Hˆ, the hat denotes an estimate) and the size of the
population from which the harvest is taken (Nˆ), harvest rate may be estimated as:
(13.7)based on the relationship expressed in equation (13.6). Although this approach
has seen some use (e.g. Anderson and Burnham 1976), resulting estimates are
often imprecise, and the more common estimation approach involves use of
marked birds (also see Chapter 5).
The most straightforward approach using marked birds requires a sample of
ntbirds to be individually marked before the hunting season in year t. If rtof these
marked birds are shot and retrieved by hunters during the subsequent hunting
season and reported to conservation officers, or to a national bird banding data
repository, then we can estimate a new quantity, recovery rate (ft), as:
(13.8)If multiple years of bandings and recoveries are available, then the band recovery
models noted in Chapter 5 (e.g. Brownie et al. 1985) can be used to estimate
recovery rate, as well as survival rate. Estimates based on these models make full
use of recoveries obtained in all years following banding and are thus somewhat
more precise, although the simple estimator in Equation (13.8) is fairly efficient.
Harvest rate can be estimated using estimates of recovery rate and band
reporting rate, , defined as the probability that a marked bird shot and
retrieved by a hunter is reported to the conservation agency. Recovery rate is then
related to harvest rate as ftthtso that harvest rate can be estimated from
estimates of recovery rate and reporting rate as:
(13.9)Reporting rate can be estimated from reward band studies in which some bands
are clearly inscribed with a reward that can be obtained by reporting the band.
When the reward is sufficiently large that reporting rate can be assumed to
approach 1 (see Nichols et al. 1991), the relative recovery rates of standard bands
and reward bands can be used to estimate reporting rate (e.g. Henny and
Burnham 1976; Conroy and Blandin 1984).
Estimation of harvest rate associated with illegal kill is very difficult and can be
accomplished using equation (13.7) if abundance and illegal harvest can both be
estimated. Standard approaches using marked birds are not likely to be useful,
hˆtfˆt⁄ˆt.ˆt
fˆtrt⁄nt.hˆHˆ⁄Nˆ,Assessing harvest levels| 315