Bird Ecology and Conservation A Handbook of Techniques

324 |Exploitation

Box 13.3Case study: adaptive harvest management of Mallards

In North America, duck harvest is regulated by annually setting a common sport

hunting season for a number of species, with the length of the season dependent

on the current status of the duck populations and the conditions of the breeding

habitat (Nichols et al. 1995; Williams and Johnson 1995; Johnson et al. 1997;

Williams 1997). Presently, the status of Mallards (Anas platyrhynchos) is used as a

surrogate for the status of most other duck species. An explicit adaptive manage-

ment approach is used to choose the regulations each year ( Johnson et al. 1997).

The two fundamental uncertainties about population dynamics and responses to

management that are being addressed by this approach are (1) whether the effect

of harvest is additive to, or compensatory with, natural mortality; and (2) the

degree to which reproduction depends on density; four alternative models of

Mallard population dynamics capture this uncertainty. An optimal state-dependent

harvest policy is calculated based on the alternative models and the objective to

maximize cumulative harvest over an infinite time horizon, subject to a constraint

that reflects a desired minimum spring population size. Each May, Mallard popu-

lation size and the number of ponds in prairie Canada are estimated through

aerial surveys. The particular set of regulations associated with values of these

“state variables” is taken from the optimal harvest policy and implemented. The

four models all predict the population size in the next year, the observed value for

this quantity is used to adjust the weights associated with each model, and the

process continues.

The life-history information that went into building the models for management

of Mallard harvest is substantial, and is based on a number of long-term, large-

scale monitoring efforts: population size has been estimated with aerial surveys

for over 50 years; annual survival rates, harvest rates, and differential vulnerabili-

ties of adult and immature Mallards are estimated from band returns; and repro-

ductive rates are estimated from the ratio of immature to adult birds in the

harvest. Harvest of this species can be maximized, with little risk to the popula-

tion because of this extensive knowledge of the population dynamics, because the

monitoring is designed to specifically inform management, because relevant

uncertainty is incorporated directly into the process and because management

can respond annually to current conditions.

to inform the models of the population dynamics and to update their associated
weights. The increased knowledge is used in the next time step to select the appro-
priate management action. Ideally, the optimization is made actively, that is, taking
into account the potential effects of learning on future management decisions
(Walters and Holling 1990).