untitled

A harvest can be controlled either by placing a quota on offtake or by controlling

harvesting effort. The latter can be regulated by setting a hunting season or by

limiting the number of people harvesting the population. The essence of controlling

effort is that there is no direct attempt to control the number of animals harvested.

An important outcome from controlling effort is that a constant proportionof the

population is being harvested. Suppose, for example, that each day harvesters effec-

tively sample 2% of the area inhabited by a harvested species. We can mathematic-

ally depict this by saying that the catchability coefficient (q) =0.02. If hunters remove

all animals that are encountered in the area that they sample, then the yield (H) would

increase with both effort (E) and population density (N) in the following manner:

H(N) =qEN

Note that fixed effort policies have a built-in mechanism to reduce exploitation levels

should resource density decline to dangerously low levels, because harvest levels also

drop automatically with declines in resource abundance. Note also that a conserva-

tive effort level might yield a similar equilibrium harvest as more extreme effort, which

could encourage a more moderate policy (the equilibrium harvest is the point at which

the yield function intersects the net recruitment curve). Both of these characteristics

tend to have positive moderating influences on population dynamics, as we shall demon-

strate later in the chapter.

The control of harvest by quotas has an intuitive appeal because there is a direct

relationship between the prescription and the result. In contrast, with the harvest

regulated by control of harvesting effort an intermediate step has been inserted between

prescription and outcome. Administrators tend to favor regulation by quotas because

the size of the yield is directly under their control.

In fact, the disadvantages of regulating effort are more conceptual than real. Regula-

tion of effort is usually a safer and more efficient means of managing harvested popula-

tions than is regulation by quota. Harvesting a constant number of animals each year

is inefficient when the population is subject to large, environmentally induced, swings

in density. The quota must be set low enough to be safe at the lowest anticipated

density, or alternatively the size of the population must be censused each year before

WILDLIFE HARVESTING 343

Year

Predicted marten harvest

500

400

300

200

100

0

0 5 10 15

Fig. 19.9Predicted
variation in marten
harvests under a fixed
proportion harvest
policy, with annual
marten harvests set
at the proportion
(h=0.36)
approximating the
maximum sustainable
yield (H=423 martens
per year) under average
environmental
conditions, starting
from a population size
of 400 animals in 1973.

19.3.2Constant

effort harvesting

strategy