untitled

the harvesting season, the quota being adjusted according to the estimate. In addi-

tion, regulation by quota is unsafe when the quota is near the MSY. As mentioned

in the previous section, the density at equilibrium with that yield is unstable such that

a small environmental perturbation may trigger a population slide towards extinction.

If yield is controlled indirectly by limiting harvesting effort (e.g. by limiting the

number of hunters), but with no further restriction on yield per unit of effort, those

dangerous sources of instability we mention above are eliminated. A fixed effort sys-

tem will, within limits, harvest the same proportion of the population at high and

low density. Yield tracks density, the system automatically producing a higher yield

when animals are abundant and a lower yield when they are scarce. A regulatory

mechanism is built into the harvesting system itself and it is thus fairly safe so long

as the appropriate harvesting effort has been calculated correctly. That is not difficult

because fine-tuning of the appropriate effort does not destabilize the system in the

way that fine-tuning a quota can. Also, because of the built-in regulation there is not

the same need for frequent monitoring.

The traditional means of setting harvests have been fixed quotas, fixed proportion,

and fixed effort policies. In recent years, conservation biologists have argued that the

truly safest option is to practice what is called fixed escapement harvesting. The premise

of fixed escapement policies is this: rather than trying to maintain high levels of har-

vests in the face of stochastic variation in resource levels, managers instead choose

to place conservation needs ahead of that of resource users. The general procedure

is to harvest only “excess” animals above a target threshold, termed the escapement.

After recruitment takes place the excess is removed by harvesters. This guarantees

that recruitment never falls below the threshold, even if it means that no animals are

harvested in some years (Lande et al. 1994, 1997). For example, imagine that escape-

ment is set at 75 individuals, for a population with rmax=1 and K=100. Harvests

are set according to the following formula, where f(N) is the net recruitment func-

tion in the absence of stochastic variation in the environment (Fig. 19.10):

HN

fN

fN

()

( )

()

=

≤

−

0 if escape

escape otherwise

344 Chapter 19

19.4 Fixed escapement harvesting strategy

40

30

20

10

0

–10

0 50 100 150

N

Net recruitment or harvest

Net recruitment

Harvest

Stable equilibrium

Fig. 19.10Net
recruitment in the
absence of harvest in
relation to population
density, plotted relative
to a fixed escapement
harvest. The intersection
of the net recruitment
curve and the harvest
line identifies the stable
equilibrium, at which
offtake equals the
growth increment to the
population.