production. Thus, this method guards against the other type of error: determin-
ing that the exploitation is unsustainable when in fact it is. That is, if a harvest is
determined to be unsustainable using Robinson and Redford’s method, you can
be assured of that conclusion; but the method cannot be used to determine
whether the actual harvest is sustainable (Robinson 2000). It is critical that an
assessment of sustainable exploitation acknowledges what sort of error it is
guarding against.
13.3 Harvest control and management objectives
13.3.1Harvest management strategies
The PBR equation provides a target level for total harvest, but the manager has
to figure out what management actions can be implemented to achieve that har-
vest level. Newton (1998, Table 14.2) describes the advantages and disadvant-
ages of 6 different harvesting systems: free-for-all, fixed quota, fixed effort,
variable quota, fixed percentage, and fixed escapement. In a free-for-all system,
there is no control of the harvest, which makes it easy to administer, but poten-
tially a significant conservation concern. As the harvesting systems improve in
their ability to conserve the resource, the administrative burdens and monitor-
ing costs increase. In a fixed quota system, the same number of animals are
allowed to be removed each year; this may be relatively easy to administer in
small areas with good enforcement effort and results in a constant yield, but can
be very unstable, leading to sharp population declines if the population size
drops below a critical level. Stepping to a variable quota system, where the
number harvested each year depends on the population size, greatly increases
the ability to conserve the resource, at the cost of increased monitoring
and enforcement. The PBR calculation implies either a variable quota system
(equation 13.4) or a fixed percentage system (equation 13.5, with FRretained).
These systems are much more robust to uncertainty than the fixed quota
system. The decision about what harvesting system to employ depends heavily
on the specifics of the situation: local laws and customs, enforcement ability,
avenues for communication with the users, monitoring potential, and other
political pressures. Managers need to combine these considerations with the
ecological properties of the harvesting system to identify the best course of
action. Two suggestions can be offered: first, management actions that are
intended to affect the harvest rate (relative to the population size) will be more
protective of the resource (without sacrifice of harvest potential) than actions
intended to affect the total annual harvest; and second, management actions
that depend on the state of the system (especially population size or density) will
310 |Exploitation