Harvest management - Ecological Applications: Toward a Sustainable World - page 193

may want to see an overall reduction in moose, perhaps because they are overex-
ploiting their own food resources, and so sets a higher quota (or vice versa if the
population has declined due to adverse weather). Similar approaches are used around
the world for harvests of game birds, waterfowl, mammals, and even reptiles such
as crocodiles, with quotas of animals permitted to be taken from each hunting area
(often quite small in extent) set according to local circumstances.

7. 2 . 2 Management
by fi xed effort - of
fi s h a n d antelopes

Theory tells us there is less risk of overexploitation with fi xed effort than fi xed quota
strategies (Box 7.1); as a result many harvests are managed by legislative regulation
of effort. And this is despite the diffi culty of measuring and controlling effort in a
precise manner. For example, an agency may issue a fi xed number of gun licenses
with the expectation that this will produce constant hunting effort from year to
year, but the skill of the hunters is uncontrolled. Similarly, regulating the size and
composition of a fi shing fl eet, as a means of controlling effort, leaves the weather
to chance.
Consider again the Pacifi c whiting (Figure 7.4). The fi shery for this species is
shared between Canadian and US fi shing fl eets. But in the 1990s the two national
agencies could not agree how to share the catch and, as a result, the total allowable
catch (TAC) was exceeded, probably contributing to reductions in the size of the
stock during these years (Figure 7.4a). In 2003, a treaty was signed to improve
management of Pacifi c whiting, with 73.88% of the TAC going to the USA and
26.12% to Canada. The method of determining the TAC is of the fi xed effort variety,
using what is known as the '40-10 harvest strategy'. This rule automatically imposes
the constraint that fi shing effort should not exceed that required to produce the
MSY - taken as equal to recruitment rate when the population is 40% of unexploited
biomass (i.e. 0.4 B 0 ) - and fi shing ceases if the stock biomass drops to 10% of unex-
ploited biomass (0.1 B 0 ). Between 0.4 B 0 and 0.1 B 0 there is a straight-line effort curve
(as for E M in Figure 7.3a), but w ith the added safeguard that at low population density
(10% or less of B 0 ) no catch is taken at all. Ishimura et al. (2005) carried out simula-
tions of a population model (like that used for population viability analysis in
Section 5.4) to test the effectiveness of the 40-10 rule for Pacifi c whiting. They con-
cluded that the rule worked well - producing acceptably large annual catches, with
small variation in catch from year to year, and only a small number of years when
the fi shery would have to be closed.
The Saiga antelope ( Saiga tatarica ) has been hunted for centuries in the deserts
of Central Asia for its meat, hide and horns. Until 1990 the population was inten-
sively managed by the Soviet state, but management broke down and poaching
increased after the break-up of the Soviet Union. Milner-Gulland (1994) produced
a simulation model, based on extensive information about birth rates and death rates
and environmental variation, to test whether fi xed quota or fi xed effort management
regimes would be best for the antelope's future management. Males are worth more
than females because their horns, used for medicine, are much more valuable than
meat. Thus, the model considered the consequences of including different propor-
tions of males and females in the catch. Allowing a constant number of individuals
to be taken is administratively more straightforward, but this is a poor strategy for
the long-term survival of the population (Figure 7.7a). To avoid the well-known risk
of overexploitation in a fi xed quota harvest, Milner-Gulland suggests that hunting
would have to be kept to a very low level (about 2% of the population per year, or