Harvest management - Ecological Applications: Toward a Sustainable World - page 190

gun-days may yield 100 birds when the population is small, but 200 birds when it is twice as large,
and 300 birds when it is three times as large). A plot of harvesting rate for constant effort is thus
a straight line, increasing linearly with density and starting at zero (with zero effort). Four fi xed
effort lines are plotted in Figure 7.3a. There is an 'optimum' effort giving rise to the MSY, E M , while
efforts both greater ( E H ; steeper effort line) and smaller than this ( E L ) provide smaller yields. The
relationship between yield from the harvest and effort is shown in Figure 7.3b: the MSY is gained
with effort E M .
Both a fi xed quota ( H M ) and a fi xed effort ( E M ) can be used to provide the MSY. But the really
good thing about the fi xed effort approach is that the equilibrium (where harvest rate equals recruit-
ment rate) is stable, in contrast to the unstable equilibrium of the fi xed quota. Now, in contrast to
Figure 7.2, if density drops below N M (Figure 7.3), recruitment exceeds the harvesting rate and the
population recovers. In fact, there needs to be a very considerable overestimate of E M before the
population is overexploited and driven to extinction ( E OV in Figure 7.3).

(a) (b)

E H

E OV

E M

H M MSY

E L

N H N M N L E H E M E L

Population size ( N ) Effort

Fig. 7.3 Fixed effort sampling. (a) The maximum sustainable yield ( H M ) is obtained with
effort E M , leading to a stable equilibrium at density N M . At higher (steeper dotted line) or
lower efforts, there are also stable equilibria where the effort lines cross the recruitment
curve, but the yields are less than the maximum sustainable. The very high effort line ( E ov )
does not cross the recruitment curve; in other words, harvesting rate is always greater than
recruitment rate, the population is overexploited and will be sent extinct. Curves, arrows
and dots as in Figure 7.2. (b) The relationship between yield from the harvest and effort
applied. The MSY occurs with an effort of E M .

Despite all its shortcomings, the MSY concept dominated resource management for
many years in fi sheries, forestry and wildlife exploitation. Prior to 1980, for example,
every one of the 39 management agencies for marine fi sheries around the world
managed on the basis of an MSY objective (Clark, 1981), and in many harvests the
MSY concept is still the guiding principle.

7. 2 Harvest
management in
practice - maximum
sustainable yield
(MSY) approaches

7. 2 .1 Management by
fi x e d q u ota - of fi sh
and moose

On a specifi ed day in the year, the hunting season (for a given species) or a fi shery
is opened and the cumulative catch is logged. The permitted quota is set at the
estimate of MSY for the exploited species and, when the quota has been taken, the
hunt is closed for the rest of the year. The ideal situation would occur where con-
siderable data are available on net recruitment and density - so that a plot like that
in Figure 7.2 can be used to estimate the MSY. But such data are rarely available
and, even when they are, the pattern does not necessarily conform to the idealized
form in Box 7.1. Thus, Figure 7.4a shows estimates for both recruitment (of