untitled

This escapement policy is obviously safe, at least in principle. It only becomes unsafe

when we are uncertain of the abundance or recruitment. Hence, wildlife or fisheries

managers would usually have to forecast abundance at the beginning of the harvest

period from past levels of abundance, based on past harvest statistics themselves. To

demonstrate how one might model a fixed escapement strategy, we shall again use

the stochastic recruitment model for martens. The fixed escapement (ν=723) was

chosen to approximate the marten abundance at which the MSY occurs under aver-

age conditions of prey abundance and weather. The equation of change in marten

abundance (N) in the next time interval is defined below:

Although marten abundance varies considerably less (Fig. 19.11) than was the case

for fixed quota (Figs 19.5 and 19.6) or fixed proportion (Figs 19.8 and 19.9) poli-

cies, harvest varies even more according to this policy (Fig. 19.12).

N

gN Z fN Z

gN Z fN Z

t

ttt tt

ttt tt

+ =

<−

−−

1

0

( , , ) ( , )

( , , ) [ ( , ) ]

if

otherwise

εν

εν

WILDLIFE HARVESTING 345

1000

800

600

400

200

0

0 5 10 15

Year

Predicted marten abundance

Fig. 19.11Predicted
variation in marten
abundance under a
fixed escapement
harvest policy, with an
escapement (ν=723)
appropriate to the
maximum sustainable
yield (H=423 martens
per year), starting from
a population size of
400 animals in 1973.

800

600

400

200

(^0051015)
Year
Predicted marten harvest
Fig. 19.12Predicted
variation in marten
harvests under a fixed
escapement harvest
policy, with marten
escapement (ν=723)
appropriate to the
maximum sustainable
yield (H=423 martens
per year) under average
environmental
conditions, starting from
a population size of 400
animals in 1973.