Environmental Engineering Reference
In-Depth Information
Fig. 7.8 Monthly squid
catches by licensed
vessels in the Falkland
Islands where a
constant escapement
management strategy is
used. Note that there
are two peaks in
catches each year - this
is because there are two
fi shing seasons
(February-May and
August-October).
Dotted lines (1984-86)
represent estimated
rather than actual
catches. The total
annual catches have
held up well, indicating
that the constant
escapement strategy is a
sustainable approach.
(After des Clers, 1998.)
35,000
30,000
25,000
20,000
15,000
10,000
5000
0
1983
1985
1987
1989
1991
1993
1995
Year
breeding individuals before breeding has occurred. It has a downside, however,
and that is the expense of continuous monitoring through the hunting season.
Constant escapement is particularly appropriate for annual species that mature
together, breed once and then die, because these lack the buffer provided by imma-
ture individuals in longer-lived species (Milner-Gulland & Mace, 1998). The Falk-
land Islands Government uses a constant escapement strategy for the annual Loligo
squid. Stock sizes are assessed weekly from mid-season onwards and the fi shery is
closed when the stock falls to 0.3-0.4 B 0 . After 10 years of this management regime
the squid fi shery is holding up well, indicating that the approach is sustainable
(Figure 7. 8).
Note that the fi shery for Pacifi c whiting (Section 7.2.2), a nonannual species,
also has an element of constant escapement in that the fi shery is not opened in a
year when the stock has fallen below 0.1 B 0 . And the approach can be used for
mammal hunting too. Stephens et al. (2002) used simulation models to compare the
outcomes for a population of alpine marmots ( Marmota marmota ) of fi xed quota,
fi xed effort and constant escapement harvesting. In the latter case, harvesting only
occurred during years in which the population exceeded a given threshold and
exploitation continued until that threshold was reached. These social mammals are
hunted in parts of Europe but the model was based on extensive data available from
a nonhunted population. Stephens' team found that constant escapement harvesting
provided the highest mean yields and this was coupled with a low risk of
extinction.
7. 2 . 4 Management
by constant
escapement - in
space
A different way to approach a constant escapement strategy is to place spatial con-
trols on fi shing or hunting, as opposed to the temporal controls of closing a season
early or not having a season at all. While nature reserves around the world have
usually been established to conserve biodiversity, some could also play a role in
harvest management. The protection of 10% of a hunted population in reserves (e.g.
shell fi sh in coastal marine reserves, reef fi sh in coral reef reserves, Saiga antelope
in national parks) is potent ially equivalent to providing a fi xed escapement of 0.1 B 0 .
However, to be truly equivalent, the unexploited reserve should then be the source
of recruits to the harvest at large. And this depends on whether the reserves are
naturally sources or sinks of recruits, as determined by dispersal powers in the face
of factors such as ocean currents. These ideas were discussed in the context of dis-
persal behavior in Section 4.2.4.
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