Environmental Engineering Reference
In-Depth Information
Fig. 7.13 Changes over
25 years in (a) mean
body weight and (b)
horn length of 4-year-
old bighorn rams on
Ram Mountain, Alberta,
Canada. (After Coltman
et al., 2003.) (c)
Changes in mean size-
at-maturity of cod
( Gadus morhua ) in one
area of the northwest
Atlantic. Solid circles
represent mean size-at-
maturity during each of
1959-64, 1965-69,
1970-74 and 1975-79.
Simulated values for the
same time periods are
for populations in
which body size-at-
maturity is assumed to
be heritable (diamonds)
or where variation is
not determined
genetically but simply
according to random
environmental effects
(squares). The actual
change in size-at-
maturity corresponds
closely with the model
with heritability,
indicating that it is an
evolutionary response
to size-selective fi shing.
(After Baskett et al.,
2005.)
(a)
110
100
90
80
70
60
50
(b)
80
70
60
50
40
Year
(c)
70
60
50
40
30
20
1950
1960
1970
1980
Year
cases where harvest mortality is low, protected areas may provide no benefi t, and
may even reduce overall yield because part of the fi shery is removed from the
harvest. When they compared the use of nonharvested areas with more traditional
management approaches, Baskett and his team found that increasing the minimum
size limit protected to some extent against both evolution to small size-at-maturity
and lower yields. But only when a maximum size limit was also imposed (fi sh above
this size must not be caught) were the benefi ts comparable to those provided by
protected areas. Maximum size limits cannot by applied to certain fi shing methods
but, on the other hand, such an approach would be feasible in terrestrial mammal
hunts, if hunters would agree to forego the largest trophies. But this is an unlikely
outcome when, for example, bighorn sheep trophy hunters are prepared to pay
hundreds of thousands of dollars at auction for a permit to hunt (Coltman et al.,
2003). Quite a conundrum.
7. 5 A broader view
of harvest
management -
adding economics to
ecology
The most obvious shortcoming of a purely ecological approach to harvest manage-
ment is its failure to recognize that when harvesting is a business enterprise, the
monetary value of the harvest must be set against the costs of obtaining it. I noted
for pest management that the aim is usually not to go the whole hog, but to reduce
the pest population to a level at which it does not pay to achieve yet more control
Search WWH ::




Custom Search