Environmental Engineering Reference
In-Depth Information
When the data do not contain sufficient information, dynamic models often give
outlandish parameter values with huge standard errors. You should always there-
fore check that your results make intuitive sense (for example, that the initial
population size is about what you would expect) and are reasonably precise.
Where
several species
can be targeted by the same harvest effort, the species with
lower intrinsic productivity will be overexploited first, partly because they tend to
be larger and more profitable, and also because they are simply more vulnerable.
This leads to a progression of local extinctions at any given location (Roberts
1997). Thus, in principle, the species profile of offtake contains information about
the state of the harvest system. This is undoubtedly true in broad terms: for
example, an offtake consisting entirely of small rodents, where once large ungu-
lates and primates were commonly hunted, almost certainly indicates overex-
ploitation (Rowcliffe
et al
. 2003). This principle has been applied to monitor the
state of global fisheries, based on the idea that fishers only turn to species with low
trophic level (plankton-feeders) once all the large predatory species are gone. The
average trophic level of landings can thus be used as an index of overexploitation.
Pauly
et al
. (1998) used this index to show that virtually all marine and freshwater
fisheries monitored by the Food and Agriculture Organisation had been overex-
ploited since 1950, especially in the northern hemisphere. This marine trophic
index has since been adopted by the Convention on Biodiversity as a means of
monitoring progress towards sustainable management of fisheries (Pauly and
Watson 2005).
There are some important barriers to the widespread use of this approach. First,
the
resolution is low
, requiring fairly dramatic changes in offtake structure before
clear patterns can be detected. It is likely that the most vulnerable species will be
heavily overexploited or extinct by the time the changes are detected. Second, and
exacerbating the first problem, broad spatial coverage in the offtake data (for
example, from markets with large catchments) may
obscure local problems
.
Widespread overexploitation in some parts of the catchment may be undetected
because vulnerable species continue to appear from more recently exploited areas.
Finally, if profiles are monitored at a remote end point of trade rather than at the
point of harvest, they will reflect the outcome of a chain of transactions, with the
potential for considerable
distortion of the original profile
that was harvested. It
is hard to generalise about how this might affect conclusions. This approach there-
fore has potential only as a large-scale monitoring tool for the detection of overex-
ploitation when it has already happened rather than a smaller-scale management
tool for the prevention of overexploitation.
Changes over time in the distance travelled from base to reach hunting or fishing
areas can be related to population depletion. In the extreme case, hunters may
