Environmental Engineering Reference
In-Depth Information
sizes, long periods of no data (due to the stock being protected and unhunted)
before management began, catastrophes (e.g. epidemics) and deterioration in the
environment. The most robust strategies should be able to respond appropriately
to these situations by changing their catch limits so that true stock sizes do not fall
below the threshold, but still be able to maximise yields (with low annual variabil-
ity) if the stock is healthy. The simulation approach demonstrated that strategies
that appeared to be precautionary and based on our best understanding of the
situation did not always perform well when their underlying assumptions were not
met—so it is always a good idea to test strategies thoroughly against a range of
scenarios. One key feature of the winning strategy was that the harvest quota
responded to the precision of the data available—if the data were more uncertain,
the catch levels were lowered.
This strategy was adopted by the IWC. It is here that implementation uncer-
tainty came into play—despite all the testing and the likelihood that this man-
agement strategy would enable whaling to be sustainable, political and welfare
considerations meant that the strategy was not adopted. The IWC currently still
has a whaling moratorium, and some countries are still whaling outside of the
international regulatory framework (Figure 7.6).
Source
: Kirkwood and Smith (1996).
Of course if a management action is tested in impossible circumstances it will
fail. The idea of this exercise is not so much to demonstrate this as to give a feel for
the range of possible outcomes. If this is done quantitatively, it will include a
weighting of models by their fit to the data, which can then give a probability dis-
tribution of possible outcomes under each scenario (e.g. Hilborn and Mangel
1997; Box 4.5). This is likely to give a different result to just using best estimates,
and one that is a better reflection of our true state of knowledge.
7.5.2.5 Making the decision
The result of the evaluation of the different actions is a performance index for each
action against each criterion. It is relatively easy to rank actions according to their
performance against each indicator. However, the criteria for success are not
generally measured using the same units, and so there needs to be a way of decid-
ing the weight to be given to each. For example, how do you determine the relative
weight to be put upon the effectiveness of a management action in improving the
livelihoods of poor families as against its ability to maintain a minimum population
of the exploited species? If criteria are expressed as constraints (such as population
size of prey at least 50% of carrying capacity, a basic level of offtake guaranteed for
all) they are much easier to include in weighting exercises than those expressed as
continuous variables (such as maximise yield, minimise variability). Either a man-
agement action fulfils the constraint or it doesn't, and if the basic criteria absolutely
