Harvest management - Ecological Applications: Toward a Sustainable World - page 198

presence of gill nets. Most fi shing nations have no such program at present (Lewison
et al., 2004).

7. 2 .7 Ecologist's role
in the assessment of
MSY

The ecologist's role in harvest management is in stock assessment : making quantita-
tive predictions about the response of a harvested population to alternative manage-
ment approaches and addressing questions such as whether a given fi shing effort
will cause the spawning stock to decline, or whether nets of a smaller mesh size
will allow the recruitment rate to recover. In the past, it has often been assumed
that stock assessment could be done simply by careful monitoring of total catch and
fi shing effort. But you have seen how diffi cult it is to determine the MSY and to
devise strategies to capture it in a sustainable manner. Certainly, complex mathe-
matical methods exist for performing the necessary calculations, but these are
themselves based on assumptions (not necessarily tested or true) about the underly-
ing dynamics of the population. However, management decisions must be made,
and the best possible stock assessments should be the basis. At the same time, we
should set about fi lling our knowledge gaps. The so-called 'adaptive management'
approach involves seeking a balance between, on the one hand, probing for informa-
tion (directed experimentation) and, on the other, minimizing losses in short-term
yield while avoiding long-term overfi shing (Hilborn & Walters, 1992). The process
is 'adaptive' because according to the monitored outcome of some appropriately
cautious management regime, the approach can be fi ne-tuned and tried again. Ecolo-
gists are crucial in this process because who can better appreciate the uncertainties
and provide appropriately enlightened interpretations?
Finally, and to be brutally realistic, managing most marine fi sheries to achieve
maximum sustainable yields will be very diffi cult to achieve. There are generally
too few researchers to do the work and, in many parts of the world, no researchers
at all. The term data-less management has been applied to situations where local
villagers follow simple prescriptions to make sustainability more likely - for example
locals on the Pacifi c island of Vanuatu were provided with some simple principles
of management for their trochus ( Tect u s nilo t ic u s ) shellfi shery (stocks should be
harvested every 3 years and left unfi shed in between) with an apparently successful
outcome (Johannes, 1998).

7. 3 Harvest models
that recognize
population structure

The models of harvesting described in Box 7.1 and Section 7.2 are known as 'surplus
yield' models. They are both useful and used by managers, but they have a variety
of limitations, as you have seen. Perhaps their biggest drawback is that they ignore
population structure, and this is bad for two reasons. The fi rst is that 'recruitment'
is not the simple process envisaged in MSY curves, but a complex outcome of adult
survival and growth, adult fecundity, and juvenile survival and growth, each of
which may respond independently to changes in density and harvesting. The second
drawback is that most harvests focus on a portion of the population (e.g. fi sh that
are large enough to sell, mature trees for buildings or furniture, large deer with
antlers worth displaying). The approach that takes these complications into account
involves the construction of models like those already described for population
viability analysis (Section 5.4). These usually take the form of matrix models encap-
sulating survivorships and reproductive rates in an age-structured population. In
fact, I have already mentioned the results of such models in this chapter - recall the
Saiga antelope example (Figure 7.7), where a harvest including more males could