Environmental Engineering Reference
In-Depth Information
2
In Chapter 1, we show how a basic model of population dynamics can help us to
understand the principles of sustainability, and in Chapters 4 and 5 we cover the
practicalities of a range of methods for assessing and predicting biological sustain-
ability. These concepts and techniques all require some form of biological infor-
mation on the exploited population. At the simplest end of the spectrum, changes
in population size can be used as a direct measure of the state of the system. More
complex model-based methods might require the estimation of other parameters
such as intrinsic growth rate or age-specific rates of survival and productivity. The
range of parameters relevant to exploited species is summarised in Table 2.1. In this
chapter, we summarise the basic classes of method available to measure each of
these key parameters. The state of the art in most of these techniques requires com-
puter-intensive statistical analysis, but fortunately a large amount of software is
available that makes these analyses accessible, much of it free. For detailed instruc-
tions on analysis, we therefore refer readers to these programmes and associated lit-
erature (Section 2.7). The information provided in this chapter is primarily
intended to equip you with the knowledge necessary to make informed choices
about appropriate techniques and robust survey designs for your fieldwork.
Just occasionally, it is possible to observe all the individuals in a population, giving
a point estimate for population size with no uncertainty attached. With a
complete
census
of this kind, we do not accept the possibility that our estimate may be
wrong, and make no attempt to estimate how far out it might be. If you're going
to use this approach, you therefore need to be absolutely convinced that your
observations are complete and unbiased. In practice this is possible only for very
conspicuous species in small areas. If we wish to say something about larger
populations (as will almost always be the case for exploited species), it is far better
to accept that we cannot observe all individuals with certainty and
sample
the
population in a way that allows us to make robust inferences about the entire
population, based on the observed sample.
However, this approach brings with it the issues of
bias
and
precision
, which
should be understood if sampling methods are to be applied appropriately. Ideally,
