Environmental Engineering Reference
In-Depth Information
chi-squared test. Chi-squared tests are particularly useful for exploration of social
data, where variables are often categorical (divided into categories rather than on
a continuous scale) and the data are frequencies (such as the number of people
saying yes or no to a question). Chapter 3 gives some useful statistical resources,
and examples of how comparisons between sampling units have been used in
the literature to look at social sustainability, while Chapter 2 discusses statistical
methods from a biological perspective.
Another form of comparison is with a
reference point
, defined
a priori
from a
theoretical model or from a policy prescription. Examples of social policy pre-
scriptions include that no household must live on less than $1 a day, or every
household must have access to fresh water. Biological sustainability is also often
assessed against reference points. A commonly used approach when data are scarce
is to compare the current catch, effort or population size with a theoretical thresh-
old defining the limit to sustainability (Box 4.1). This idea is the basis of several
popular sustainable catch indices (Robinson and Redford 1991; NMFS 1994;
Slade
et al
. 1998; Wade 1998; Robinson and Bodmer 1999), although none of
these indices is fully rooted in an explicitly defined population model (Milner-
Gulland and Akcakaya 2001). While this issue can be overcome, there are a num-
ber of more fundamental problems with the use of simple biological indices. These
problems don't entirely negate the use of reference points, but it is important to
understand the potential pitfalls if you do use them.
First, the interpretation of catch in relation to maximum sustainable yield
(MSY) is
ambiguous
. Harvest
less
than MSY may indicate unsustainable offtake
from a small, overexploited population (false sustainability), while harvest
greater
than MSY may be the result of sustainable effort during the early stages of a new
harvest (false unsustainability). To avoid this ambiguity, you need to know the cur-
rent population size. Given this information, false sustainability will be defined by
a current population below half the carrying capacity. False unsustainability may
be suggested, but not unequivocally defined, by a current population close to the
carrying capacity.
This also highlights the issue that simple indices assume harvest and population
are at
equilibrium
, while the real world is dynamic. One way around this is to
avoid basing the comparison on catch alone by using a reference point that
expresses catch as a proportion of current population size. This is a safer option
because it requires absolute catch to fall if the population declines, and this
approach has been adopted by US National Marine Fisheries Service to define
limits to allowable by-catch of marine mammals (NMFS 1994; Wade 1998).
However, the approach can still be misleading if it is applied at a single point in
time. For example, if harvest happens to be unusually light when you make the
assessment, you may be unpleasantly surprised later when things return to normal
and the population is overexploited. Avoiding this requires continual monitoring
and the analysis of trends (Section 4.3).
Reliance on MSY as a reference point provides no buffer against inevitable
uncer-
tainty
in parameter values and offtake estimates, or against
random fluctuations
in
