Assessing current sustainability of use - Conservation and Sustainable Use - page 140

Fig. 4.1 Male saigas bear horns which are used in traditional Chinese medicine.
Hence they are selectively hunted. Photo © P. Sorokin.
small one, so the actual number of individuals may be less important than their
size. Fisheries often target particular size classes (individuals small enough to fit on
a plate, for example), and changes in the size structure of the population can affect
population dynamics quite profoundly (Jones and Coulson 2006). However, just
as for changes in age structure, changes in size structure can be hard to interpret—
is a population small-sized due to severe overharvesting of large animals or due to
improved recruitment as a population recovers? To answer these questions, more
information is needed. The important point here, though, is that the structure of
the population is important because it impacts upon its productivity (i.e. the
number of new individuals produced per individual present), which determines
the sustainable harvest rate. Indeed, some people, particularly in fisheries, suggest
that the reproductive potential of a population, modelled using information on
both population size and structure, is a better sustainability indicator than abun-
dance alone (Katsukawa et al . 2002).
In species such as mammals, where size is less important as a determinant of sur-
vival or fecundity, independent of age (at least in females), trends in size structure
of the harvest are less informative about sustainability because they are not strongly
related to population dynamics. They may, however, indicate long-term genetic
changes, which could be significant. For example, Coltman et al . (2003) showed
reductions in the body and horn size of bighorn sheep due to hunting, which also
affected the productivity of the population.