Environmental Engineering Reference
In-Depth Information
4.3.2 Trends in ecosystem structure
Overexploitation of one or a number of species is likely to have knock-on effects on
the rest of the ecosystem. Ecosystem-level effects of fishing are a major field of
enquiry in marine systems, where fishing disrupts food supplies, damages habitat
and injures or kills individuals of non-target species such as turtles or seals (Tudela
2004). Many governments (including the EU and the USA) are now emphasising
the importance of an ecosystems approach to management of fisheries, which inte-
grates fishing into the wider management of the marine ecosystem. One reason
why marine protected areas are seen as a useful management tool is that they can
address these ecosystem-level effects of fishing through excluding fishers from par-
ticular areas (Sumaila et al . 2000). In terrestrial conservation, less has been done on
the wider effects of hunting, although Redford (1990) coined the term 'the empty
forest' referring to bushmeat hunting that removes mammals from otherwise
apparently pristine forests. Empty forests may lack the pollination and seed dis-
persal services provided by animals (e.g. Forget and Jansen 2007), while a study in
Boliva (Roldan and Simonetti 2001) showed that heavily hunted forests had far
less trampling of tree seedlings than hunted forests, all of which are likely to have
knock-on effects on tree recruitment.
It's one thing to demonstrate that hunting has effects on other components of
an ecosystem, but quite another to develop indices of sustainability for these kinds
of effects, and then use these indices as measures of management effectiveness.
However, progress is being made. For example, both the International Council for
the Exploration of the Seas (ICES) and the Commission for the Conservation
of Antarctic Marine Living Resources (CCAMLR), which are influential inter-
national fisheries research bodies, have working groups on the ecosystem effects of
fishing activities (see Section 4.6).
4.3.3 Trends in catch per unit effort
Catch data are often much easier to obtain than population size, so it is intuitively
appealing to treat catch alone as an index of population size and monitor it in order
to detect declines. However, inferring sustainability from catch raises an additional
problem as well as those involved in using population size directly. The assumption
that catch is proportional to population size may be badly mistaken, for a variety
of reasons. This can result in a stable catch despite a greatly reduced population.
Relying on declining catch to signal overexploitation can in this case be catastrophic,
resulting in massive overexploitation before the problem is detected. Conversely,
declining catch may simply reflect changing harvester effort driven by social or
economic circumstances.
Effort has a very specific meaning in harvesting theory. Rather than the collo-
quial meaning, it is all the inputs that are put into harvesting. At its most basic,
effort is the time spent hunting or foraging, but it also includes the type and effi-
ciency of the weapon used to kill an animal, the mode of transport used to get to the
harvesting location, etc. Effort on its own is not a useful indicator of sustainability,
 
 
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