21
Kottelat and
Whitten (1996)
contend that
introductions in
South-east Asia have
been conducted
haphazardly in an
effort to strengthen
depleted wild
populations or to
improve the genetic
quality of a
population.
While some fisheries personnel feel that the introduction of alien species
increases freshwater diversity and thus "improves" it (Baluyut 1983, Kottelat
and Whitten 1996), the majority of ecologists see introduced species as a threat,
rather than a benefit, to ecosystem integrity (Sala et al. 2000). Given that food
security priorities frequently take precedence of environmental issues, the current
and potential impacts of alien species are typically either overlooked or ignored.
FAO 236 (1983) reflects belief that has been held by many in the aquaculture
community.
Clearly, while aquaculture has the potential to provide low-cost protein
to those who need it most, it is not without its cost. There is increasing evidence
that aquaculture has contributed to the nutrient loading of water bodies and the
introduction of IAS (McCrary et al. 2001, Santiago 1994). The introduction of
alien species may lead to rapid increases in fish yields, but it may also cause
consumer preferred native species to go to the point of extinction (Bailey 1980,
Fernando 1976). In order to maximize the full potential of aquaculture, we must
address the externalities that arise from the utilization of potentially invasive
species. Furthermore, fisheries managers need to determine whether the benefits
of alien species introduction are actually greater than the costs. If direct and
indirect costs are taken into consideration, it is quite plausible that very few
introductions of alien species actually resulted in increased fisheries productivity.
Furthermore, in many instances, rehabilitations of the existing fishery might
have been at least as cost-effective and had fewer deleterious effects (Eccles 1985).
"However, notwithstanding the risks associated with the stocking of exotics in lakes and reservoirs, the fact
remains that it has been the only successful means of maximizing fish production in reservoirs in the Southeast Asia
the costs of dam removal, leeves and drainage structures. According to the 2001
Blue Millennium report, aquaculture in inland waters presents the greatest threat
to biodiversity (IDRC et al. 2001). The majority of donor funding has gone to
aquaculture projects instead of to restoring or maintaining capture fisheries (Baird
2001). In some communities, traditional capture fisheries are more commonly
practiced then aquaculture (Baird 2001). This can lead to the dependency on
introduced species, both native and non-native, both posing challenges to genetic
and species diversity.
Other development activities, dam building for example, can facilitate
the growth of aquaculture. According to the International Rivers Network, over
the past ten years, construction of more than 100 large dams has been proposed
in Southeast Asia (IRN 2003). Dams change the natural flow of rivers, which
often leads to the destruction of fisheries habitats. When a dam is built on a
river, an artificial lake (reservoir) is created, and habitats are created that support
still-water (lenthic) organisms, rather than species frequenting flowing (lotic)
water. For example, dam-induced changes in water flow, barriers to downstream
spawning and feeding, and increases in sedimentation are known to favor
lacustrine (still-water) species instead of riverine species of fish (Bhukaswan
1980). Reservoirs are thus stocked with lacustrine fish, often alien, to sustain or
build a fishery (Usher 1997, Bhukaswan 1980). Some of these alien species are,
or become invasive.
Many aquaculturists acknowledge that fish do escape and that they are
nearly impossible to contain indefinitely. Fish are introduced into natural systems
deliberately as a natural resource and as forage, and can escape ponds and cage
or can be accidentally released during transport and handling. Kottelat and
Whitten (1996) contend that introductions in Southeast Asia have been conducted
haphazardly in an effort to strengthen depleted wild populations or to improve
the genetic quality of a population.
(Baluyut 1983)."