Use of wild fish and other aquatic organisms as feed in aquaculture in the Asia-Pacific 113
systems where clusters of cage farms exceed the carrying capacity of the waterbody.
The environmental effects can be both direct and indirect. The direct effects result
in fish kills, not only of the farmed fish but also of wild stocks. The latter results in
conflicts with the artisanal fishers who make a livelihood from fishing in the waterbody
(Abery et al., 2005). Moreover, intensive feeding and the accumulation of excessive
amounts of nutrients tend to elevate the levels of ammonia and at times, even toxic
hydrogen sulphide, which may not cause direct mortalities, but can stress the stock so
that it becomes susceptible to disease. The adverse impacts can often be remedied by
siting the cage systems in different areas of the waterbody and reducing the feeding
intensity (Box 12).
In addition, the feeding of fish as a direct food source to cultured stocks is known
to be even more environmentally damaging than feeding of pellet feeds because of
the likelihood of lower digestibility. However, there is limited evidence to show the
efficacy of pellet feeds as opposed to trash fish/low-value fish as feed. Often, gross
conversion efficiencies are used for comparative purposes. However, this approach
does not take into account the moisture content of trash fish/low-value fish as feed,
which amounts to about 70–75 percent.
Similarly, little is known about the efficacy and resultant environmental effects of
the use of farm-made feeds. This is understandable, as the composition, method of
preparation and feed management of farm-made feeds are diverse (De Silva and Davy,
1992; De Silva, 1993; Tacon and De Silva, 1997), and more often than not, the quality
of feeds used could differ between adjacent farms culturing the same species. As such,
comparisons become difficult if not impossible. The environmental effects of the use
of farm-made feeds are also difficult to evaluate, and to the authors’ knowledge no
such studies have been made. However, the small-scale farmers are the best judges of
the efficacies and the cost-benefits of the feeds they use. The recent shift to the use
of compound feeds in catfish farming in the Mekong Delta and the shift to the use of
poultry processing waste in snakehead and catfish farming in Thailand, perhaps are
evidence of the increased efficacy that farmers obtain by such changes.
In China, current techniques for finfish culture in ponds and cages are believed to
result in 30 percent of feeds being wasted or uneaten by the cultured stock. Compared
with artificially formulated feeds, the discharge of nitrogen and phosphorus into the
environment by using trash fish/low-value fish as feed is three to four times higher. The
uneaten feed together with the excreta of cultured fish impacts the culture environment,
FIGURE 22
A very unhealthy practice in Asia: feeds in drums, improperly sealed and exposed to the
elements for a week or more, a practice that could lead to loss of quality and even make
the feed rancid, and accordingly result in reduced performance of the stock and lowered
feed conversion efficiency