338 Organic waste recycling: technology and management
7.6.3 Food for other aquatic and amphibious herbivores
The concept of harvesting aquatic weeds in situ can be extended to include other
herbivorous animals.
Ducks, geese, and swans
Ducks, geese, swans, and other waterfowl forage on vegetation, controlling weeds
on the banks of waterways and often clearing aquatic weeds and algae from small
lakes, ponds and canals. In Hawaii, USA, 65 Chinese white goslings were placed in
a 1 ha pond, completely covered with dense paragrass (Brachiaria mutica) and
cattail (Typha spp.) that annually grew 1.8 metres above the water. Despite the
failure of mechanical and chemical controls to manage the weeds for several
previous years, the geese cleared them out in 2.5 years. Also, in 1967, 100 mute
swans were added to Nissia Lake near Agras in northern Greece. The lake was
being used to produce hydroelectricity, but the turbine inlets were clogged with
aquatic vegetation. When the swans were placed in the inlet area, they cleared it
within a few weeks (NAS 1976).
These waterfowl can control aquatic weeds noticeably only in small bodies of
water such as farm ponds and small lakes. In a larger waterway the number of
waterfowl needed to solve the weed problem makes their use impractical.
Nonetheless, waterfowl could be used to supplement other weed control efforts
such as the use of herbivorous fish or mechanical harvesting.
In addition to being weed-control agents, ducks and geese produce nutritious
eggs and highly prized meat. Thus they provide an additional income to farmers
that raise them at little extra cost. But, if not carefully managed, ducks and geese
can become pests for some crops (especially grains) neighbouring their
waterway. Where sanitation is poor, salmonellosis sometimes decimates ducks,
geese, and swans, and this disease can be transmitted to humans. Due to the
outbreaks of avian influenza (bird flu) diseases in several countries worldwide
which affect the poultry and humans, the practice of using this type of animals
for weed control is not attractive and should be avoided.
Turtles, rodents and manatees can be used as weed control agents, but they
cannot be used for food as they are endangered species and are not usually
consumed by people.
7.7 Wastewater treatment using aquatic weeds
Most aquatic treatment systems consist of one or more shallow ponds in which
aquatic weeds are grown to enhance the treatment potential. Typically, each