In most countries with water
hyacinth problems, three methods have usually been employed for its control –
manual and mechanical harvesting, biological and chemical control. The system
adopted however, depends on several considerations. Even within the same
country, different methods must be considered based on specific locations and workability. in our case, here in Nigeria,
experience on the water hyacinth problem in past fifteen years confirms the
need for a mixture of strategies in tackling the problems in the wide variety
of water bodies that are already infested. In doing so, the cost effectiveness
and environmental impact of whatever control strategy is employed must be borne
in mind.
Harvesting is a useful method for
water hyacinth control but there are serious bottlenecks. To be effective, the
harvesting method has to be fast enough to outpace the reproductive potentials
of water hyacinth. According to Khan and Thyagarajan (1988), under the most
favourable conditions, 10 plants can multiply to 600,000 in only eight months.
To be effective therefore, the harvesting rate must be equal to or more than
the growth rate.
So far, mechanical harvesting has
not been a continuous• exercise in Nigeria. Most of it has been done to
combat a surge or to open u already
blocked waterways. There is bound to be a recurrence except harvesting is
backed up with regular mopping. Utilization of harvested plants is also suggested
to solve the problem of disposal and also to meet at least partially the cost
of harvesting.
Biological Control
It is believed that on the long-term
basis, biological control will •be relatively cheaper than other methods of
control. By far, the most successful 18 Table I Summary of losses caused by
floating weeds in ECOWAS Region Source: ECO WAS Report (1995) agents are the weevils, Neochetina
eichhorniae. In 1992, rearing of the weevils
started at the Nigerian Institute for Horticulture (NIHORT) under the auspices
of the National Committee on Water Hyacinth. Releases were made in 1994 at
Iwopin (Lekki Lagoon). Releases are also in progress at various sites along the
River Niger by NIFFR (National Institute for Freshwater Fisheries Research)
under the Nigerian/German Kainji Lake Fisheries Promotion Project. Work on
biological control in Nigeria
has been seriously
Hampered by funding.
Other biological control methods involve
using herbivorous animals—grass carp, other herbivorous fish, manatees, and
other herbivorous animals. For the past few years, this author has advocated
the use of the grass carp (even as a pilot project). But no fund has been made
available for the project.
The grass carp has been shown to be
a very effective method of water hyacinth control in several countries
including Indonesia, Philippines, Guyana (Soerjani, 1984; Buruah,
1984).
In the process the weed is cleared
ündone gets sizeable fish thereby contributing to fish production and improving
the economy of the riverine people who now regard Water hyacinth as a menace.
According to Soerjani (1984), in Indonesia, in a Water hyacinth monoculture,
fish of 6 — 8cm with a density of 4 16,000/ha reduced water hyacinth population
of about 1,200 ton/ha with 48 — 83% compared to the potential growth in two
weeks.
Chemical Control
Though chemical control of water
hyacinth is possible, the main objections to its large-scale use in the
Nigerian situation are:
i). The rural population use the
creeks, rivers and lagoons for domestic and drinking purposes in view of lack
of Sector Low Estimated Losses High Estimated Losses (millions US$) (millions
US$) 28 564 8 7 14 36 72 Fisheries Fleath Hydro-energy Agriculture Total 75 150
19 pipe-borne water. Indeed over 80% of people of Ilaje area depend on this as
their main source of water for life. The nearly 30 fishing villages on the
periphery of Lekki Lagoon (a major area of water hyacinth infestation) drink
the water directly.
ii). The lagoons and rivers are
major sources of fishing and nursery grounds for early development of many
species of fishes and shrimps. Such aquatic animals and sessile mollusks will
be contaminated.
iii). Chemical spraying will result
in serious pollution problems as the water weeds decay.
iv) The unforeseen toxic effects of
residual chemical on aquatic organisms and man.
Indeed, according to Obeid (1984),
after chemical spraying of water hyacinth in the White Nile, the local people
who live close to the river complained of stomach ache and intestinal
troubles—effects that were traced to the chemical spraying. Harley (1989) also
warned that there was an environmental cost in using herbicides. Residues of
herbicides in the water and sediments may affect the aquatic environment and
kill fish directly or by reduced levels of dissolved oxygen caused by decaying
weeds. The pity of it all is that the chemical control treatment must be 20
continued indefinitely or the infestation will regenerate from scattered
plants and seeds and the on-slaught on the environment will continue.
I have never been completely against
the use of chemicals but it must be limited to areas where they are absolutely necessary
and will do only a little havoc. Such usage must also be borne on tested
experiments. The occurrence of water hyacinth in Kainji Lake
raises a fresh problem for use of chemicals. The Kainji Dam so important in the
hydroelectric supply of Nigeria must be protected. In case
of a major surge in the dam area, a buffer zone can be created around the dam.
Appropriate chemical can then be sprayed around this buffer zone to keep the
water hyacinth in check and prevent it from fouling up the dam. The dead weeds
can then be mopped up. Details of these must be jointly examined by NEPA and
NIFFR before the exercise is initiated.
For the rest of the River Niger in
Nigeria, it must be a combination of biological control, manual and mechanical
harvesting, adequate monitoring and burning up of the weeds when certain parts
of the River Niger dry up. Utilization as a Method of Control Utilization of
water hyacinth for a wide range of purposes has been suggested as
a method of checking and exploiting
its growth and spread. Jayanth (1987) listed the following uses for water Hyacinth
utilization:
(a) As manure
(b) For producing processed feed for
pig, cattle and other animals.
(c) As a source of protein,
gibberellic acid etc.
(d) As a vegetable for human
consumption.
(e) For extraction for fibre or
making paper arid other products.
(f) As a renewable source of energy
in the form of biogas rich in methane.
(g) For waste water treatment. Realising that
fresh water hyacinth contains nearly 95% water, most of which has to be
extracted before usage, the economic viability of the economic uses must be
seriously considered, In any case, utilization must never be considered as an
option for controlling the menace of water hyacinth