Irrigation in the eleven River Basin Authorities and in particular in the South Chad Irrigation Project and the Baga Polder Project is increasing and demands high yearly investments for aquatic weed control in canals and drains. If the weeds are biologically controlled by the grass carp (Ctenopharyngodon idella Val.), the costs will be turned into profit, particularly when the fish production (resident fish plus grass carp) is harvested and sold for food. The use of irrigation canals and drains for aquaculture in the form of fish polyculture may be a wise step towards increased fish production. This paper highlights the concept of fish production from aquatic weed control and concludes that it is a proven profitable venture in several countrie

Okafor, L.I.

English

Aquatic Commons

7,
0-).scaic:h
.
TfJ idu91.1.fi, Ni_geria
_gerian fish
t- be Da': by aquacui-
-uo:.:°r.es and in
Bc,ga Pol6ev
3 for Egli
a :La,,ore
the co
°,.lhe fish -,produ...
WEEDS
-ing ,,doo.sl: and oossibly has
Basin
, 7::.lgat,on Project
:oject L'Jo ff:=1A1KIfo hi,Jh yearly
weed s Tf the
controi17.H hy chs. giass <sari- 07..tc,nophugolon
s will b, c:ovned into pcofit,
(resident fish. !Tdus grass carp) is harvested
a a16 KOr food, use or oanals and drains for
J..:Iaoulcuie in the fo,:m of fish polyculture will be a wise step
tres iilcresed fizh production. ThLs paper highlights the concept
of fish production from aquatic weed control and concludes that it is
a oven profitable venture in several countries.
L. Lot river basin irrigation schemes in Nigeria, many of the
kilometers of ditches, canals and reservoirs are known to be
overgrown with submerged, emerged and floating weeds and by
filamentous algae partly due to increasing eutrophication. In order
to maintain water flow,and irrigation, most of the weeds in these
waterways have to be removed by one method or by a combination of'
methods.
All weed control practices can be broadly classified into three
(Lkobundu, 1980). These are chemical, cultural and biocontrol methods.
C ;ICAL CONTROL METHOD
Although submerged, emerged and floating weeds in the waterways could
be removed by chemical means, many chemical control methods are
hazardous to ecological balance of the aquatic ecosystems and to
the farmer himself.
CTILTURAL CONTROL METHOD
Te traditional method of removing water weeds is cuttingby hand and
fin;, out. However, hand weeding cannot be applicable in channels
up by dense masses of submerged filamentous algae and species
Ceratophyllum demersum L. This weed has been reported to-be
1:17t. in Lake Chad (Okafor, 1980). The usual thing will be to
loloy a system of mechanical cutting from a boat.
Both hand and mechanical cutting have the serious disadvantage in
that weeds begin to grow again as soon as they are cut, and with
many water weeds regrowth can be rapid. Veaetative fragments of most
aquatic weeds will grow rapidly if they are left in the water unless
removal operations are very thorough. Indeed'attempts at mephanical
control may merely serve to spread the infestation. Such work is -
very time-consuming particularly in 'raking out cut weeds, and arduous.
It is therefore becomina increasingly difficult to find men willing
to undertake work of this nature and the cost of employing them is
rising continually.
68
7iETHOD
It is recognised that grass carp (CtepppLar-;ng)pn it 1.) -
herbivocous fish can control growth of -z,cuati uFis
suc.!e&ssfully used in the Netherlands ,Docuet, ;';'77; 7a ,
i982), Switzerland (Muller, ,979), United I
Rob&c), ;970; Scott et al, 1971 and Buclla,0 1981), Trr -
,Taf_J_;i1-levi 1973Y, the Soviet Union (Al ley, 1976) F- h, ,Gharably
' al, 102).
Reasons for the considerable interest t ahis Asiatic herbivorous
fish or phytophagous fish all over bL id include:-
First of all, and what seems to ' c1 esSential
is the fact that it is a plant-f :(Lig species.
feeds on macrophytes and algae ah-1 c, nbt a highl
cnnsumer for it eats quite a af aquatic ecir,
Another important thing is its ability to
climatic oonditions since.it comes from a c 1,i
hot summers and severe winter conditions.
Other important factors are:
its fast rate of growth
its bia s¡ze
its resistance to handling
easy to culture including artificial propagation
(g its good taste when used for food.
It is primarily used for biological control of the overgrowing water
reservoirs, canals and channels.
In Egypt, Gharably et al (1982) demonstrated that the use of grass
carp for the control' of weeds in and drains forms an attractive
'alternative to the traditional weed controb manual,
mechanical and chemical; and is more economical and that the
growth of grass carp stocked in ca.els and drains enables a
considerable annual harvest to be cropped without affecting the
capacity needed for proper weed control. In this way the grass carp
can contribute to the protein consumption of the Van Ion
(1980) reported that the grass carp shows most prers;._.,..tive because
of its fast growth and good marketability, and that it is not bound
to one food source but it is a. polyphagous, non-selective feeder,
especially in temperatures over 20°C.
FISH PRODUCTION/HECTARE
Maembe (1981) reported that Lake Chad is one of the richest fishing
grounds in the wcrld with 80-100 kg/ha yield. However, it has beeir;
shown in Egypt that growth of grass carp will result in a yield of
360 kg/ha/year (Khattab et ali.1982) for human consumption without,
interferina with the control of aquatic weeds. Van Zon et al (1982)
reported that even if the purPose of grass carp production in
irrigation systems will be liMited to weed control and some extra_
protein production, the yield could easily be around 500 kg/ha/year_
(without additional feeding or fertilization).
ECONOMIC GROWTH
The grass carp obtains its nutrient requirements completely from
eating aquatic plants and does not require any expensive dry pelleted
fish-food.
69
-enc. with
sosiE, -ouW7ries, the method of biologic& control with grass carP
is not a., iowed because of the danger of the possible reproduction and
nat)_,..7:cI:.sation of the species, following introduction. In 1975, a
ne od sterile hybri D' gras5 carp and bighead (Aristichthz
noL)ilis R1,7.11.) was developef tarian and Krasznai, 1978) which can
be usad for the-control of squ',.tic -egetation withbut the risk of
unanted o-Jerpopulation. Ttr is c-r,aaper, more effective and more
acceptable ir view,,point of L =orriental protection.
SUMMARY
In the river basin schemes c I1crria, it is :common practice to
control aquatic weeds in irr roi canals, channels and drains
manually and mechanically. l L:d_ar to avoid the hazaxds of chea?er
chemical weed control on huma i aalth, fish production and the
quality of irrigation water. EV).7i to reduce costs and increase't1-7.e
efficiency of aquatic weed contol, the grass carp could be introluced
as an alternative method. The d.elopment of a sound weed controL
progamme with grass carp, ad-yosted to the specific conditions
el:sping in most river basin irrigation systems will result in
increases in fish production. This will be a desirable
as the strongly increasing Nigerian fish demand has largely
to be met by aquaculture. This successful venture has been found
pfoitable even in African countries..
)bundu, I.O. (1980) Weed control strategies for multiple cropping
systems of the humid and subhumid tropics. Proc. Inter. Wee
Control Conf., IITA, Ibadan pp 80-100,
Alley, D.S. (1976) The role of phytophaggus fishes in the
reconstruction of commercial fish fauna and biological improve-
ment of water. J. Ichthyologia, 16(2): 216-229.
Bouquet, H. (1977) Grass carp in the Netherlands. Proc, 8th Brit.
Coarse Fish Conf., 108-111,
Buckley, B.R. (1981) Practical problems with the use of grass carp
for weed control. Proc. Conf. on Aquatic weeds and their
control, 125-129.
El Gharably, Z. El., A.F. Khattab and F.A.A. Dubbers (1982)
Experience with grass carp for the control of aquatic weeds ir
irrigation canals and drains in Egypt. Proc, 2nd Inter. Symp,
on Herbivorous Fish. Novi Sad, Yugoslavia, 17-26,
Janichen, H. (1973) Die biologische Krantung, Institute
Rinnenfischerei Berlin-Friedrichshagen 44 p. (in German).
Knattab, A.F., A. El Gharably and F.A.A. Dubbers (1982) Aquatic
weed control in irrigation canals by means of grass carp. Conf.
on the Role of Research in River Basin Development, Cairo, Egypt,
Manan, T. and Z. Krasznai (1978) Karyological investigations on
Ctenopharyngodon idella and Hypophythalmichthys nobilis and
their crossbreeding. Aquaculture Hungaria 1, 44-50.
Maembe, T.W. (1981) Final report on the improvement of fish
processing and transportation on Lake Chad. FAO, Rome.
Muller, R. (1979) Aquatic weed control using grass carp. Swiss
Federal Institute of TEchnology E.A.W.A.G. News, 10, 1-2.
70
':/eede ro'cert:_a:
111 Cliap /P,J11)er ,,,nted
tìe°=0;-'n cxn leed OTTDR,
Tecembe-,:
Riemens. The esiti.: 5:ock±n ifor weed
Control in 17.11e 1Netheriands. 2K16. cr. 3:71up, on
ierbivorous
Sc)tt, B. and T.O. Robson (1970) Efficiency of grass carp
(Ctenopharync-Odon idella Val.) in controlling submerged water
weeds. 'Nature, 226, 870,
Scott,'B., D.G. Gross, R. E. Iszard and T.O. Robson (1971) Recent
work on grass carp in the United Kingdoff from the stand point
of its economics in controlling submergeld aquatic plants.
Próc. European Res. Council 3rd Inter. Symp. on Aquatic Weeds.
106-116,
Vaic Zon, J.C.J., J.E. Blom and E.A. Huisman (1982) Irrigation
systems and protein source.' Proc. 2nd Inter. Symp. on
Herbivorous Fish. Novi Sad. 133-139.
71


Fish production from aquatic weeds

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