ABSTRACT
This study was designed to evaluate the larvivorous potential of some indigenous fishes for the control of mosquito larvae and by extension, the adult mosquitoes. Twelve (12) fish species were collected from streams, rivers, ponds, swamps, ditches, reservoirs, lakes and dams in parts of northern Nigeria using hand nets, cast nets, drag nets and malian traps. These fishes were evaluated in the laboratory against the number of Culex quinquefasciatus larvae that each can consume in 24 hours, from which their predatory indices (PI) were computed. Similarly, the ability of nine of the fish species to prey on mosquito larvae in outdoor concrete ponds was evaluated for 12 months. Fish hardiness was measured by the ability to survive without being fed for five months (20 weeks), ability to survive under extreme pH, and ability to survive in different water volumes for 13 weeks. The fish species were bred under static and flow-through systems. Furthermore, Clarias galmaensis hatched in the laboratory using ovaprim hormone was tested for larvivorous efficacy at 5, 7 and 9 months. All the fish species evaluated preyed on mosquito larvae, but the consumption rate varied with species, the differences in the consumption rates among the 12 fish species were significant at P<0 .05.="" as="" by="" consumed="" day="" each="" fish="" follows:="" i="" larvae="" mean="" mosquito="" number="" of="" per="" pi="" species="" the="" were="">Poecilia reticulata
265.00/53; Clarias gariepinus 101.36/223; Clarias galmaensis 49.40/988; Epiplatys spilargyreius 45.00/18; Hemichromis bimaculatus 35.96/187; Oreochromis niloticus 35.12/144; Leptocypris niloticus 29.33/88; Schilbe mystus 26.60/125; Synodontis ouemeensis 24.67/74; Brienomyrus niger 17.00/51; Astatotilapia burtoni 7.80/39 and Brycinus nurse 3.29/28. The number of larvae consumed by each of these fish species was lower when commercial feed was added but t-test showed that the differences were not significant (P>0.05). Except for ponds that contained Brienomyrus niger, Brycinus nurse and Synodontis ouemeensis, all other ponds were devoid of larvae throughout the test period of 12 months. The lowest pH tolerated by the fish was 3.16, by Astatotilapia burtoni and the highest was 11.85, by Brycinus nurse. No fish species had mortality of more than 50 percent when starved in the laboratory for five months. Mortalities recorded among the different fish species kept in 10, 20 and 30 litres of water for 13 weeks were generally low; in six of the 12 fish species no mortality was recorded; in all, only 15 (5.21%) out of 288 fish died. Poecilia reticulata bred in the static and flow-through system and Epiplatys spilargyreius in the laboratory all year round. Poecilia reticulata, Clarias gariepinus, Clarias galmaensis, Epiplatys spilargyreius, Hemichromis bimaculatus, Leptocypris niloticus, Oreochromis niloticus and Schilbe mystus showed promise as good predatory agents for mosquito larvae with the potential that could be harnessed for the control of mosquitoes and consequently the diseases they transmit.CHAPTER ONE
1.0 INTRODUCTION
1.1 Background of the Study
Mosquitoes are ubiquitous insects found in nearly every climatic region of the world (AES, 2000). They are vectors of malaria, yellow fever, dengue and dengue hemorrhagic fever, filariasis/elephantiasis, chikungunya and encephalitis. Malaria is endemic in more than 100 countries and forty-one percent of the world’s population lives in areas where the disease is transmitted (CDC, 2007; USAID, 2007). There are approximately 200,000 cases of yellow fever with 30,000 deaths every year world-wide (Sumodan, 2001; AMCA, 2005). Yellow fever occurs mainly in western sub-Saharan region and occasionally in some eastern countries; about 90% of all cases are found in Africa (TMB, 2004). Dengue is the most important tropical disease, after malaria, in 1998; more than 2.5 billion persons lived at risk (Gubler, 1998). It is estimated that there are 50 to 100 million cases of dengue fever and 500,000 of dengue hemorrhagic fever each year (WHO, 2006; McCormack, 2007). More than one billion people are threatened by lymphatic filariasis, the estimated at risk population is 390 million, which accounts for approximately one‐third of the global burden, thirty-eight percent of the at‐risk population are children under 15 years (GAELF, 2008). Encephalitis is a viral disease that causes inflammation of the brain and spinal cord; there are different types of encephalitis. Not only can mosquitoes carry diseases that afflict humans, they also transmit several diseases and parasites that dogs, sheep and horses are very susceptible to (AMCA, 2005). Mosquito-borne diseases have plagued civilization for thousands of years and throughout history; no insect has contributed more to human discomfort, disease and death than the mosquito (IDPH, 2007).
A lot of human activities are geared towards improving human health and a peaceful environment. Among the challenges facing human beings particularly in the developing countries is the menace of mosquito and mosquito–borne diseases which are on the increase despite of steps being taken to address the problem. Methods used for mosquitoes and mosquito-borne diseases control in Nigeria include, chemotherapy, removal and proper disposal of water retaining containers in the environment, draining marshes, removal of aquatic weeds and debris which shelter and protect mosquito larvae, chemicals, the use of mosquito repellents, use of mosquito nets, the use of mosquito screens on windows and doors in houses. However, some of these methods of mosquito control are often associated with problems. When the insecticidal properties of Dichloro-diphenyl-trichloroethane (DDT) were discovered in the 1940s, it became the principal method for Aedes aegypti eradication programmes in the Americas (WHO, 1997). The use of chemicals such as DDT and Hexachlorocyclohexane (HCH) are nowadays discouraged due to their residual effects, widespread resistance, soaring price and other operational difficulties (Goddard, 2003; Ghosh et al., 2005a). Proper disposal of water-holding containers has not been very successful. The use of repellants in the form of coils, creams, lotions, or in pressurized cans is expensive and not accepted by some people. The use of mosquito screens on windows and doors in houses is also expensive. Increase in irrigation schemes, dams for generating hydroelectric power, car washing activities, excavation projects, construction activities, road construction, damaged roads, wells, and floods all contribute to increase in potential mosquito breeding sites. Other factors that increase the number of mosquitoes or diseases associated with them include resistance of mosquitoes to some common previously effective insecticides, human drug resistance, increasing cost of treatment, non-availability
of cheap and effective drugs, poor drainage, lack of trained and dedicated scientist and specialists who work on mosquitoes, lack of vaccine breakthrough, poverty which hinders many people from accessing treatment, purchasing nets and insecticides. Dragonflies, birds, bats, dragonfly nymphs and diving beetles which are natural predators of adult mosquitoes and larvae are inadequate as their predatory effect is minimal in nature. The bacterium, Bacillus thuringiensis var. israelensis (Bti), which is known to kill mosquito larvae, is expensive to purchase and apply, it does not kill the pupae, it is effective only on standing and not moving water; must be reapplied often, and not all mosquito species respond to it, Bacillus sphaericus (Bs) is highly effective on Culex but not on Anopheles larvae (Gouge et al., 2001; Goddard, 2003). Therefore, in spite of efforts and money invested in combating mosquitoes, their number and diseases associated with them seems to be on the increase. Thus, mosquito control is far from being successful in Nigeria as there is hardly any method that is perfect in eliminating them. However, some fish species are known to eat mosquito larvae if they have access to them and Global experience has shown that in many situations fish are effective in controlling mosquitoes. Fish have been used as early as 1903 in some countries. The World Health Organization (WHO) recognizes the roles of larvivorous fish either alone or in an integrated approach for control of mosquito-borne diseases (WHO, 2003). In Nigeria biological control of mosquitoes using fish has remained unexploited because information on fish species with mosquito larvivorous potential is lacking, thus the mosquito control value of its vast fish population has remained unknown, undetermined and unutilised. Therefore, the essence of this study is to search for potential larvivorous indigenous fish species that can be used to control mosquito larvae towards the integration of biological control (using fish) into the overall mosquito control programme in the country.
The use of larvivorous fish can greatly reduce mosquito population in the environment; this will reduce mosquito nuisance and bites thus leading to a decrease in mosquito-transmitted diseases. The use of fish to control mosquito larvae will improve the health status of many Nigerians. The use of fish to control mosquito is more economical than chemical control methods (WHO, 1982; GMVC, 2002; WHO, 2003). Fish are environmentally friendly, acceptable and have no deleterious ecological consequences (GMVC, 2002; NVBDCP, 2007). Fish need no special equipment for introduction and maintenance (GMVC, 2002).
1.4 Aim and Objectives
This study is aimed at evaluating the predatory potential of some fish species on mosquito larvae from parts of northern Nigeria.
The objectives of this research are to determine:
i. The predatory potential of some fish species from parts of northern Nigeria on mosquito larvae.
ii. The rate of consumption of mosquito larvae by selected fish species in the absence and presence of commercial fish feed.
iii. The ability of the selected fish species to survive in different water pH levels.
iv. The ability of the selected fish species to survive in different water volumes.
v. The ability of the selected fish species to survive without being fed for five months.
vi. The reproductive potentials of the selected fish species in captivity.
i. Fish species indigenous to northern Nigeria are not good mosquito larvivorous agents.
ii. There is no significant difference in the rate of mosquito larvae consumption by the selected fish species in the absence or presence of commercial fish feed.
iii. Changes in pH do not affect the survival of the selected fish species.
iv. Change in water volume does not affect the survival of the selected fish species.
v. The ability of the selected fish species to survive is not affected by withdrawal of feeding.
vi. The reproductive potential of the selected fish species is not affected by captivity.
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