Gambusia affinis also known as mosquito fish. Copyright: US Centers for Disease Control and Prevention

Roloffia, a species of killifish.
Copyright 1997 Anthony C. Terceira


Nothobranchius
Copyright 1997 Jim Robinson

BIOLOGICAL CONTROL

Overview
Malaria continues to kill millions of men, women and children in third world and developing nations. The extent of this devastation is plain to see when one looks at the world map showing where malaria exists today.

FIG. 1 Estimated incidence of clinical malaria episodes caused by local transmission of
Plasmodium falciparum in 2004. Adapted from a map in the World Malaria Report 2005
(WHO, 2005), with the assistance and permission of the World Health Organization.

From: Walther and Walther (2007) Annals of Tropical Medicine & Parasitology, 101(8): 657-672

Prevention of disease transmission by mosquitoes and other insects requires a multifaceted approach, which includes killing mosquitoes with pesticides and larvicides. Biological control has received less interest over the last 3 decades, but now regaining interest because of the resistance of mosquitoes to the chemical approach.

Biological control may also be achieved by the use of predators that prey on mosquito larvae in stagnant pools. The use of larvivorous fishes (fishes that feed on larvae, such as Gambusia affinis), has been employed successfully for many decades. However, Gambusia is an aggressive fish that has displaced many indigenous fishes, thus causing environmental concern in areas where they are introduced. Using larvivorous fishes is an approach employed only in countries where Gambusia has already been introduced or is naturally present. Another drawback of using conventional fishes is that mosquitoes primarily breed in stagnant bodies of water which dry up seasonally. The introduction of Gambusia is effective only during the wet season and will require restocking after the dry season passes and the next rainy season sets in.

Annual Killifishes
Poseidon promotes the use of a different larvivorous fish, called annual killifishes. These fishes are found in freshwater and can maintain permanent populations even in temporary bodies of water. Adult and juvenile annual fishes die during the dry season and reappear when the rainy season starts. Killifishes are able to maintain stable populations because of a unique development cycle in which the embryos laid during the wet season can undergo a state of suspended animation, thus allowing them to survive adverse conditions such as drought and high temperatures. Populations of these fishes have been found in pools as small as imprints made by elephant hooves and in areas that previously experienced drought lasting as long as five years. Annual killifishes feed on the larvae of mosquitoes, which also hatch during the rainy season. With an adequate fish population in temporary pools, it is theoretically possible to produce adequate biological control in special bodies of freshwater.

Besides their unique life cycle, there are advantages to the use of annual killifishes:

• Their small size (approximately five to seven cm.) enables them to seek predators in shallow corners of the pool and under/in between the leaves of water plants.

• Their small size also makes them less suitable as a food source by local human population that co-inhabit these areas.

• They are rarely aggressive and have never been found to develop populations in permanent bodies of freshwater.

• The sturdy eggs can be mass-produced, thus making it possible to carry millions of embryos for dissemination in difficult terrain usually found in malarious areas.

Historical Perspective
In 1952, the noted ichthyologist, George Myers, made the first field observation that annual fishes may have potential in biological control of mosquitoes since mosquito bites were diminished in areas normally populated by these fishes. It was not until two decades later in 1978 when Jonathan Matias and Jules Markofsky, then with the Orentreich Foundation in New York, that this idea was again re-introduced during the Columbia University Seminars on Pollution and Water Resources. It took yet again another two decades when Nova Pacific Research Foundation (Philippines) received a grant from Conservation, Food and Health Foundation (Boston, Massachusetts) to undertake laboratory studies on the predatory activity of annual fishes against mosquito larvae. The results of this research are highlighted here to demonstrate the practical use of annual fish as a biological control method.

Tests on predatory activity of annual fishes.

Table 1. The relationship between the length, weight and larval consumption in mature adults of annual fishes from South America and Africa. Mean and standard Deviation of 5 fishes per test.

The data in Table 1 show that different species have different rate of consumption, with the South American species of C. whiteii showing the highest consumption. There were no sex differences in total food consumption, except in C. whiteii where the females showed 4 times higher food consumption despite being half the size of the males.. These fishes can live almost exclusively on mosquito larvae when no other food is present. However, ponds are inhabited by many species of insect larvae and other life forms that serve as food during the rainy season. The data demonstrate that these fishes are predatory to mosquito larvae and may serve as a biological control in isolated pools of fresh water.

Figure 1. The relationship between feeding rate, weight and length in the annual fish,
Nothobranchius guentheri. Each point represents the value for each fish in the study.

The results for N. guentheri showed that the larger the fishes, the greater the larval consumption rate.

Research on understanding the mechanisms that control the onset of duration of arrested development (also known as suspended animation or diapause) has been undertaken by Poseidon scientists for several decades. The level of knowledge about the biology of these fishes has reached the stage where it is now possible to mass produce annual species for biocontrol trials. For more information about annual fish biology please CLICK HERE.

Poseidon’s goal is to embark on mass production of annual killifishes to support future use of these fishes for biological control of mosquitoes and insects. The control of suspended animation also has major implications in studies on health, disease, and aging. Mass production will also enable the company to support the expansion of research in this area for possible applications in biomedical sciences.

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