Haplochromine Cichlids of Lake Victoria

The haplochromine cichlid fish of Lake Victoria demonstrate both the exuberance of species radiation and the tragedy of mass extinction, the first to occur during historical times. These tiny, colorful fish, which constituted 80 percent of the fish biomass in Lake Victoria prior to 1978, now account for less than 2 percent (Kaufman, 1992). This decline has been caused by a combination of human influences.

The cichlids are a very large and diverse family of freshwater, perchlike fish characterized by certain anatomical features (such as having a single nostril on either side of the snout, and an interrupted lateral line system) and the high degree of parental care devoted to their offspring (Keenleyside, 1991). Cichlids are native to Africa, the Middle East, Madagascar, Sri Lanka, the southern coast of India, and Central and South America. Cichlids have also been accidentally or intentionally introduced in many other parts of the world, often with disastrous effects on the native fish populations because of predation or competition.

In Lake Victoria, the cichlids of the hap-lochromine genus form a vast "species flock," a group of species derived from a common ancestor. The haplochromines have radiated to utilize a diverse array of feeding strategies. There are bottom-dwelling detritus-eaters, algae-scrapers that feed along the rocky shoreline, snail-crushers and others that extract the snail from its intact shell, insect-eaters, zoo-plankton-eaters, prawn-eaters, fish-eaters, scale-scrapers, and, most notoriously, the pedophages, which specialize in eating the embryos and fry of other haplochromines (Goldschmidt, 1996). Since all of the Lake Victoria haplochromines are mouth-brooders, this means that the pedophage must wrest its meal from the mother's mouth.

The haplochromines are also notable for the rapidity with which they radiated to occupy the many niches of Lake Victoria. This lake is located in East Africa, bordered by Tanzania, Uganda, and Kenya. Lake Victoria is the second largest body of fresh water in the world (the largest being Lake Superior in North America). Although the lake existed in very ancient times, climatic changes during the Pleistocene led to its complete desiccation, which may have lasted for 5,000 years. All of the modern species of haplochromines endemic to Lake Victoria have evolved since the lake refilled, within the last 12,000 years— an example of extremely rapid evolution (Johnson et al., 1996). The diversity of adaptive speciation and niche separation among the haplochromines has earned Lake Victoria the nickname "Darwin's Dreampond" (Goldschmidt, 1996).

More than forty species of haplochromines from Lake Victoria are listed as extinct by the International Union for the Conservation of Nature (IUCN)(Hilton-Taylor, 2000), but Harrison and Stiassny (1999) caution that not enough is known about many of the species even to determine their correct taxonomy. At the current level of knowledge, it may be impossible to determine whether isolated populations of some species have survived. For now, many can be listed as "missing in action." It is safe to say that the species flock is in critical danger of losing a large portion of its diversity.

Examples of the diverse species of hap-lochromines that are missing in action include species occupying all of the ecological niches. Haplochromis ishmaeli, a green-bodied fish with splashes of red on its dorsal fin and tail, was a snail-eater. H. ishmaeli used its pharyngeal mill, a crushing device located in its throat, to pulverize the mollusks that it swallowed whole. In contrast, H. degeni, a species extinct in the wild, extracted the snail by banging and shaking the shell to dislodge its resident. H. mega-lops was a zooplanktivore, feeding on microscopic crustaceans. It vacuumed up its tiny prey by rapidly protruding its funnel-shaped mouth and sucking in water and zooplankton. Other haplochromines, such as H. macrog-nathus, an elongated fish with a silvery body, were streamlined predators that preyed upon haplochromines and other small fish. Some haplochromines, such as H. welcommei, which may still lurk in the waters of Lake Victoria, require only pieces of their victims. They ambush their prey and roughly scrape their meal, the scales, off of its body. H. microdon, with its rainbowed belly and tiger-striped dor-sum, was a pedophage. It targeted a mouth-brooding female and used its body to ram into the mother with such force that the brood would be expelled from the safety of her mouth. H. microdon then had its meal. Many of the fish that are missing in action have not yet been assigned scientific names but are still known by colorful, descriptive monikers assigned by their discoverers, such as H. two stripe yellow green.

Prior to the arrival of Europeans at Lake Victoria in the 1850s, human impact was limited to subsistence fishing and low levels of agricultural and human waste runoff. Settlement by Europeans increased the demand for fish, and the volume of runoff from lakeside industry and agriculture soared. Deforestation and soil erosion also contributed to increased flow of sediments into the lake.

With increased fishing pressure, the catch of large fish (especially the tilapiines Ore-ochromis esculentus and O. variabilis) dwindled. To satisfy the demand, nets of smaller and smaller mesh were employed to catch the smaller and smaller fish. The result was the decimation of mature and immature populations alike. The small haplochromines were harvested to make fishmeal but were not as desirable as the larger species. In the 1950s a proposal was made to stock the lake with nonnative species, the predatory Nile perch (Lates niloticus) and plankton-eating Nile tilapia (Oreochromis niloticus). The proponents of these introductions sought to utilize the haplochromines as food for the perch that would be a more marketable fish crop. Opponents to these introductions raised sound arguments, but they fell upon deaf ears (Fryer, 1960).

The most obvious problem with the introduction of predators is based on the simple principle that an adequate prey supply must be available to support the population. In a closed system, a rapidly growing predator population can be expected to exhaust its food supply, at best leading to a boom and bust cycle. Anticipating this disastrous outcome, scientists urged caution, fearing that introductions of nonna-tive fish would jeopardize rather than enhance Lake Victoria's commercial fishery.

Indeed, the critics' predictions have been borne out. The native tilapine species, which had been the most important commercial species, already in decline because of over-fishing, also became the prey of the Nile perch. Competition for food with the introduced Nile tilapia further strained the native tilapi-ines. With the diminishing catches of tilapia, human and perch appetites turned to the hap-lochromines (Ogutu-Ohwayo, 1990).

In the pristine Lake Victoria, diversity was advantageous for the haplochromines, enabling them to make full use of the lake's opportunities. However, populations within each species were limited, and the practice of mouth-brooding made the population more vulnerable to predation (the death of a mouth-brooding female also results in the loss of her progeny). The predatory perch, which reproduce at a much higher rate than the native cichlids, contributed to the decimation of the hap-lochromine populations. Having eaten themselves out of their fish food supply, they now consume prawns and cannibalize their young (Goldschmidt, 1996). It seems unlikely that a population dependent upon cannibalism can survive.

Although the Nile perch have been vil-lainized for their role in the decline of the native fish species in Lake Victoria, the greatest share of the blame falls directly to humans. Overfishing and indiscriminate taking of young fish have diminished the breeding populations. Agriculture, industry, and deforestation have resulted in increased soil runoff and direct discharge of nutrient-rich material into the lake. Algae are nourished by this runoff, and they proliferate. The algal blooms block the sunlight, and decay of the algae consumes oxygen. This process is called eutrophication. Massive fish kills in 1984 were attributed to unusually large algal blooms following storms that stirred up nutrient-rich sediment. Deoxy-genation and acidification of the water, coupled with physical clogging of the fishes' gills, killed many fish. In addition, toxins produced by the algae have been implicated in these events (Ochumba, 1990). Algae also clog the intake filters of water purification plants, increasing filtration costs to local consumers. The increased nutrient loads have also supported the proliferation of water hyacinth (Eichhornia crassipes), an exotic plant that chokes waterways and further contributes to eutrophication (Baskin, 1994).

Deoxygenation of the water has made the deeper levels uninhabitable for many species. The haplochromines, which seem to be more tolerant of decreased oxygen availability than the Nile perch, may take refuge in the deeper waters. This tactic may help shield them from predation, but fluctuations in oxygen content in this already oxygen-poor environment may be fatal, and the added stresses of life under these conditions may be detrimental to growth and reproduction (Kaufman and Ochumba, 1993). Eutrophication has also led to decreased underwater light levels. Some scientists have hypothesized that closely related hap-lochromine species may have difficulty recognizing other members of their own kind, and thus be vulnerable to accidental inter-species breeding (Seehausen et al., 1997)

Ultimately, few have benefited from the changes in Lake Victoria, except the industrial fishing and fish export industries, and even they are now facing declining catches. From the point of view of the average local fishermen, the current state of affairs in the lake is dismal. Catching the large perch requires stronger, more expensive nets, yet the market price for Nile perch is lower than for the elusive native species. Unlike the smaller native fish, the large, oily perch must be smoked for preservation, and increased demand for firewood has contributed to deforestation (Barel et al., 1985). As fish catches have diminished, and most of the fish caught are destined for export, this primary protein source has become inaccessible to much of the local population; protein malnutrition is a growing health problem. There is also concern that loss of snail-eating haplochromines may lead to an increase of bilharzia, a parasite carried by snails, which causes severe human disease (Kaufman, 1992).

Goldschmidt (1996) found that by the late 1980s, approximately 70 percent of previously documented species were missing from a sampled area, in comparison with prior survey results. It is believed that more than 50 percent of the known species of Lake Victoria cichlids may have become extinct since their first recognition within the last 200 years. Remnant populations of some species may persist in satellite lakes and streams, but human creation of interlake waterways threatens to allow the entry of perch into those refugia (Kaufman and Ochumba, 1993). Captive breeding programs have been established for a few of the threatened species, but much of the remarkable diversity of the Lake Victoria haplochromines has already been lost. It is unlikely that human intervention will come soon enough to salvage what remains. Lake Victoria must serve as a tragic lesson about the dangers of meddling with ecosystems and the consequences of irresponsible resource management. Nevertheless, Lake Victoria is not called a dreampond erroneously. There is evidence that new species may yet be arising, selected for as a result of the current environmental pressures.

—Julie Pomerantz

See also: Conservation Biology; Endangered Species; Evolution; Evolutionary Biodiversity

Bibliography

Barel, C. D. N., et al. 1985. "Destruction of Fisheries in Africa's Lakes." Nature 315: 19-20; Barlow, George W. 2000. The Cichlid Fishes: Nature's Grand Experiment in Evolution. Cambridge: Perseus; Baskin, Y. 1994. "Losing a Lake." Discover 15: 72-81; Fryer, G. 1960. "Concerning the Proposed Introduction of Nile Perch into Lake Victoria." East African Agricultural Journal 25: 267-270; Goldschmidt, Tijs. 1996. Darwin's

Dreampond: Drama in Lake Victoria. Cambridge: MIT Press; Harrison, I. J., and M. L. J. Stiassny. 1999. "The Quiet Crisis: A Preliminary Listing of the Freshwater Fishes of the World That Are Extinct or 'Missing in Action.'" In Extinctions in Near Time, edited by R. D. E. MacPhee, pp. 271-331. New York: Kluwer/Plenum; Hilton-Taylor, Craig. 2000. 2000 IUCN Red List of Threatened Species. Gland, Switzerland: IUCN Publications Service Unit; Johnson, T. C., et al. 1996. "Late Pleistocene Desiccation of Lake Victoria and Rapid Evolution of Cichlid Fishes." Science 273: 1091-1093; Kaufman, L. 1992. "Catastrophic Change in Species-rich Freshwater Ecosystems." BioScience 42, no. 11: 846-858; Kaufman, L., and P. Ochumba. 1993. "Evolutionary and Conservation Biology of Cichlid Fishes as Revealed by Fau-nal Remnants in Northern Lake Victoria." Conservation Biology 7, no. 3: 719-730; Keenleyside, Miles H. A. 1991. Cichlid Fishes: Behaviour, Ecology and Evolution. London: Chapman and Hall; Ochumba, P. B. O. 1990. "Massive Fish Kills within the Nyanza Gulf of Lake Victoria, Kenya." Hydrobiologia 208: 93-99; Ogutu-Ohwayo, R. 1990. "The Decline of the Native Fishes of Lakes Victoria and Kyoga (East Africa) and the Impact of Introduced Species, Especially the Nile Perch, Lates niloticus, and the Nile Tilapia, Oreochromis niloticus." Environmental Biology of Fishes 27: 81-96; Seehausen, O., J. J. M. van Alphen, and F. Witte. 1997. "Cichlid Fish Diversity Threatened by Eutrophication That Curbs Sexual Selection." Science 277: 1808-1811.

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  • SONJA
    What are haplochromine cichlid?
    3 months ago

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