The Riddle of Lake Victoria

Lake Victoria is a hotbed of biological activity. Recent developments in research on Lake Victoria illustrate how lines of study can cross and intermingle, creating almost a feeding frenzy in scientific output.

Nile perch (Lates sp.) from Lake Victoria in the Shedd Aquarium. Photo by Ad Konings.

Studies of Lake Victoria are not new. Many decades ago, researchers like P. H. Greenwood of the British Museum of Natural History recognized the fascinating diversity of cichlid fishes in Lake Victoria. Subsequent work, spearheaded by Frans Witte and his students and colleagues (e.g., Tijs Goldschmidt; Ole Seehausen) at the University of Leiden further elaborated on the marvels of this lake, the largest body of tropical freshwater on the planet.

It was because of this history of investigations in Lake Victoria that the disastrous effects of the introduction of Nile perch (Lates sp.) has become frighteningly obvious. The exact history of when and who introduced the Nile perch is a little murky, but the essence is that sometime in the 1950s the introduction was made in an attempt to increase fishery production. The thinking at the time was that a large predator would eat the smaller fishes (i.e., the numerous small cichlids), thereby concentrating their biomass into an easier-to-catch and more profitable product. Although it took a while to establish itself, eventually the Nile perch population exploded, decimating cichlid populations. In his book Darwin's Dreampond, Tijs Goldschmidt describes the dramatic reduction in cichlid numbers and, even more alarmingly, whole species, in the mid-1980s. By 1987, 100 of the 110 known species of haplochromines from the sublittoral zone had vanished.

The early 1990s showed little change. More recently however, starting in the late 1990s and continuing today, things are once again changing in Lake Victoria (Witte et al., 2000). Fishermen report substantial increases in cichlid catches. But the cichlids they are catching now are not the ones they were catching before. So what's happening?

When the Nile perch numbers exploded, so too did the fishery for them. There is now a very intense fishery on Nile perch such that the Nile perch numbers are declining. As they decline, numbers of cichlids are increasing. However, the cichlids that are increasing the most are zooplanktivores. The detritivores and phytoplanktivores are not. In fact, much of the cichlid catch now is made up of two species of zooplanktivores, namely Haplochromis (Yssichromis) pyrrhocephalus and H. (Y.) laparogramma. This contrasts to catches in the late 1970s where detritivores and phytoplanktivores made up the bulk of the catch.

The question of which species should survive and which should go extinct is a long-standing issue in evolutionary biology. The tragedy of Lake Victoria has, for better or worse, provided scientists with a first-hand opportunity to examine this process in action. Witte et al. (2000) argue that some of the best evidence for understanding extinction is through examination of pairs of similar species. In this case, they compared H. (Y.) heusinkveldi and H. (Y) pyrrhocephalus. Prior to the Nile perch, both species were equally abundant and similarly distributed around the lake. Both are similar in morphology. However, H. (Y) heusinkveldi has not recovered, while H. (Y) pyrrhocephalus has. Why? Witte et al. argue that there are subtle differences in the visual system of these two species: H. (Y) heusinkveldi is well-equipped for seeing small particles while H. (Y.) pyrrhocephalus sees and eats larger particles. Concomitant with the Nile perch increase - and likely related to it - there have been dramatic changes in the water chemistry of the lake, including eutrophication, algal blooms and a general decrease in water clarity. It would appear that these factors have been more detrimental to H. (Y) heusinkveldi than to H. (Y) pyrrhocephalus. However, it is also very possible, as the authors point out, that the fishes we see today may not be the fishes we see tomorrow, as Lake Victoria continues to change.

Complexity and change are nothing new to Lake Victoria. That there was (or had been) enormous biodiversity was clear. The puzzle was: how did it get that way and how long had it taken? Researchers have attempted to examine this question for some time; indeed it has been regarded as one of the holy grails of evolutionary biology to explain the radiations of cichlids in the Great Lakes of East Africa. However in 1997, Thomas Johnson and colleagues argued that this diversity must have been produced at an astounding rate, namely all of it in about 12,000 years - a rate never imagined before for vertebrate speciation. I summarized their argument in a previous article (Coleman, 1997). In brief, using geologic techniques, they argued that the lake must have been dry 12,000 years ago. Combined with an earlier study by Meyer et al. (1990) that suggested that all cichlids in the lake came from a single ancestor, i. e., there had not been multiple invasions of cichlids from nearby waters, therefore somehow this single species of cichlid gave rise to 300-500 other species in the span of 12,000 years.

As so often happens in science, once a statement like that is made, the glove has been dropped and other scientists weigh in on the validity of the techniques and evidence used and attempt to refute such an incredible claim. This is how science works best: the healthy antagonism of independent workers ensures that unwarranted assumptions are questioned, faulty techniques are revealed and inaccurate data are rejected. It is important to keep in mind that because we were not present and taking data at the time that many evolutionary events took place, much of our knowledge about evolutionary biology must be inferred through indirect means. Indirect methods are particularly prone to problems.

Sandra Nagl and coworkers (2000) have questioned the key belief that all cichlids in Lake Victoria came from a single ancestral species that re-colonized the lake after it dried out. By examining a broader range of species from the surrounding area (Lakes Edward, George and Albert), they show that indeed, it is highly unlikely. They argue that the ancestors of the Lake Victoria cichlids have been present in the rivers of East Africa for roughly 1.4 million years. These fishes were trophic generalists and had substantial genetic polymorphism (diversity within the gene pool). When these fishes entered the refilling lake, they were able to rapidly adapt to numerous niches because of the genetic polymorphism already present in the population.

Geoffrey Fryer (2001) goes further, questioning numerous subtle contradictions and inconsistencies in the original work, as well as that of Nagl, et al. (2000). He questions the geophysical evidence, pointing out that while the lake might have indeed diminished dramatically, it is one thing to say the lake was smaller and quite another to say it was completely dry. Further, he points out that if Lake Victoria did indeed dry out completely, and was re-colonized by a relatively small fauna, as Johnson suggested, then not only do we have to explain the cichlid diversity, but we also have to explain all the other endemic fauna in the lake as well. Though not as dramatic as the cichlids, these other fishes (Clariidae, Mochokidae, Mastacembelidae, Characidae, Mormyridae, Cyprinodontidae and Cyprinidae) have all produced endemic species, and in some cases, endemic genera in Lake Victoria. Have these lineages also undergone extraordinary rates of evolution?

The bottom line from all of these studies, and many more besides, is that we do not yet know what exactly has gone on or is going on in Lake Victoria. One thing is for sure; the Lake contains a remarkable diversity of cichlid fishes and once we understand how and why they got that way we will understand a lot more about the complex world of cichlids.