Gatonye Gathura And Cosmas Butunyi
6 October 2008
Nairobi — In what could be a first in the world, scientists have caught evolution in the act of creating a new species in Lake Victoria as animals and plants try to cope with increased pollution and the effects of climate change.
The scientists have tabled evidence indicating that it is not pollution and over-fishing alone that are responsible for the disappearance of some fish species in Lake Victoria and the evolving of others like the cichlid into new species.
Researchers from Tokyo's Institute of Technology and the Swiss Federal Institute of Aquatic Science and Technology have observed the cichlid, also called Pundamilia nyererei, evolve into a new species better adapted in sighting its prey and predator.
In a study published in the journal Nature last Tuesday, the researchers say this explains the very rapid loss of pundamilia in Lake Victoria over the past 30 years, falling by half from 500.
Researchers looked at two species, conspicuous by their red or blue colours. They determined through lab experiments that certain genetic mutations helped some fish adapt their vision at deeper levels to see the colour red and others in shallower water to recognise shades of blue.
The researchers showed that the eyes have adapted to this difference so that fish that live in deeper water have a pigment in their eyes that is more sensitive to red light, while shallow-water fish were sensitive to blue. The study says the eye adaptations have also affected mating patterns.
In clear waters, the study says, the colour that appears brighter shifts from red to blue gradually with depth, and red and blue fish stick to their zones, cementing their genetic differences.
In murky waters, the transition from red to blue happens much quicker and blue and red fish sometimes interbreed, destroying species differences.
Drastic fall
Local researchers at the Kenya Marine and Fisheries Research Institute (Kemfri), say surface water temperatures at the lake have been rising, resulting in a drastic fall in dissolved oxygen.
"The water temperatures are higher than they were 10 years ago," says Dr John Gichuki, Kemfri director at Kisumu.
Apart from an overall reduction in the water in the lake, the researchers say, the strength and pattern of the winds have also changed. Normally, the lake's water is turned by the wind twice a year, injecting oxygen into the water. "This has not been the case recently," Dr Gichuki says.
The winds are not strong enough, resulting in water at the bottom remaining unturned, restricting the entry of oxygen.
The available oxygen is eventually used up and fish that are not tolerant to the new conditions flee the "dead zones" to the top layers, where they are easy prey.
The effects of climate change are being reflected in the fish composition and densities, according to Dr Jembe Tsuma, another researcher at Kemfri.
While Nile Perch, which requires high oxygen conditions, has been on the decline, there has been a resurgence of species which are tolerant to low oxygen conditions, but have a low commercial value.
Must change
"The mud fish now accounts for over 20 per cent of the fish landings in the Nyanza Gulf," Dr Tsuma explains.ecent surveys found that 2.93 tonnes of Nile Perch are landed per square kilometre, down from 11.08 four years ago.
As the fish evolve, researchers say the fisher folk communities must also change. "Due to higher temperatures, there is increased water loss and lower lake levels, thus reducing productivity," says Ms Marisa Goulden, a researcher at the Tyndall Centre of the University of East Anglia.
This comes in the wake of another startling revelation that tonnes of water hyacinth and hippo grass are decomposing at the bottom of Lake Victoria.
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