Scientists raise alarm over invasive species decline in East African great lake

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A fisherman carries two large Nile perch fish at Lihunda Beach in Lake Victoria on June 26, 2016. Image: Tonny Omondi

By Halima Abdallah

Editor’s note: Uganda-based reporter Halima Abdallah of The East African developed this story at an environmental journalism workshop led by Eric Freedman, director of MSU’s Knight Center for Environmental Journalism which publishes Great Lakes Echo.

Will the commercial viability of Lake Victoria and its ecosystem be sustained? This is the question arising from re-emergence of low value native species like dagaa against dwindling stocks of high-value species like the Nile perch.

A new study, “Nile Perch and Transformation of Lake Victoria” published in the Africa Journal of Aquatic Science 2016 shows that haplochromines — diverse groups of small fish found in East Africa’s lakes — now form 60 per cent of the lake’s biomass. The other 40 per cent is made up of Nile perch, tilapia, catfish and lungfish, among other species.

“The changes in fish stocks are attributed to changes in key environmental variables,” said Anthony Taabu Munyaho, the lead researcher and director at National Fisheries Resources Research institute in Uganda.

“Overfishing, habitat degradation, death of animal life in the lake due to lack of oxygen, pollution and invasive species are the main drivers,” he added.

Even though the study finds that the haplochromines biomass is rising, it still falls below 83 per cent, the volumes recorded in the 1950s.

Back then, the Lake’s water was unpolluted, but commercial productivity remained inconsequential as the fish catch was mainly for domestic markets.

The introduction of Nile perch in 1962 (some studies suggest that it could have been illegally introduced in 1954 by Uganda Game and Fisheries Services as breeding population was discovered in 1963) transformed Lake Victoria into one of the most productive freshwater lakes in the world. However, the scientists did not know the ecological impact of the development.

Nile perch spread into other lakes like Lake Tanganyika and Lake Kyoga. In 1999, Lake Victoria alone had a production capacity of 1.6 million tonnes of Nile perch, but in 2016 only 803,000 tonnes were registered.

Endemic species now extinct

Kenya, Tanzania and Uganda’s production capacities for Nile perch currently stand at 54,000; 393,000; and 365,000 tonnes respectively, according to the study. In 1999, dagaa volumes stood at 200,000 tonnes but in 2016 researchers say they have recovered at least 1.3 million tonnes.

This rise in volumes is attributed to declining numbers of Nile perch.

Before introduction of Nile Perch, there were up to 500 endemic species but these gradually went extinct, leaving only a few to survive.

“Nile perch eats Nile tilapia and haplochromines  and so Nile perch dominated the lake,” said Dr Taabu.

Scientists argue that the depletion of native fish biomass by Nile perch may have been the source of eutrophication of the lake, which also kills the fish.

This story originally appeared in The East African and is republished here with permission.

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