Scientists raise alarm over decline in Nile perch stock in Lake Victoria

Friday December 16 2016

A fisherman carries two large Nile perch fish

A fisherman carries two large Nile perch fish at Lihunda Beach in Lake Victoria on June 26, 2016. PHOTO | TONNY OMONDI 

By HALIMA ABDALLAH

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.

Rapid population growth around the lake basin increased nutrients loading through deforestation, agriculture and growth of cities. The Nyanza gulf is the most densely populated and highly polluted.

After the Nile Perch explosion in the 1980s, the fish population came to be dominated by only three species — Nile Perch, Nile tilapia and dagaa — which fisheries productivity increased 10 fold.

In Kenya, for instance, Nile perch was discovered in 1975 while catches of catfish and lungfish had collapsed in 1977, falling by 84 per cent and 98 per cent respectively.

Haplochromines collapsed totally by 1983 while Nile perch had increased 10 times. Similar trends were recorded in Uganda and Tanzania. 

There are currently one million people employed directly and indirectly in the fishing industry in spite of the collapse of some fish processing factories due to low Nile perch and Nile tilapia catch has.  By 2014, beach value was estimated at $600 million, although Nile perch made up to only 25 per cent of the total catch, which was valued at $300 million.

Lake ecosystem

Low Nile perch catches have seen some factories close shop, resulting in low exports and unemployment in the value chain.   

Annual fish yields from Lake Victoria are estimated at one million tonnes valued at over $400 million, which contributes to 2 per cent, 3 per cent and 2.8 per cent  to the GDPs of Kenya, Uganda and Tanzania respectively.

In re-examining the role of Nile perch in the transformation of Lake Victoria, researchers concluded that after more than half a century since its introduction into the lake, significant changes have occurred. This, they said, calls for measures to restore the lake’s ecosystem and replenish the dwindling stock of fisheries resources of Nile perch and Nile tilapia.

“It may be difficult to accept that a single species could have such a profound impact on such a large and complex ecosystem, but Nile perch appear to have been the catalyst for these sweeping changes. Most of the recent changes in Lake Victoria have been man-made and there can be little doubt that the deliberate introduction of Nile Perch was the main driver behind them,” the researchers concluded.