Lake Victoria is under threat and the very people this water source is supposed to serve are the ones threatening its existence. Today, let's explore why the Nile Perch is almost non-existent in the lake.
By Mathias Mugisha
More fish factories will close, turning fishermen into economic refugees. Out of the over 20 fish factories in Uganda, only 15 are still operational. In Jinja, two of the four factories have closed down. All the factories are operating below capacity.
Overfishing, catching immature fish, environmental degradation and pollution on Lake Victoria are killing the Nile perch, which forms the backbone of commercial fishing.
The young fish are starving and suffocating to death. The National Fisheries Resources Research Institute (NaFIRRI) predicts doom if nothing changes.
Information from NaFIRRI indicates that micro-organisms in the lake on which the young fish feed are also dying. This is brought about by the floating algae bloom on the lake that is poisoning the fish and cutting off oxygen supply to marine life.
Elias Muhumuza, a research technician at NaFIRRI, says Nile perch needs a lot more space and dissolved oxygen than other fish.
The lake is dying along with fish because of over fertilisation (eutrophication). This is the process by which a water body becomes rich in dissolved nutrients from fertilisers or sewage, thereby encouraging the growth and decomposition of oxygen — depleting plant life like algae and harming other organisms in the lake.
Algal blooms are normally a result of water pollution (normally caused by untreated waste from industrialisation, urbanisation and agriculture) which are normally experienced in some bays of Lake Victoria like Kitubulu and Murchison bay.
This has an effect on fish and other organisms in the water because the over-growth of algae due to a lot of nutrients and presence of suspended solids in the water reduces light penetration/ transparency, which has a lot of negative effects on all aquatic organisms.
Decomposition/decay of dead algae or other organisms consumes a lot of oxygen and hence competes with the fish, their prey and other aquatic organisms for dissolved oxygen, affecting the stability of the whole aquatic ecosystem.
“The result from the above scenarios is that the fish will die or will be under stressful conditions (due to lack of oxygen and food), meaning they will not be able to reproduce and grow well (the survivors), hence reduced stocks.
A man offloads smuggled fish from neighbouring Tanzania. PHOTO/Mathias Mugisha
Most young Nile perch stay in shallow waters near the shores to avoid high winds and being eaten by bigger Nile perch.
Nile perch is dying young because the organisms the fish feeds on also die,” explains Muhumuza.
Records from NaFFIRI show that the Nile perch has declined from an average of 1.2 million tonnes from 1999 to 2007, to about 800,000 today; while silver fish (mukene) has increased from about 400,000 to one million tonnes over the same period.
Other groups, mainly Nile tilapia (ngegge) and haplochromines (nkejje) have increased from 300,000 to about 600,000 tonnes. However, the trends show that when you compare the two, nkejje seems to have increased while ngegge declined.
“The increase in the smaller fish species is attributed to the decrease on the Nile perch which preys on them. Nile perch is the top predator. Once the number of predators goes down, its prey increases,” says Muhumuza.
Currently, mukene accounts for about 70% of Lake Victoria biomass, overtaking nkejje which accounted for 80% of the lake biomass when the Nile perch was introduced into Lake Victoria in the 1950s and 1960s for sport fishing.
The Nile perch (Latesniloticus) is a species of freshwater fish in the Latidae family of the order Perciformes. The
Nile perch can weigh up to 250kg with an average length of a mature fish averaging 121-137cm (48-54 inches), although many fish are caught before they can grow this large.
Though the introduction of the Nile perch was an ecological disaster that has led to the extinction of some of the indigenous fish species, it increased fish stocks in the lake 10 times, according to NaFFIRI.
The Nile perch was introduced along with two other tilapia species to boost the indigenous tilapia stocks in Lake Victoria which had declined.
The indigenous tilapia has since become extinct.
The pollution on the lake has also greatly affected the fish species.
According to Dr. Dismus Muhumuza, head of aquaculture research at NaFIRRI, the Nile perch was brought with four foreign tilapia species namely: Oreochromis niloticus, Tilapia zillii, Tilapia rendalli and Oreochromis leucostictes.
These species competed with the native tilapia species of Oreochromis variabilis and Oreochromis esculentus as the Nile perch hunted both of them with its biggest appetite directed at the nkejje.
Today figures from NaFIRRI show that Oreochromis variabilis is rare while Oreochromis esculentus is extinct.
The alteration of the native ecosystem had disruptive socioeconomic effects on local communities bordering the lake. Large-scale fishing operations, while earning millions of dollars from exports, have displaced many local people from their traditional occupations in the fishing trade and brought them to fishing villages where scarcity of fish has turned them into economic refugees.
On the flipside, Nile perch stimulated the establishment of large fishing companies and boosted sport fishing tourism in the region, with Murchison Falls considered one of the best fishing destinations in the world.
Every year, anglers from all over the world converge here to hook out Nile perch, weigh the fish and take their pictures with the catch, before releasing them back into the water.
But now, the tide has turned against the Nile perch, with man as its greatest enemy, threatening its existence and the livelihoods of the over 30 million people who depend on Lake Victoria.
“If this trend continues, more factories will have no choice but to close. There will simply be no fish left to sustain the factories,” Muhumuza concludes.
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