HARARE – AT five in the morning on a Friday, 25-year-old fisherman, Reuben Kadzira stood on the banks of Lake Chivero.

He picked a handful of fish from his fishing nets.

He had many weeks of fruitless fishing expeditions on the western part of the lake, a place that he has spent many years aching out a living.

There have been dramatic declines in the level of his harvest in recently, and like many other fishermen, he is worried.

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“The catch has been bad,” He says with open abandon. “It could have been the cold weather driving the fish into the deep waters,” says Kadzira.

But a much more worrying phenomenon is taking place in Harare’s biggest source of water and protein.

Unknown to fishermen like Kadzira scientists say the depleting catches are not a result of the migration of fish into deeper territory.

They have discovered an alien species that has been decimating stocks at a terrific rate, and is now threatening those who have depended on its stocks for decades.

The Nile Perch, a fish type that is also known as Niloctous, has swamped local indigenous species, reducing drastically fish populations.

The Nile Perch is a territorial predator, which can decimate other fish types in their thousands.

It can reach up to 20 kilogrammes in weight.

Zimbabwean biologist, Neil Deacon, says the Nile Perch has an insatiable appetite for other fish, which has changed characteristics of Lake Chivero’s aqua culture. But there are other considerations as well.

“ There are a number of challenges with regard to maintaining fish populations in lakes like Chivero,” Says the biologist, “These may also regulations and marine protected areas, harvesting fish at the right age to protect the breeding cycles and maintainance of fresh water environments.”

He says it had been hoped that by introducing the

Nile Perch, fish output would improve on the lake, which would translates into economic gains and fish sporting activities for fishermen.

But the effects have been dire.

Native species are now threatened with extinction, including the ecologically important tiny organisms that other fish species feed on.

The introductions changed habitat, trophic dynamics and water clarity.

The change in water clarity is thought to be responsible for hybridisation of the haplochromines, another type of fish, and affecting fish diversity.

In Lake Kariba, Nile Perch has been introduced, and fish stocks in Zimbabwe’s biggest water body are also threatened, according to official reports.

Kariba’s local economy was until 2000s based on native as well as exotic fish.

Lake Kariba is the main supply of fish within Zimbabwe.

Smaller reservoirs and rivers in communal areas probably contribute around 2 000 tonnes, assuming yields of 100 kg/hectare.

Privately owned reservoirs are said to be generally, not exploited significantly, so yields are on average, less than 50 kg/hectare giving a very rough total production figure of 3 000 tonnes according to the Zimbabwe Fish Producers Association.

Fish produced by aquaculture makes a negligible contribution to total fish supply and has been estimated to be in the region of 750 tonnes per annum. Yields from the rivers also make a negligible contribution to total fish supply.

Fish is an important food product and is on demand in the country.Estimates for the current supply of fish product in the country vary. Import data is not readily available either from imported countries or Zimbabwe since April 1993. Estimates from Namibia indicate that approximately 3,500 tonnes of frozen horse mackerel is enters the country every month. South Africa , Tanzania, Kenya and Mozambique are other countries that imports fish into Zimbabwe.Information provided by companies in Zimbabwe varies but ranges between 1,500 – 2,000 tonnes.

With the decimation of the scanty fish resources through marauders many problems have surfaced among them border conflicts attributable to the migratory and trans-boundary nature of the fishery resource. Of the recent past, the shortage of fish catch has led to the communities [both in Zimbabwe and Zambia] trading accusations of getting into each other’s territories and poaching fish.

Pollution might have increased the species richness of the lakes or might have caused local extinction of some species and out-migration of many species which are not tolerant to polluted water.

Environmentalists suggest that species the distribution is a result of different tolerances and responses of organisms to the biology as well as the physio-chemical conditions of the environment.

“Invader species might have decreased local species richness or might have caused local extinction of some species, but pollution of the lakes could also be a factor playing havoc with the fish stocks,” submits Professor Christopher Magadza, a fresh water scientist.

The perch is effectively reared in ponds as other fish species like tilapia. Unlike Tilapia, Nile Perch is considered more sophisticated in terms of management. One time Director of the Kariba research Station Magadza says the most trying aspect of rearing them is that Tilapia feeds on plants and artificial food like bread and cakes, while Nile Perch feeds on tilapia and aquatic insects.

Worried about the decline in fish in the lake, the department of National Parks and Wildlife Authority, The University of Zimbabwe (UZ) and Great Zimbabwe University (GZU) have started carrying studies to determine the extent of damage the species has had on local fish breeds in the country’s prominent lakes, Chivero and Kariba.

According to Gary Stafford, a conservationist at Lake Chivero, fish quantities have reached alarming levels, pointing out that a team of scientists and students are searching the lake to establish the extent of fish drops. Although the conservationists attribute other factors to the decline, he believes alien species can reduce fish stocks drastically.

Stafford who is working together with UZ, GZU and National Parks recommends that in order to avoid this discord, conservation measures such as co-management units that focus on conclusive studies to establish the nature of fish reduction, curbing over-fishing, the purging of illegal fishing and pollution control should be considered.

Deep-water cages located almost a kilometre or a boat ride offshore should also be advised. These cages hold fish (Tilapia) ranging from two to six months old. In this type of environment, strong currents flush throughout the cages throughout the day, and create favourable conditions for the fish.