Fishing:Natural lakes

Inland water fishing in Zambia. Image credit mg.co.za

• Fresh water lakes are mainly found in Tanzania, Malawi and Zambia.
• Zimbabwe does not have natural lakes, all are man-made.
• Lake Tanganyika in Tanzania and Lake Malawi are the largest and deepest rift valleys in the region.
• These large lakes provide almost all the inland commercial fish in Southern/central Africa.
• Lake Malawi has about 250 different fish varieties whereas Lake Tanganyika has about 230 species.

• Deep fresh water lakes like Tanganyika and Lake Malawi usually form two layers of water at different temperatures.
• The top layer has warm waters as the sunlight can penetrate causing water to circulate.
• This layer is also rich in oxygen.
• The bottom layer is cold as sunlight cannot reach it.
• Therefor water is stationary and lacks oxygen.
• The bottom layer is rich in nutrients from plants and organisms that die and drop to the bottom of the lake and decay.
• Nutrients released from these decaying organisms feed plants in the warm upper layers where fish flourish.
• The temperatures of the two layers are maintained even in winter.
• Strong winter winds result in the mixing of water from the top to the bottom layers of deep lakes.
• Nutrients from the bottom of the lake rise to the surface, resulting in an increase in plant life and fish food.
• Therefore fish multiply during this period and fishing during this time is difficult.
• Often large quantities of fish die during the winter season, for example in Lake Chivero, in Zimbabwe, Lake Tanganyika and Lake Malawi.
• High levels of pollution and low temperature may be the cause.
• The most plausible reason is that the deoxygenated water from the lake bottom contains poisons formed when bacteria extract oxygen from sulphates.
• These poisons rise to the upper layers of the lake in winter, causing death of fish.

To access more topics go to the O Level Geography Notes page