ZIMSEC O Level Geography Notes: Natural Resources: Fishing: Fishing:Natural lakes
- 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.
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