Chemical and physical characteristics of aquatic systems influence the productivity of the lakes. The productivity of a water body determines the carrying capacity of a given aquatic system. The fluctuations in productivity will be reflected in fish population fluctuation as shown in Tables 1 and 2.
Electric conductivity (expressed in μ.mhos) is an index of total ionic concentration which is also correlated to alkalinity. On the basis of conductivity, lakes can be grouped under three types:
This group includes lakes with surface in flows of low alkalinity, e.g. Lakes Victoria and Malawi (= Nyasa). This group includes several lakes with inflows from swamps such as Kyoga, Nabugabo, Bangwelu, Mweru and George as well as the impounded Lake Kariba. In addition, the group includes several small and deep volcanic lakes of south-western Uganda such as Lakes Bunyonyi and Mutanda. Fish production in these lakes is generally high, e.g. Lake Kyoga (389 kg ha-1, Lake George 156 kg ha-1 and Lake Mweru Wantipa 100 kg ha-1). It should be noted that in the more ecologically mature Lakes Malawi and Victoria, fish productivity is less than 50 kg ha-1.
Included in this group are Lakes Turkana (=Rudolf) and Rukwa which are more or less closed basins in the Eastern African Rift Valley. Also placed in this group are Lakes Tanganyika, Kivu, Edward and Mobutu (Albert) in the open basin of the western arm of the Eastern African Rift Valley. Fish production in these Lakes is rather low. In the case of Lake Tanganyika, fish production of the order of 22 kg ha-1 where as the morpho-edaphic index (MEI) of Lake Kivu is 4.5 (see Table 1).
Placed in this group are the small shallow lakes with a depth less than 4 m. These lakes which are associated with bicarbonate salt deposits, include Lakes Magadi, Natron, Manyara, Eyasi and Nakuru. Besides, these lakes are associated with saline springs and are characterized by water level shrinkage. These highly saline lakes have generally an impoverished fish fauna and very low fish production.
The productivity of African Great Lakes is less than that of many African wetlands and shallow lakes. Nevertheless, these Great Lakes contribute a lot of fish to the people living in the constituent drainage basins and provide employment through various industries connected with exploitation of fish. It is noted, for example that several commercial activities exist around Lakes Victoria, Turkana, Malawi and Tanganyika, namely: fishing, netmaking, boatbuilding, engine repair, fish processing, marketing and distribution, as well as shopkeeping.
Comparative fish production magnitudes for some African Great Lakes are given in Table 3. It is useful to note that in many instances estimates of resource potential changes as the exploited system moves from an under-exploited phase to a state of overfishing. Consider the fish yield potential estimates determined for Lakes Tanganyika and Victoria during the 1970's and early 1980's. It appears that the exploited fish stocks of the Great African Lakes are becoming less resilient due to increasing stress caused by greater fishing effort, intra-specific and interspecific competition, climatic changes related to deforestation and lower water-table levels.
