State of World Fisheries and Aquaculture 2004 - Library

Highlights of special FAO studies
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this can provide information on the relative stability of the system that can be related
to both stock size fluctuations and overall productivity (Figure 43).
Susceptibility of species to fishing
Underneath the apparent stability in system yields of the SADC freshwaters a
bewildering array of changes can take place (Figure 44). While examples of serious
declines of single stocks can be found, many fluctuate independent of effort. Biological
characteristics play a role here, and some species are particularly “susceptible” to
fishing: for instance large, slow-growing species such as the large, predatory Lates
species in Lake Tanganyika clearly declined as a result of fishing; or species with
particularly vulnerable stages such as the large cyprinid species that are easily caught
during spawning migrations in Lakes Mweru, Malawi, Victoria and Tana. Most species,
however, are remarkably resilient to increased effort and this characteristic is related to
the system variability. The more a species is adapted to pulsed environments, the less
relevant management becomes from a biological perspective. “Resilient” species such
as tilapias have long dominated many African freshwater systems. Recently, however,
shifts towards pelagic fast-growing, short-lived and “highly resilient” species, such as
freshwater clupeids, have taken place in many lakes.
Selectivity and scale of operation of fishing patterns
Small-scale fisheries are able to adapt rapidly to changing circumstances, through
change and diversification of fishing methods. In Lake Mweru, in response to the
disappearance of large-sized Oreochromis mweruensis in the 1970s, the complete
gillnet fishery decreased its mesh size in just a few years. Strong year-classes formed
after years with high flood-pulses, and large-sized O. macrochir reappeared despite the
increased effort. Not being caught by the dominant smaller mesh sizes, they formed