INSHORE FISHERIES AND FISH POPULATION CHANGES IN LAKE KARIBA 1 J. Kolding, B. Musando and N. Songore 1. INTRODUCTION Since the creation in 1958 of Lake Kariba, situated on the Zambezi River and shared between Zambia and Zimbabwe, substantial changes in both its fisheries and in the fish communities have been observed. Although probably one of the best studied fresh water systems in Africa (Karenge and Kolding 1995a), the sustainable exploitation levels of its fish communities are still largely unknown (Anon. 1992, Kolding 1994). Fear of overfishing, or at least indications of fully exploited resources, has repeatedly been expressed (Marshall 1981, Marshall et al. 1962, Kenmuir 1982, Marshall 1985, Machena and Mabaye 1987, Marshall and Langermann 1988, Moyo 1990, Scholz 1993, Sanyanga et al. 1995, Sanyanga 1995, Machena and Kwaramba 1997), whereas other studies have contested these views (NORAD 1985, Ramberg et al. 1987, Marshall 1992, Machena et al. 1993, Kolding 1994, Karenge and Kolding 1995a). Most of the attempts to calculate sustainable yields (Marshall et al. 1982, Marshall 1985, Moyo 1986, Moyo 1990) are from classic fisheries stock assessment models based on catch and effort variables, with the underlying basic assumption of ecological stability and constant regenerative capacity. Most of the results from these analyses have proven of dubious value (see Box 5.2 in Volume I). Lake Kariba is not a stable system as most other small or medium sized lakes in Africa. Karenge and Kolding (1995b) showed that the environment, in terms of the changing hydrological regime, explains a large proportion of the variability in catch rates (CPUE). They concluded that Lake Kariba was an allothropic riverine lake where productivity was largely driven by the nutrient pulses carried by the annual floods. The question is therefore how much of the observed changes can be attributed to fishing activities and how much is due to natural environmental fluctuations. Another important management issue, particularly on the Zambian side, is the high fishing pressure and changing fishing pattern in terms of increased use of small mesh sizes and customary use of illegal fishing methods such as drive fishing (Kutumpula). It is widely believed that such uncontrolled development is a potential sign of overfishing and poses a threat to the biodiversity (FAO 1992, Lowe-McConnell 1994, Pitcher 1995). On the other hand, in highly variable systems, susceptibility to increased fishing effort is thought to be low, while resilience is high and recovery potential is rapid. Lake Kariba is a man-made grand-scale ecological laboratory with a unique chance to observe and monitor the intricate pathways of natural succession under exploitation, and a relatively good catch and effort monitoring scheme has been in place since its creation. In addition, the inshore fisheries of Zambia and Zimbabwe have evolved differently and have been subject to different types of management regimes (see Bourdillon et al 1985, Malasha 2003, Jul-Larsen 2003, and Overå 2003 for a detailed historical account and analysis). Since the overall fishing effort, catch rates and fishing patterns on the two sides of the lake are very different due to different management and enforcement, a comparative study may shed some light on the impact that these different fisheries have on each side of the same ecosystem. After a brief description of the physical and biological environment, and the different management regulations in place, we will use the long-term time series of commercial and experimental catch rates available to describe and discuss the observed development and changes on both sides of the lake. From this comparison we will evaluate the impact of fishing on the fish stocks relative to their natural succession and fluctuations in the environment. 1 pp 67-99 In: Jul-Larsen, E., Kolding, J., Nielsen, J.R., Overa, R. and van Zwieten, P.A.M. (eds.) 2003. Management, co-management or no management? Major dilemmas in southern African freshwater fisheries. Part 2: Case studies. FAO Fisheries Technical Paper 426/2. FAO, Rome.