Assessment of multispecies cichlid fisheries of the Southeast Arm of Lake Malawi, Africa

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Abstract
The multispecies cichlid fisheries of southern Lake Malawi have been managed by control of fishing effort, based on Gulland's (1961; Fishery Invest., Lond., Ser. 2, 23: 1–52) modification of the Schaefer (1954; Bull. inter-Am, trop. Tuna Commn, 1: 27–56) stock assessment model, together with continual monitoring and adaptation to changes in the fisheries as they occur. Schaefer yield estimates are updated in this paper. For the demersal trawl fishery, which exploits 160+ species, maximum sustainable yield (MSY) is estimated to be 1355 tonnes/year at an effort level of 620 boat days, and effort is controlled at a level of approximately 500 boat days. MSY for the Chambo (three Oreochromis spp.) ringnet fishery is estimated to be 2190 tonnes/year at an effort level of 11 360 ringnet pulls. Walter's (1986) equilibrium yield model has also been applied to the Chambo fishery and indicates maximum yields of between 1800 and 2500 tonnes at approximately 10000 ringnet pulls. Catch rates in both fisheries are influenced by a number of factors apart from fishing effort. In the demersal trawl fishery the effects of changes in species composition, variability in recruitment, a change in mesh size, and a decline in efficiency of certain fishing units can all be detected in catch and effort data. In the Chambo fishery, catches are related to changes in annual mean lake level three years previously. A hypothesis is proposed here which links wind-induced mixing of nutrient-rich deep water with improved survival of planktivorous juvenile fish. The use of simple stock-assessment models combined with close observation and monitoring is effective in managing fisheries for tropical, short-lived species.

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