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SynopsisOtoliths and scales were used for age and growth determination of Oreochromis andersonii from the Okavango Delta, Botswana. Marginal increment analysis showed that an annulus was formed in both the scales and otoliths during the dry summer period. Using scales, the growth of 0. andersonii was described by L, = 285.27(1-e-"-*6"+*.()~)) mm SL and using otoliths by the equation L, = 267.48(1-e-' *'(1+2.'8)) mm SL. Maximum age estimates of 10 years using scales and 13 years using otoliths were obtained and the growth curves were significantly different (p < 0.01). Age estimation using scales tended to over-emphasise growth in 0. andersonii resulting in larger predicted lengths-at-age. For this reason, otoliths are considered to be more reliable and suitable than scales in determining the age and growth of this species.

Sectioned otoliths were used for age and growth determination of Oreochromis macrochir, a common cichlid species from the Okavango Delta, Botswana . The Okavango Delta is a vast inland wetland ecosystem which receives annual floodwaters from the highlands of southern Angola . Floodwaters reach the northern areas of the delta between January and March and the southern areas between June and September each year. Samples were collected from sites which receive flood waters between May and August . Marginal zone analysis showed that an annulus was formed between October and December during the dry, summer period . Maximum age estimates of eleven years were obtained and growth was described by the 3 parameter von Bertalanffy model as I t = 215 .24(1-e -0 .42 (t +1 .08 )) mm SL . 29

The Pongolapoort Dam in Zululand, South Africa has regulated the flow of the Phongolo River to its floodplain downstream since 1970. Ecological surveys of the floodplain fish communities were conducted before (1974-6), during (September 1983) and after (August 1984) a severe drought to assess the response of the fish stocks. During the three study periods the floodplain lake communities varied. Fish populations were markedly reduced during the drought. The redistribution of fish on the floodplain after the drought was rapid. Colonization of previously dry lakes was mainly by juvenile fish and the mean size of the fish was lower in these lakes than in the refuge lakes. A few species successfully spawned during the drought, although the natural mortality of the young of the year was probably greater due to increased competition and predation in restricted habitats. Large scale spawning in most species occurred immediately after the drought was broken by a large flood.