Abstract
Growth curves of Oreochromis shiranus in two small reservoirs, Chisombezi (2·2 ha) and Mbvoniha (3·6 ha), were constructed by fitting a seasonally oscillating von Bertalanffy growth function to tag‐recapture data. Annual average numbers were calculated for each length class from gillnet cpue data corrected for selectivity. The lengths at first maturity were estimated from samples, and annual brood numbers were deduced from the linear relation between female length and brood number. Annual instantaneous mortality rates were estimated by linearized length‐converted catch curves for lengths >90 mm, and for lengths between 10 and 90 mm by inference from the theoretical annual brood numbers. Estimated mean biomass was 84 and 106 kg ha −1 , while production was 417 and 872 kg ha −1 year −1 in Chisombezi and Mbvoniha, respectively. Thus, the production‐biomass ratios were high at 5.0 in Chisombezi and 8.2 in Mbvoniha.