Abstract

Life history traits, including age, growth, reproduction, recruitment pattern and mortality rates of L. gossei (Burgess & Axelrod, 1973) locally known as Chisawasawa from the south east arm of Lake Malawi were studied between March 2005 and February 2006. A total of 1208 male and female fish ranging from 58 – 146mm SL and 73 – 200mm TL were sampled on monthly basis during the ongoing fishing trials carried out by the Department of Fisheries using a research vessel R.V. Ndunduma. The vessel pulls a gulloppur bottom trawl net with a 23m head rope and a 38mm cod end mesh. A total of 659 sagitta otoliths were used to age fish. Mean monthly gonadosomatic index was computed to determine the peak breeding period. Size at maturity was estimated by fitting a logistic ogive. A recruitment pattern of the fish was constructed using length frequency data. Mortality rates were estimated using the catch curve analysis. Validation of age was done by using a combination of analysis of marginal increment and length frequency distribution.The results of age validation assumed that a single opaque ring was laid down annually in the otoliths. The maximum age for L. gossei was estimated at 6+ years (175 ± 1.14mm mean TL). Growth was best described by the 3-parameter von Bertalanffy growth model as Lt = 150.06(1 – e-0.27(t + 1.94)) r2 = 0.95 SL or Lt = 204.59(1 – e-0.24(t + 2.28)) r2 = 0.96 TL. Growth was fastest in the first and second year and decreased progressively thereafter.Fish bred between November and August with a peak breeding activity from February to March/April and a trough between September and October. The mean size at maturity for females was estimated at 135.95mm TL or 104.05mm SL. Using length-at-age data fromthis study, age-at-50% maturity was estimated at 2.14 years. The condition factors increased with spawning activities and were significantly different among months (p < 0.05). The recruitment pattern of L. gossei appeared to be continuous throughout the year, but showed increased recruitments during the period of May to June with a peak of 22.46% in May. The mean value for total mortality (Z) from the catch curve analysis was estimated at 0.63 year-1, natural mortality (M) was 0.38 year-1, fishing mortality (F) calculated (F = Z – M) was 0.25 year-1 and the exploitation rate was 0.40. The age and growth information showed that this species is relatively long lived and, slow-growing, reaching sexual maturity after two years. The slow growth rate and precocial breeding habits imply that L. gossei populations are relatively stable and could remain the same in the future if effort is kept constant. However, once the fishery is over fished, it would require a long period for the stock to rebuild to levels that could support sustainable catches. A management strategy for the south east arm of Lake Malawi, therefore, has to place emphasis on the maintenance of the spawning stock of the species.

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School	: Bunda College of Agriculture
Issued Date	: 2006

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