Changes in fish species diversity, size structure and distribution in the trawlable demersal zones of Lake Malawi, Malawi

A study was conducted to assess temporal and spatial changes in the trawlable demersal zones of Lake Malawi. Data from surveys conducted in 1998 and 2020 targeting 120 stations covering a surface area of 9647.97 km was used. Trawling speed of Research Vessel Ndunduma was restricted to 4.6 km/hr. Length frequency distribution was modelled with the probability density function for determining the likelihoods in the gamma distribution. Parameters for modal length and logistic modelling were guessed and Solver in Microsoft Excel 2021 was used to generate the best of fit values through iteration with GRG Nonlinear approach. The study determined fish diversity using the Shannon and Weiner relationship. The recent survey recorded fewer fish species (149) against 158 sampled in the previous survey. The overall catch rates in 2020 and 1998 ranged from 3.8 kg/0.5 h to 2003.8 kg/0.5 h and 28.7 kg/0.5 to 1,884.3 kg/0.5 h, respectively. Overall fish density in the 2020 and 1998 surveys was calculated at 11.7 tons/km and 7.5 tons/km, respectively representing a 35.6% drop. The study has revealed temporal and spatial shifts in the fish stock composition, distribution and abundance which necessitates urgent management interventions to prevent further fisheries resource losses. Efforts to regulate mesh sizes of the cod-ends of trawlers are encouraged just like the initiative of introducing a closed season for the commercial operators.