Modelling the current status of Lake Malawi fish stocks, an inland lake in East Africa

Abstract

Inland rift valley lake systems have been sustaining humanity for a long time. Only recently have commercial fisheries entered these systems and for Lake Malawi, this occurred in the mid-1970s upon successful experimental trawl fishing. Lake Malawi with the highest diversity of freshwater fishes in the world has had fisheries for centuries. Previous assessment of the impact of commercial trawl fisheries revealed significant changes in stock composition and overall biomass. The study evaluates the state of fisheries resources using novel techniques developed using integrated assessment methods amenable to data poor fisheries. Data from biomass assessment surveys and landed catch from 2016 to 2019 were examined to determine the status of selected fish stocks over time. Stock Synthesis for Data Limited tools in R environment were used to run the models. The Surplus Production Models and Statistical catch-at-age models that were used to examine alternative hypotheses on life history parameters on the key stocks exploited primarily by the commercial trawl fishery and evaluate long-term trends on these populations. The study results revealed that Mcheni (Rhamphochromis spp.), Ndunduma (Diplotaxodon limnothrissa) and Utaka (Copadichromis virginalis) stocks are within the sustainable limits, while Chambo (Oreochromis karongae), Chisawasawa (Lethrinops gossei), Mlamba (Bathyclarias nyasensis) and Kampango (Bagrus meridionalis) appear to be overfished in recent years, though Kampango may have recovered recently. Given, the large uncertainties with productivity of most tropical fishes with climate change, as well as large uncertainties due to inaccurate and untimely data submissions, it is recommended that a systematic monitoring and evaluation program, like the one being conducted in Lake Malawi should be developed for other inland lake systems. If stocks are facing overfishing by both the small-scale and commercial trawl fisheries, limits to overall catch and size should be implemented possibly through output controls such that the fisheries remain sustainable for the long term.