Hydrological and ecological significance of deep pools in the Likangala and Domasi Rivers for post-recession recovery of Lake Chilwa fisheries in Malawi

Abstract

Lake Chilwa in southern Malawi undergoes periodic drying that severely impacts fisheries, aquatic biodiversity, and the livelihoods of millions. Recovery of the lake fishery depends largely on deep pools in influent rivers, which serve as critical fish refugia during lake recession. These pools are increasingly threatened by environmental degradation, unsustainable irrigation, and poor fishing practices. This study examined the role of deep pools in tributary rivers in maintaining fish biodiversity and supporting fisheries recovery during and after lake drying. Deep pools in the Likangala and Domasi rivers were mapped and assessed for productivity and species diversity between December 2018 and 2019. Baseline water quality data from 2015/16, covering dry and wet seasons at 16 sites, were analysed using standard methods, together with Environmental Water Flow assessments based on a desktop reserve model. Water quality varies by river, season, and channel type, with significantly higher concentrations of most parameters during the dry season. Hydrochemical analysis showed bicarbonate dominance, suggesting mineral denudation driven by CO₂-charged runoff as the main controlling process. Clear hydrochemical gradients distinguish deep pools from open channels, confirming deep pools as chemically and biologically distinct habitats. Desktop Reserve Model results indicated that the Domasi River is in better ecological condition, while the Likangala River shows over-commitment of water resources. In both rivers, deep pools maintained fish biodiversity and acted as key refugia and breeding habitats during lake recession through maintenance of water volume at a quality amenable for fish production. However, fish biomass, productivity, and species diversity were significantly higher in Likangala River pools, reflecting their larger size and depth, higher water volumes, and better community-based fisheries management. This shows that favourable ecological conditions alone are insufficient without effective governance. Fish production rose from about 5 000 tonnes before the 2017 recession to a record 42 000 tonnes in 2024. Recovery followed flood-pulse dynamics, enhanced by cyclone-driven nutrient inputs and an extended closed fishing season. Despite this, unregulated water abstraction, catchment degradation, agricultural runoff, weak governance, and invasive species threaten sustainability. Strengthening local management, community awareness, multi-sectoral coordination, and long-term monitoring is essential to sustain fishery resilience under increasing climate variability.