A Prognosis of the Chemical Causes of Vulnerability of Lakes Malawi and Malombe Fish Resources

There is evidence that ammonia, hydrogen sulphide and dissolved oxygen are implicated in fish kills occurring on the Lake Malawi shores exemplified by mortalities recorded on Karonga shores. Exposure to fish is influenced by strong winds and water temperature stratification causing upwelling and fish mortalities in surface waters. The vulnerability of fish resources along Karonga lake shore areas to physico-chemical factors was computed from secondary sources data obtained in 2000 to 2016 regarding lethal and sub-lethal limits; this has helped to broaden our understanding of aquatic risk factors in this area. Equilibria relationships of ammonia (NH3 + H2O = NH4 + + OH-; K = 10-4.74; 2.70-4.28 μg.L-1 at depth of 100 to 200 m) and hydrogen sulphide (H2S = HS- + H+; K= 10-7.01; 54.9-1 82.5 mg/L at a depth of 0-150 m) are imputed to have been implicated in the cause of a spate of fish kills, which extended to Nkhata Bay, Nkhotakota, Salima and Mangochi. Uranium fallout from Karonga might also be implicated but further analysis on actual exposure is needed to confirm its effects on fish resources. Inferences regarding in fish vulnerability have been drawn from studies done elsewhere on similar freshwater fish but under controlled conditions. While fish catches temporarily improved in 2007-2008 and 2010-2011, changes in fish diversity have been consistent, such as the disappearance of Ntchila (Labeo mesops), decline of Chambo (Oreochromis species) and the dominance of sardine type Usipa (Engraulicyprus sardella). Therefore, other sources of risks are overfishing and climate change; the latter is evidenced by warming of the deep water columns, reduced dissolved oxygen and declining water levels in both lakes Malawi and Malombe. The significance of fish by-catch recorded as ‘others’ indicates that a high number of fish species remain scientifically undescribed, hence the need for further taxonomic studies. Fish catches as per caput fish supply for Karonga District (19.5-38.0 kg) are above the current national average of 4.0 to 5.8 kg. Since fish provides Karonga communities with affordable animal protein, it significantly contributes to food security for the town. More than 5,500 people depend directly on fishing, representing 10% of the population of Karonga. Fisheries are a driver to rural commercialization for businesses of up to 22,000 people; hence, it is critical to people’s livelihoods and incomes at the local level. Lake Malombe has experienced enrichment from the catchment which also shows that potential risks exist from the same chemical substances.