Abstract

There is evidence that ammonia, hydrogen sulphide and dissolved oxygen are implicated in fish kills occurring on the Lake Malawi shores of Karonga town. Exposure to fish is influenced by strong winds while water temperature differences cause upwelling and fish mortalities in surface waters. The vulnerability of fish in Karonga town 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; 54.9-82.5 mg.L-1 at a depth of 0-150 m) are imputed to have been implicated in the cause of a spate of fish kills.  Uranium fallout might also be implicated but further analysis on actual exposure is needed to confirm this. Inferences of 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 over-fishing and climate change; the latter is evidenced by warming of the deep water columns, reduced dissolved oxygen and declining Lake Malawi water levels. 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 kg) are above the current national average of 4.0 go 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. Therefore, this study confirms what has commonly been observed for Karaonga town; that fish as a nutrition and income source should be factored into studies dealing with risks to livelihoods of Karonga town communities; even where this might require indirect methods of assessment.

Key words: Fish catches, Karonga town, Lake Malawi, fish diversity, per caput fish supply, thermodynamic concentrations (H2S/HS-; NH3/NH4+).