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

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

July 29, 2020 Biological Science 0

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

Author (s) Details

Dr. Orton V. Msiska, PhD
COFAD-GOPA-UMODZI Consortium GIZ-Program Sustainable Fisheries and Aquaculture GV Fish Aquaculture Value Chain Project in Malawi, AVCP P.O. Box 833, Mzuzu, Malawi.

Mr. Geoffrey Z. Kanyerere
Fisheries Department, P. O. Box 593, Lilongwe, Malawi

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