| Summary: | The diet of Oreochromis niloticus from hypereutrophic Lake Chivero and oligo-mesotrophic Lake Kariba was studied in 2017, with focus on the consumption of cyanobacteria. Water and fish were sampled to investigate the relationship between water quality and the diet of 131 fish using microscopy methods. Thirty-six fish were sampled in the second sampling phase to determine the presence of toxin producing cyanobacteria in their stomach contents using PCR methods. Lake Chivero had higher conductivity values, phosphates and total phosphorus levels while temperature and transparency were lower than those recorded in Lake Kariba. Lake Kariba recorded significantly higher water temperatures in June, October and December than Lake Chivero (p<0.05). Cyanobacteria, Microcystis spp. and Cylindrospermopsis sp. dominated in Lake Chivero and Lake Kariba water respectively. Lake Chivero fish were 0.491 times more likely to have empty stomachs than fish from Lake Kariba. Microcystis spp. had the highest relative abundance in fish stomach contents for both lakes and months. Percent relative abundance of Microcystis spp. ranged from 96.51% to 100% for fish from Lake Chivero and 46.74% to 100% for Lake Kariba fish. Stomach fullness index and abundance of cyanobacteria in the stomach contents were influenced by water oxygen percent, dissolved oxygen and temperature, transparency and phosphates for fish from Lake Chivero. For Lake Kariba fish, the stomach fullness index and abundance of cyanobacteria were influenced by total phosphorus, temperature, phosphates, transparency and chlorophyll a. Qualitative molecular analysis of the stomach contents showed the presence of potentially toxin producing cyanobacteria, Microcystis spp., in stomachs of fish from both Lake Kariba and Lake Chivero.
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