Histopathological disturbances in two fish species Oreochromis niloticus (Linnaeus 1758) and Labeo barbus pellegrini (Bertin et Estève 1948) from downstream mining sites in the upper Ulindi and Elila river basins, Eastern DR Congo

The effect of intensive mining on the health status of fish living in the upper basins of the Ulindi and Elila rivers
was investigated between June 2018 and March 2022 by comparing the reproductive, hepatic and gill systems
of fish collected from sites upstream and downstream of the mining sites. Biometric indices and organ
histopathology are biomarkers that were studied at two different trophic levels using two fish species
Oreochromis niloticus and Labeobarbus pellegreni. Histological evaluation was performed on the liver, gonads and
gills of both species. Water and sediment samples were analysed for total mercury (T-Hg) concentration by
(atomic absorption spectrophotometry). Histological changes were assessed semi-quantitatively, and the results
were graded according to the severity of the histological responses. The results showed that L. pellegrini
accumulated more T-Hg than O. niloticus. The T-Hg level was higher in the organs of both species during the
rainy season than during the dry season in the following order: Gonads > Liver > Gills and exceeded the
WHO/FAO recommended limit (T-Hg = 1.0 mg/kg wet weight) only in the testes and gills of L. pellegrini
collected at the downstream sites of the Ulindi and Elila rivers. This does not necessarily reflect the level in the
carcass. Taken together, the results suggest that mercury accumulated downstream of gold mining operations
is altering the health of fish populations and that L. pellegrini can be considered a sentinel species for
monitoring this pollution. To protect fish health, it is recommended that certain mining practices such as the
misuse of mercury be avoided throughout the Congo River basin