The fish isn't native to Lake Kivu at all — it's the Lake Tanganyika sardine, deliberately introduced to Kivu in 1959, where it became the lake's most important catch.
Isambaza are small, silvery freshwater sardines from Lake Kivu, lightly floured and deep-fried whole until crunchy enough to eat bones and all. Fishermen bring them in each morning around Rubavu and Kibuye, and lakeside kiosks pile them into bowls to share at sunset with a cold drink.