LAUSR

Extensive transfer of tilapia between lakes throughout East Africa has often led to hybridisation with indigenous fish populations. The endemic Oreochromis hunteri of Lake Chala, an isolated crater lake near Mount Kilimanjaro, is potentially susceptible to introgression from a species formerly identified as Oreochromis korogwe, introduced ~ 30 years ago. We combined whole-body geometric morphometry on 104 specimens of both taxa with molecular phylogenetic analysis of mitochondrial loci from 15 O. hunteri and 9 O. cf. korogwe specimens to assess whether hybridisation has occurred. Using fishes from Lake Jipe and Nyumba ya Mungu reservoir, we expanded our analysis to all four Oreochromis species currently inhabiting the Upper Pangani River system to determine the closest relative of O. hunteri, and hence the possible source population of the ancestral species that colonised Lake Chala. Our results indicate no interbreeding occurs between O. hunteri and O. cf. korogwe, and suggest O. jipe to be the closest living relative of O. hunteri. The introduced O. cf. korogwe is a phenotypically uniform but genetically variable population, the identity of which remains unknown. The high haplotype diversity of O. hunteri is consistent with fossil evidence indicating that its ancestor colonised Lake Chala at least 25,000 years ago.