Hybridisation between native Oreochromis species and introduced Nile tilapia O. niloticus in the Kafue River, Zambia

The Nile tilapia Oreochromis niloticus has been introduced throughout Africa outside its native range for aquaculture purposes. Hybridisation between escaped O. niloticus and native Oreochromis species is of concern due to potential negative effects on wild genetic resources for conservation, aquaculture and capture fisheries. We document the 2008-2010 extent of O. niloticus established in the Kafue River, Zambia, test for hybridisation with two native species, O. andersonii and O. macrochir, using eight microsatellite loci, and evaluate losses in genetic diversity. We genotyped parental O. niloticus (n = 6), O. andersonii (n = 22) and O. macrochir (n = 28) to provide genetic baselines, and compared these to 33 fish from the Kafue River that possessed hybrid phenotypes. Analysis using the program STRUCTURE suggested that most specimens were of hybrid ancestry, representing an admixture of O. niloticus, O. andersonii and O. macrochir. Using the program NEWHYBRIDS, one individual was identified as probably being a recent hybrid. The state of tilapia biodiversity in the Kafue River suggests that non-native tilapia introduction may have negative impacts on the genetic resources available for aquaculture and fisheries. Managers should carefully account for these risks when considering further exotic introductions to regions where non-native tilapia have not yet become established.