Rapid growth of a locally-endemic tilapia may enable persistence in an African lake invaded by Nile tilapia

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The introduction of non-native species can lead to competition with native species for key resources, driving the decline and extinction of endemic biodiversity. Recently, a newly discovered and evolutionarily distinct lineage of Korogwe tilapia (Oreochromis korogwe) was reported from small lakes in southern Tanzania. This small-bodied lineage is potentially threatened by introduced Nile tilapia (Oreochromis niloticus), an invasive large-bodied congeneric with a pan-tropical non-native distribution. Nile tilapia is known to dominate ecologically-similar native tilapia in competitive interactions, preventing access to resources such as food and shelter. We therefore hypothesised that competition between Nile tilapia and Korogwe tilapia could limit access to resources by the native species and hence reduce their growth rate, a key determinant of fitness. In this study, tilapia were collected from Lake Rutamba in two field seasons, and individuals were classified using microsatellite DNA genotypes as O. niloticus, O. korogwe or interspecific hybrids. Recent growth rate of these individuals was determined by measuring the distance between scale circuli. In contrast to expectations, we found that native O. korogwe overall had a faster growth rate than the invasive O. niloticus, with hybrids showing growth rates more similar to O. korogwe. We propose that in Lake Rutamba the persistence of O. korogwe could be partially enabled by a faster growth rate than the large-bodied invasive O. niloticus. Based on these results, we suggest that predictions of the effects of invasive species on native biodiversity may benefit from information on relative fitness, in addition to ecological niche overlap.