Tilapia species are able to hybridize in captivity or in
natural conditions following introduction of alien species. Here, we report
a case of hybridization between two (and perhaps three) autochthonous
species, Tilapia zillii and T. guineensis, resulting from a modification of their environment due to
human activities. The man-made dam Ayamé (Côte d'Ivoire) transformed
the River Bia into a lake (Lake Ayamé, area: 197 km2). In this new
environment, both tilapia species have hybridized. In order to study
hybridization along the Bia System, 25 allozyme loci have been screened in
470 specimens from ten locations. Two loci LDH-1 and PGI-2 were diagnostic between the
two different species and can be used to identify hybrids in each sampling
location of the River Bia basin. The percentage of pure T. zillii individuals in Lake
Ayamé varied from 18.1% in Ebikro to 4.2% in Kétesso while
percentage of pure T. guineensis varied from 81.3% in Yaou to 50.0% in Ebikro. The
proportions of hybrids varied from 8.3% in Yaou to 31.9% in Ebikro.
The existence of private alleles observed in tilapia from Lake Ayamé
compared to what has been observed in both reference populations, T. zillii and T. guineensis led
to the hypothesis that a third species could have been engaged in this
hybridization. This third species is likely to be T. busumana, formerly present in the
lake but now rarely observed. If this natural hybridization is due to the
dam construction and the correlated ecological changes, then other
hybridizations could result since many dams have been constructed within the
range of these species in Côte d'Ivoire.