Reproductive Isolation Between Two Sympatric Dace, Rhinichthys atratulus and R. cataractae, in Manitoba

The Mink and Valley rivers, Manitoba, contain two sympatric species of dace (Cyprinidae: Rhinichthys). These fishes, although interfertile, have never been reported to hybridize in nature.Seasonal isolation is only partially developed, since overlap in time of spawning exists. During spawning, adults of the two species tend to occupy different habitats, the Great Lakes longnose dace Rhinichthys cataractae cataractae being most abundant in water velocities faster than 45 cm/sec and the western blacknose dace R. atratulus meleagris in water velocities less than 45 cm/sec; but some overlap exists. However, the place of spawning of the two species is distinctly different. Nests of longnose dace were found in water velocities exceeding 45 cm/sec with large stones, whereas nests of blacknose dace were found in slower water velocities in gravel. The patterns of spawning behaviour of these two cyprinids reveal marked differences and obvious incompatibilities. Males of both species actively defend territories prior to spawning but show distinct preferences for conspecific females over other females. Observations further indicate that females are receptive only to patterns of courtship behaviour shown by males of their own species.The combination of different places of spawning and patterns of behaviour is considered to be a powerful barrier to hybridization.