Morphological differences between the Teleosts Couesius plumbeus (Lake chub) and Rhinichthys cataractae (Longnose dace) and their hybrids from Alberta

Marked external and osteological differences exist between the related cyprinids Couesius plumbeus and Rhinichthys cataractae. C. plumbeus differs from R. cataractae in several external features, such as eye size and mouth shape, any one of which permits identification of individuals. C. plumbeus has postcleithral and nasal bones while R. cataractae does not. Many differences in bone shape also exist. Natural hybrids between the two species from Upper and Lower Kananaskis reservoirs, southwestern Alberta, are intermediate in the average of their external and osteological characters. Some characters in the hybrids, however, are so variable that they fall within the range of both parental species. An individual hybrid may resemble one parental species in one diagnostic character and the other parental species in another. The largest individuals known of the three forms from the Kananaskis system are hybrids.