Asian carps are classified as either bighead carp Hypophthalmichthys nobilis or silver carp H. molitrix by multiple presumptively diagnostic morphological characteristics; however, hybrids pose a dilemma. Fish sharing the morphological characteristics of both species were observed in an Illinois River backwater (Calhoun County, Illinois) approximately 5 mi (8 km) upriver from the confluence with the Mississippi River as well as in two locations in Pool 26 of the Mississippi River (Madison County, Illinois). Biopsied tissues from individuals exhibiting mixed morphological features were analyzed at four diagnostic allozyme loci (ADH-1*, sMDH-A*, CK-A*, and sSOD-1*) via starch gel electrophoresis. This comparison revealed a high percentage of hybridization (22.5%) from an indiscriminate sample of 120 fish. Moreover, an unexpected percentage (12.5%) of individuals identified in the wild as either parental bighead carp or silver carp by gill raker morphology were genetically identified as hybrids. Finally, two levels of hybridization were detected, first-generation hybrids (F-1) and post-F-1 hybrids, revealing the onset of extensive introgression and the potential for a hybrid swarm. Variation in the amplified COII domain of mitochondrial DNA indicated a strong directional bias of hybrids (88%) containing silver carp maternal lineages. Morphologically, F-1 hybrids were often identifiable (88%) by the presence of twisted gill rakers, but post-F-1 hybrids were difficult to identify with any appreciable certainty. This result creates concern where taxonomic assignment is critical for management or monitoring, warranting a more extensive and intensive examination of this phenomenon in North American waters. Finally, prior observations in aquaculture have shown reduced jumping behavior, fitness, and condition of fish resulting from post-F-1 matings between these species. This is the first confirmed presence of wild post-F-1 individuals of Asian carps in the United States and, although further monitoring is needed, a hybrid swarm may ultimately decrease invasion success as introgression continues.
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