Chapter 3: Hybrids of Bigheaded Carps Genus: Hypophthalmichthys
Hybridization between closely related species of cyprinids (e.g., species of Hypophthalmichthys) is not unusual (Schwartz 1981). Silver carp Hypophthalmichthys molitrix are known to hybridize and to produce viable offspring with both bighead H. nobilis and largescale silver H. harmandi carps (Chan and Fan 1988; Mia et al. 2002). We found no literature confirming hybridization between largescale silver carp and bighead carps, but it seems probable that they would hybridize. Hybrids of silver and bighead carps are often used in aquaculture because bighead carp produce insufficient milt late in the season (Mia et al. 2002). Both crosses (♀ bighead carp × ♂ silver carp and the reciprocal cross) are diploid and are fertile (Brummett et al. 1988). Hybrids of bighead and silver carps often strongly resemble one or the other of the parent species. The inadvertent use of hybrids or backcrosses as broodstock and the resultant introgression has been identified as a problem in aquaculture in Asia (Kohinoor et al. 2002). In 422 hatchery broodstock identified morphologically as silver carp in Bangladesh, 8.3% had bighead carp allele(s) at one or more of three microsatellite loci (Mia et al. 2005). Mia et al. (2005) suggested that the complex genotypes indicated introgression or generations of hybridization. Scientists should be aware of the potential presence of hybrids when performing research on wild bigheaded carps. In China, hybrids of bighead and silver carps are found in the wild, but they are unusual and usually attributed to escapement from aquaculture (B. Yi, Institute of Hydrobiology, Wuhan, China, personal communication, 2004). In a study of the early development of bighead, black Mylopharyngodon piceus, grass Ctenopharyngodon idella, and silver carps involving the examination of thousands of fishes captured from the Yangtze River, hybrids were never found (Yi et al. 2006). Mair (2003) argues against the use of hybrid bighead carp × silver carp in culture in Asia because they might escape and contaminate wild stocks, causing introgression.
Hybrids between bighead carp and silver carp are common in at least some parts of the United States. These are not likely to be the result of escapement of artificially induced hybrids because neither silver carp nor bighead carp hybrids are in use in aquaculture in the United States. Five percent of the adult bighead carp caught by Chapman in the lower Missouri River in summer 2004 were hybrids. Reciprocal hybrid crosses (♂ bighead carp × ♀ silver carp and the reciprocal cross) were confirmed by genetic analysis. The presence of large numbers of wild-spawned hybrids implies that bighead and silver carps often spawn in the same place at the same time in American waters. Verigin et al. (1979) noted a high degree of hybridization between bighead and silver carps in the Syr’Darya River (Kazakstan, Tajikistan, and Uzbekistan) where the fish are also introduced.