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Figure 30. Silver Carp, SIUC 35334, 204 millimeter SL, from New Madrid County, Missouri.
The Silver Carp (fig. 30) is a large fish; individuals have been reported to reach about 1.3 m (Xie Ping, 2003) and weigh over 35 kg (Li and Mathias, 1994). The species is deep bodied and laterally compressed, with a ventral keel that extends forward from the anus almost to the junction of the gill membranes (fig. 31). The lateral line is complete and curved ventrally with 91-124 small scales. The eyes are large and located low on the head. The mouth is large, terminal, and somewhat oblique. The first gill arch has more than 100 gill rakers. The rakers are thin, branched, and fused into a sponge-like apparatus. The pharyngeal teeth are in one row (0,4-4,0) with striated surfaces (fig. 32). Barbels are absent. The dorsal fin is short with ii-iii (7) rays and lacks a thick spine-like ray. The anal fin has ii-iii (11-14) rays. The dorsal surface of the body is olivaceous to grayish black and the sides are silvery (figs. 33 and 34). Meristics for Silver Carp are given in appendix B.
Figure 31. Ventral keel of Silver Carp.
Figure 32. Pharyngeal teeth (0,4-4,0) of Silver
Silver Carp fingerlings exhibit an unusual morphological adaptation to low oxygen conditions. Adamek and Groch (1993) report that hypertrophy of the lower lip of Silver Carp fingerlings can appear within 24-48 hours of oxygen depletion. This rapid morphological adaptation serves to increase intake of the surface-water layer, and disappears within 24 hours after the return of normoxia. Sexual dimorphism in Silver Carp is subtle. Both sexes develop fine tubercles along anterior pectoral fin rays; however, in females these tubercles are only present on the distal half of the rays (Opuszynski and Shireman, 1995).
Presence of a ventral keel differentiates the Silver Carp and Bighead Carp from all native cyprinids except the Golden Shiner (Notemigonus crysoleucas). The Golden Shiner has larger and fewer lateral-line scales (39-51) than Silver Carp and Bighead Carp (>60). Additionally, the Golden Shiner typically has five pharyngeal teeth per side (in a single row), whereas Silver Carp and Bighead Carp have only four teeth in a single row (per side). Small juvenile Silver Carp may resemble native shad (family Clupeidae), but can be differentiated by the presence of a lateral line and usually <14 anal rays (versus no lateral line and >16 anal rays in shad).
Figure 34. Adult Silver Carp, 438 millimeter SL, from the U.S. Fish and Wildlife Service
Silver Carp is most similar to Bighead Carp; however, it has silvery sides in contrast to the mottled sides of the adult Bighead Carp. Additionally, the Silver Carp has long, thin gill rakers that are fused to form a sponge-like apparatus (fig. 7a in Key) and a ventral keel that extends from the anus to the anterior part of the breast, almost to the junction of the gill membranes (fig. 31). The Bighead Carp has long, thin gill rakers that are not fused and has a keel that extends forward only to the base of the pelvic fins. When apressed (held flat against the body), the pectoral fin of the Silver Carp does not extend to the base of the pelvic fins, whereas the pectoral fin of the Bighead Carp extends beyond the pelvic-fin origin. Typically, the eyes of the Bighead Carp are slightly lower on the head than those of the Silver Carp; however, this characteristic may not be diagnostic. The name “Bigheaded Carps” is often applied collectively to include all the Hypophthalmichthys species. For example, in some publications the Hypophthalmichthys molitrix is referred to as “Bighead Carp.”
A closely related species, the Largescale Silver Carp (Hypophthalmichthys harmandi; Sauvage 1884) occurs in Hainan Island, China, and northern Vietnam (PRFRI, 1991; Chen, 1998). The Largescale Silver Carp was previously treated as a subspecies of Silver Carp (H. molitrix harmandi; Yen, 1985), but it is now considered a distinct species. Largescale Silver Carp has fewer lateral-line scales (78-88) than Silver Carp and more anal rays (15). To date, we are unaware of the occurrence of Largescale Silver Carp in the U.S.
Silver Carp has been artificially hybridized with Common Carp (Makeyeva, 1968, 1975; Makeyeva and Verigin, 1974b) and Grass Carp (Andriasheva, 1968). Silver Carp is also reported to hybridize with Bighead Carp both naturally (Verigin and others, 1979) and through artificial means (Tang, 1965; Green and Smitherman, 1984). Larval development of Silver Carp X Bighead Carp hybrids was described by Mihai-Bardan (1980). Silver Carp is not known to hybridize with any native North American cyprinid species. The common name “Bighead” is used in this report to refer to Hypophthalmichthys molitrix.
Male Silver Carp attain maturity at 2-3 years in subtropical/tropical locales and 4-6 years in temperate regions (Alikunhi and Sukumaran, 1964; Kuronuma, 1968; Bardach and others, 1972; Abdusamadov, 1987). Males generally mature a year before females (Abdusamadov, 1987; Opuszynski and Shireman, 1995). Opuszynski and Shireman (1995) reviewed gonad condition through the stages of development. Fecundity is generally high, and differs among geographic regions and fish ages and sizes. Larger Silver Carp tend to have more eggs and heavier ovary masses (Verigin and others, 1990). Fecundity ranges from about 265,000-2,000,000 eggs per fish (Abdusamadov, 1987; Vinogradov and others, 1966; Kamilov and Komrakova, 1999). Egg and larval development were illustrated in Nakamura (1969) and Soin and Sukhanova (1972).
Krykhtin and Gorbach (1981) studied the downstream drift of Silver Carp eggs in the Amur River Basin of eastern Asia, and determined that spawning occurred at water temperatures between 17-26.5 °C, with peak activity at 21-26 °C. For additional information on spawning requirements of Silver Carp, see the section entitled “Spawning requirements of Chinese carps” in the Grass Carp account.
The Silver Carp is a large fish than can attain a maximum length of over 1 meter and a weight of over 35 kg (Li and Mathias, 1994; Xie Ping, 2003). It is a relatively long-lived species, estimated to attain an age of 20 years (Berg, 1964). In its native range, Silver Carp occurs in large rivers, although the species has been stocked in a variety of habitats, including warm-water lakes, fish ponds, and impoundments (Berg, 1964; Abdusmadov, 1987; Fuller and others, 1999; Xie Ping, 2003). The Silver Carp is an active fish that typically swims in the upper water layer. If disturbed, individuals will sometimes leap clear of the water. The sound of an outboard motor often causes the Silver Carp to leap out of the water and collide with boaters, causing serious property damage and human injury. This behavior has received considerable attention in the U.S. (Perea, 2002).
Oxygen consumption (per gram of body mass) has been shown to increase with higher water temperature and decrease with fish age and mass (Chen and Shih, 1955; Wozniewski and Opuszynski, 1988). In about 21 days, larvae are tolerant of oxygen conditions as low as 0.5 mg/L (Wozniewski and Opuszynski, 1988). Rai (2000) reported growth at pH 7.1-9.7. Liang and Wang (1993) reported the maximum pH for culture was 9.24. Egg hatching was delayed below pH 6.5, and increased mortality and deformation of larvae occurred below pH 6.0 (Li and Zhang, 1992). However, sensitivity to low pH decreased with age (Li and Zhang, 1992). Vovk (1979) reported a temperature tolerance of 0-40 °C. Opuszynski and others (1989) reported an upper lethal limit of 43.5 or 46.5 °C (depending on the method of calculation) and an optimum growth temperature of 39 °C for juveniles. Data on salinity tolerance is difficult to interpret. Authors have reported that Silver Carp cannot tolerate salinities >4 ppt (Waller, 1985; Zang and others, 1989), although larvae are known to migrate to brackish-water areas of the Caspian Sea where salinities range from 6-12 ppt (Abdusamadov, 1987). Both oxygen consumption and standard metabolism decrease at 3-4 ppt salinity, as less energy is needed to maintain internal equilibrium (von Oertzen, 1985).
The fused gill rakers are used to sieve plankton from the water column. Once ingested, food is ground against a cartilaginous plate by blunt pharyngeal teeth (Robison and Buchanan, 1988). Larvae feed primarily on zooplankton (Korniyenko, 1971; Nikol'skiy and Aliyev, 1974). Adults feed primarily on phytoplankton and to a lesser extent on zooplankton and detritus (Berg, 1964; Nikol'skiy and Aliyev, 1974; Cremer and Smitherman, 1980; Burke and others, 1986; Dong and Li, 1994). A primary goal of Silver Carp culture is the improvement of water quality by reduction of phytoplankton populations. However, the efficacy of this species for controlling phytoplankton is controversial (Domaizon and Devaux, 1999). Some researchers have reported that the introduction of Silver Carp has resulted in increased water quality (Kajak and others, 1975, in Lazzarro, 1987; Leventer, 1979) and, specifically, reductions in blue-green algae (Vovk, 1979; Starling, 1993). Conversely, other researchers have reported that the presence of Silver Carp increased algal biomass (Burke and others, 1986; Laws and Weisburd, 1990).
The Silver Carp is native to large lowland rivers of eastern China (Berg, 1964; Li and Fang, 1990).
The Silver Carp was first introduced to the U.S. in 1973 for the purpose of controlling plankton blooms in catfish-production ponds and sewage lagoons (Freeze and Henderson, 1982; Fuller and others, 1999). The species was subsequently raised in government and private aquaculture facilities. By 1980, specimens were found in natural waters, probably the result of escapes from aquaculture facilities (Freeze and Henderson, 1982). Subsequent escapes and contamination of Grass Carp stocking with Silver Carp may have contributed to the expansion of the species’ range (reviewed in Fuller and others, 1999). Silver Carp is now reported in 16 states and is established in the middle and lower Mississippi River Basin.
Plate 6. Distribution of Silver Carp in the United States. See Methods for details regarding data used
One of the earliest U.S. records of Silver Carp from the wild, this fish was
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