Leuciscus molitrix Valenciennes, 1844

Silver Carp are unlikely to be confused with native cyprinids due to size and unusual position of the eye. They are most similar to Bighead Carp (H. nobilis) but have a smaller head, and upturned mouth without teeth, a keel that extends forward past pelvic fin base, lack the dark blotches characteristic of Bighead Carp and have sponge-like gill rakers.

Juvenile fish lack spines in fins. Metalarvae and early juvenile are similar to Bighead Carp (Hypophthalmichthys nobilis) but the pectoral fin extends only to base of pelvic fin (as opposed to beyond in the pelvic fin in Bighead).

The species is known for leaping out of the water when startled (e.g., by noises such as a boat motor).

Nonindigenous Occurrences:

Records are available for 12 states. It is apparently established in Louisiana (Douglas et al. 1996) and Illinois; Silver Carp have been reported in Alabama, Arizona, Arkansas, Colorado, Florida, Indiana, Kansas, Kentucky, Missouri, and Tennessee. Douglas et al. (1996) collected more than 1600 larvae of this genus from a backwater outlet of the Black River in Louisiana in 1994. Burr et al. (1996) found young-of-the-year in a ditch near Horseshoe Lake and reported this as the first evidence of successful spawning of Silver Carp in Illinois waters and the United States. They felt that the species would be `established' in the state within the next ten years. Based on the occurrence of juvenile fish in Illinois waters, Pflieger (1997) felt that successful spawning of Silver Carp in Missouri seems inevitable. The current population front of adult bigheaded carp in the Illinois River is at the Dresden Island Pool and spawning may be occurring in the Marseilles and Starved Rock Pools. Established populations, with all life stages detected, exist in the Peoria Pool all the way to the Mississippi River (USACE 2017). 

In the early 1980s commercial fishermen in Arkansas had caught 166 Silver Carp from seven different sites; however, during an intensive 1980-1981 survey to determine the distribution and status of bighead and Silver Carp in state open waters, Arkansas Game and Fish Commission personnel were unsuccessful in procuring any additional specimens (Freeze and Henderson 1982). Although Arkansas state personnel did not find young-of-the-year fish, several specimens taken by the commercial fishermen were sexually mature and exhibited secondary sexual characteristics (Freeze and Henderson 1982). Nevertheless, Robison and Buchanan (1988) reported that there was still no evidence of natural reproduction in Arkansas waters. Rinne (1995) listed Silver Carp as introduced to Arizona in 1972 and denoted it as established. Apparently in reference to the same record, William Silvey of the Arizona Game and Fish Department recently informed us that the only Silver Carp documented in Arizona open waters was a population inhabiting an urban lake in Chandler during the early 1970s. However, further investigation has shown that it was most likely a Bighead x Grass carp hybrid population (P. Marsh, pers.comm.). That population, along with a large population of diploid Grass Carp, was exterminated in 1975 or 1976 by personnel from the Arizona Game and Fish Department and Arizona State University (W. Silvey, personal communication). Pearson and Krumholz (1984) documented records from the Ohio River, but they did not include it as one of the species that exist in well-established, reproducing populations. Etnier and Starnes (1993) provided information on Silver Carp, but by publication they were unaware of any records of the species in the state of Tennessee.

Silver Carp feed on both phytoplankton and zooplankton (Radke and Kahl 2002) but in contrast to the Bighead Carp (Hypophthalmichthys nobilis), which is more effective at filtering larger plankton (zooplankton), the Silver Carp’s dense gill rakers allow it to be more efficient at filtering smaller prey (typically phytoplankton) (Dong and Li 1994). While they are primarily planktivores, Bighead and Silver carp have broad, flexible diets and in some cases have been observed to feed on detritus and biodeposits (Anderson et al. 2016; Boros et al. 2014; Calkins et al. 2012).

Maturation rate of Bighead and Silver carp has been found to be related to water temperature (Kolar et al. 2007; Kolar et al. 2005). In their native range, Silver Carp reach maturity between 4 and 8 years old but are noted in North America to mature as early as 2 years old and can live up to 20 years (Kolar et al. 2007; Williamson and Garvey 2005). In the Wabash River, broad-scale migration of Silver Carp occurs in the Fall prior to overwintering, and in the Spring 1-2 months prior to the onset of spawning (Coulter et al. 2016). Spawning grounds are typically characterized by rapidly flowing (current velocity= 0.6-2.3 m/s), turbid water that ranges from 18-30 C with suspended solids limiting visibility often to 10-15 cm (Kolar et al. 2007). Spawning is often initiated by rising water levels and heavy rains in the spring (Chapman et al. 2013; Erickson et al. 2016; Kolar et al. 2007). However, rising hydrographs are not always necessary for spawning—indicating that these fish exhibit a phenotypic plasticity that may facilitate their successful establishment in novel ecosystems (Coulter et al. 2013; Kocovosky et al. 2012; Deters et al. 2012). Silver Carp fecundity is high; females weighing 6.4-12.1 kg were observed to produce 597,000 - 4,329,600 eggs per fish (Kolar et al. 2007). Williamson and Garvey (2005) observed that fecundity of six Silver Carp in the middle Mississippi River ranged from 57,283 - 328,538 eggs. It was initially believed that successful spawning required 100 km river in order to allow sufficient egg development (Kolar et al. 2007), but this notion has been disproven by several models and field observations. Evidence has shown that water quality parameters—particularly water temperature—and hydrologic factors of a river are major determinants of spawning suitability (Cuddington et al. 2014; Chapman et al. 2013; Murphy and Jackson 2013; Kocovosky et al. 2012). Given the right temperature and flow conditions, river reaches as short as 25 km may allow for the successful development of Invasive Carp eggs (Murphy and Jackson 2013).

Hypophthalmichthys molitrix has a moderate probability of introduction to the Great Lakes (Confidence level: High).

Potential pathway(s) of introduction: Dispersal, unauthorized release, escape from commercial culture
Currently, large populations of this species are already established in nearby waters connected to the Great Lakes basin including the Illinois river and the Chicago Area Waterway System (Baerwaldt et al. 2013). On June 22nd, 2017 a 4 year old male Silver Carp was found nine miles from Lake Michigan in the Little Calumet River of the Chicago Area Waterway System (CAWS). This was the first Silver Carp collected above the electrical barriers in the CAWS. The autopsy revealed that this fish originated in the Illinois/Middle Mississippi watershed and spent a quarter of its life in the Des Plaines River watershed before being caught and removed from the Little Calumet River. It is not known how the fish arrived above the electric barriers, but the autopsy revealed that the fish spent anywhere from a few weeks to a few months in the stretch of river where it was collected (Asian Carp Regional Coordinating Committee 2017). Prior to this record, the closest location to Lake Michigan at which a Silver Carp has been collected was in the Des Plaines River (river mile 290.2) at the confluence with the CSSC, north of Joliet, IL and downstream of the electric barriers (USGS 2013).

Live Silver Carp are sometimes available in live food fish markets in several major U.S. and Canadian cities, including Toronto (Kolar et al. 2005).

Hypophthalmichthys molitrix has a moderate probability of establishment if introduced to the Great Lakes (Confidence level: High).

The Silver Carp has unique, sponge-like and porous gill rakers capable of straining phytoplankton down to 4 microns in diameter (Robison and Buchanan 1988). They can feed in temperatures as low as 2.5°C (36.5°F) and can withstand low levels of oxygen (Pennsylvania Sea Grant 2013). It would be highly likely for the Silver Carp to find an appropriate food source but the amount they eat might not be sufficiently found in the Great Lakes. Recent bioenergetics modelling efforts suggest that plankton concentrations could support Silver Carp growth in productive nearshore areas and embayments (e.g. Green Bay and the Western Basin of Lake Erie), but the fish would likely be food-limited in the oligotrophic offshore regions (Cooke and Hill 2010; Anderson et al. 2015; Anderson et al. 2017).

Kolar et al. (2007) stated that the limiting factor for Invasive Carp establishment in most regions of United States would be access to a river in which Invasive Carp could successfully spawn. Several studies have found that there are several Great Lakes tributaries that have the necessary hydrologic characteristics and temperature regimes to support successful spawning (Kocovosky et al. 2012; Murphy and Jackson 2013; Chapman et al. 2013). Additionally, Cuddington et al. (2014) suggests that establishment would be likely for a small number of founding individuals (<20 fish) despite environmental stochasticity. Furthermore, the presence of only a few suitable spawning rivers on each lake may promote the establishment success given that Silver Carp would have an increased chance of finding a mate due to the fish aggregating in the relatively few nearby spawning rivers. However, establishment becomes less likely if age of first sexual reproduction is substantially delayed.

Summary of species impacts derived from literature review. Click on an icon to find out more...

Environmental	Socioeconomic	Beneficial

One of the behaviors observed in Silver Carp is responsible for significant negative impact on the recreational/tourism sector. Silver Carp regularly jump out of the water, particularly in response to outboard motors. These leaps cause collisions between boaters and fish and have been the source of numerous reports of injuries to human beings and damage to boats and boating equipment. Reported injuries include cuts from fins, black eyes, broken bones, neck and back injuries, and concussions. Silver Carp also causes property damage including broken radios, depth finders, fishing equipment, and antennae (USFWS 2006).  Jumping of Silver Carp (at least 10 feet out of the water) can result in serious injuries to boaters and it is probable that collisions between boaters and jumping Silver Carp will eventually result in human fatalities (Hoff 2004).

Silver Carp is known to harbor several disease-causing agents that pose health risks to humans. These pathogen have been mostly found in carp from different parts of Iran. They include Listeria monocytogenes (found in market and fish farm samples), Clostridium botulinum (found in 1.1% of fresh and smoked samples from the Mazandaran Province), the toxigenic fungi Aspergillus flavus, Alternaria, Penicillium, and Fusarium (found from Silver Carp and from pond water in which they were raised) (USFWS 2006). Furthermore, Silver Carp can be considered a potential carrier for Salmonella (S. typhimumium) (USFWS 2006).

These fish compete with native species that are important as sport and food species and whose decline could result in a negative economic impact on recreational angling and other industries that benefit from sport fishing, such as tourism (Kolar et al. 2005).

Hypophthalmichthys molitrix has a high potential environmental impact in the Great Lakes.

Silver Carp are efficient filter feeders that compete with virtually every fish species that forages on planktonic organisms during their early life history stages (Chick and Pegg 2001).  Interspecific competition for resources is known to cause pronounced and frequent declines in the physical condition of native fish when plankton resources are limited. Ultimately, declines in body condition may decrease potential fitness and the long-term sustainability of native fishes (Irons et al. 2007).  Meso- and microcosm studies (Domaizon and Devaux 1999b, Spataru and Gophen 1985, Starling 1993) provide supporting evidence that high consumption caused by the superior filter efficiency and large size (>35 kg) of Silver Carp, may disproportionately deplete plankton and/or alter the assemblage of zooplankton communities, consequently modifying food web structure (Irons et al. 2007, Pongruktham et al. 2010). Hypophthalmichthys spp. also can alter species composition in phytoplankton communities by promoting the dominance of species that can resist digestion (Görgényi et al. 2016).

Food web models of Lake Ontario and Lake Erie have suggested that Invasive Carp impacts on the Great Lakes ecosystem might be mitigated by several factors such as the availability of unused production that might be exploited by the carp, increased production at lower trophic levels due to high nutrients, and the potential for native piscivores to feed on larval Invasive Carp (Zhang et al. 2016; Currie et al. 2012).

These fish can excrete their own weight in 10 days (Herodek et al. op. cit.; as cited in Starling 1993). This sediment enrichment has an ultimate negative effect on water quality. Studies (Lieberman 1996; Starling 1993) demonstrated that high biomass of Silver Carp causes increases in inorganic nitrogen and phosphorus levels.  Decreases in zooplankton populations resulted in consequent increases in chlorophyll a and turbidity.

The Silver Carp has been known to be a carrier of the Asian tapeworm after the pathogen was found in Silver Carp stocks in the former U.S.S.R. and Philippines (Kolar et al. 2007, as cited in Conover et al. 2007). The Asian tapeworm, a cestode capable of being transferred to other fishes of several different orders, has minimal effects on Silver Carp but can cause severe or even lethal intestinal damage to novel hosts (Kolar et al. 2005). In addition, Kolar and others (2005) point out that this parasite has been found in several species of native North American fishes, including several endangered species.

Hypophthalmichthys molitrix has the potential for high beneficial effects if introduced to the Great Lakes.

Silver Carp are of high commercial importance in many parts of the world. According to Kolar (2005) more Silver Carp are produced than any other species of freshwater fish in the world, especially in China where it continues to grow in importance. In the US, commercial harvest of Silver Carp is increasing in parts of the Mississippi River Basin (Conover et al. 2007). The combined annual commercial harvest of bighead and Silver Carps from the Mississippi and Illinois rivers within Illinois increased from less than 600 kg per year between 1988 and 1992 to in excess of 50,000 kg per year since 1997 (Chick and Pegg 2001).  A consumer market for Invasive Carp species is being investigated in the US and fishing tournaments for Silver Carp are starting to develop in the US.

Silver Carp are frequently subjects of biomanipulation research with the purpose of cleaning wastewaters and eutrophic lakes (e.g., Domaizon and Devaux 1999b, Henderson 1978, Spataru and Gophen 1985, Starling 1993).  These filter feeding fish were utilized in Henderson’s (1978) field tests in order to determine their capabilities in controlling excessive plankton blooms and converting nutrients into usable proteins. Henderson found that the presence of the fish did affect plankton removal and stimulate nutrient uptake. Nonetheless, more recent studies (Domaizon and Devaux 1999, Spataru and Gophen 1985, Starling 1993) had contradictory results. The ability of Silver Carp to control water quality remains unknown.

Silver Carp’s ability as a biological agent for controlling cyanobacteria blooms has been widely debated. Although cyanobacteria produce toxins that can be noxious to animals and humans, Silver Carp possess natural defenses against these microcystins and are known to consume blue-green algae (Xie et al. 2004, as cited in Kolar et al. 2005). Miura (1990) has attributed phytoplankton community shifts from blue-green algae domination towards green algae to grazing by Silver Carp (as cited in Kolar 2005). On the other hand, Kucklentz (1985) found that blue-green algae, as well as total phytoplankton, increased rather than decreased after stocking Silver Carp (as cited in Kolar 2005).

In the United States, Silver Carp (as well as Bighead Carp, Largescale Silver Carp, and Black Carp) are federally listed as injurious species under the Lacey Act (18 U.S.C. 42; 50 CFR 16). Therefore, it is illegal to import or to transport live specimens, including viable eggs or hybrids of the species, across state live, except by permit for zoological, educational, medical, or scientific purposes. Violation of the Lacey Act is a Class B misdemeanor, punishable by no more than six months in jail and/or up to a $5,000 fine for an individual, $10,000 for an organization.

Each Great Lakes state and province have regulations regarding Silver Carp:

It is illegal to import, possess, deposit, release, transport, breed/grow, buy, sell, lease or trade Silver Carp in Ontario (Invasive Species Act 2015). New York prohibits the possession, sale, importation, purchase, transport, or introduction of Silver Carp. Intent to commit any of these actions is also prohibited (6 NYCRR Part 575). In Pennsylvania, it is illegal to transport, sell, offer for sale or release, or introduce Silver Carp (Pennsylvania Consolidated Statutes Title 30 Section 2508). In Michigan, it is illegal to possess, import, sell, or offer to sell Silver Carp (NREPA Part 413). Ohio lists Silver Carp as an injurious aquatic invasive species and therefore it is unlawful for any person to possess, import, or sell live individuals within the state. Dead Silver Carp can only be possessed in Ohio if they are preserved in ethanol or formaldehyde, or eviscerated (internal organs removed) (OH ADM. Code, 1501:31-18-01). Indiana prohibits the importation, possession, propagation, purchase, sale, barter, trade, transfer, loan, or release into public or private waters of live Silver Carp or Silver Carp eggs (312 IAC). Illinois lists Silver Carp as an injurious species as defined by 50 CFR 16.11-15. Therefore, Silver Carp cannot be possessed, propagated, bought, sold, bartered or offered to be bought, sold, bartered, transported, traded, transferred or loaned to any other person or institution unless a permit is first obtained from the Department of Natural Resources. Illinois also prohibits the release of any injurious species, including Silver Carp (17 ILL. ADM. CODE, Chapter 1, Sec. 805). Wisconsin prohibits the transportation, possession, transfer of, and introduction of Silver Carp (Wisconsin Chapter NR 40). Minnesota prohibits the possession, importation, purchase, sale, propagation, transportation, and introduction of Silver Carp (Minnesota Rule 6216.0250).

Use of carp as bait is prohibited in all Great Lake states and Ontario and Quebec; with Michigan and Ontario specifically prohibiting the use of “Asian carps.” (Cudmore et al. 2012).  The Invasive Carp working group recommends development of certification program(s) for baitfish to be disease-free and uncontaminated by nonindigenous species (Conover et al 2007).

Control

The Aquatic Nuisance Species Task Force and the U.S. Fish and Wildlife Service organized an Invasive Carp Working Group (Working Group) to develop a comprehensive national Invasive Carp management and control plan. The Working Group agreed that the desired endpoint of the plan is the extirpation of Invasive Carps (Bighead, Silver, Black and Grass) in the wild, except for non-reproducing Grass Carp within planned locations (Conover et al. 2007).

Monitoring of the Chicago Area Waterways System (including the Chicago Sanitary and Shipping Canal) supporting early detection allowing for rapid response is a key component of efforts to control the spread of Hypophthalmichthys molitrix from the Mississippi River system into the Great Lakes.

Biological

Safe and effective biological control of Silver Carp is not yet feasible.  Several potential technologies are being explored including: release of sterile male fish, triploid carp, transgenic alternatives (daughterless carp and Trojan genes), pheromones (sex lures or juvenile aggregation for traps), disease agents, parasites, predators.
Physical

Many types of physical barriers are being examined for potential to stop the dispersal of Invasive Carp including Hypophthalmichthys molitrix.  These include earth berms, fences, electric barriers, bubble curtains, acoustic barriers, strobe lights and high pressure sodium lights. 

The electrical fish barrier can function either as an impassable barricade or as a fish guidance system. In either case, the system consists of a series of metal electrodes submersed in water to create an electrical field capable of repelling fish. Electrical barriers have been evaluated for preventing the expansion of feral Invasive Carp populations in both the Chicago Sanitary and Ship Canal and the Upper Mississippi River System. While considered feasible for the Chicago Sanitary and Ship Canal, it was determined that electrical barriers would be less effective and less feasible on the Upper Mississippi River System. The U.S. Army Corps of Engineers (USACE) constructed a set of three electrical barriers, the first of which opened in 2002, on the Chicago Sanitary and Shipping Canal to prevent the spread of aquatic invasive species between the Great Lakes and Mississippi River basins.  Although currently in use, electric barriers are not the end-all solution to the range expansion of feral Invasive Carps in the United States. Electric barriers are not selective as to species affected.

The bubble curtain is the most elementary form of behavioral fish barrier, which in its simplest form consists of a perforated tube laid across a river bed through which compressed air is forced. The rising curtain forms a wall of bubbles that can deflect fish.   Efficacy of the bubble curtain may be enhanced when combined with light or sound. Acoustic barriers have shown promise in research trials. Bighead and Silver Carps have acute hearing and are sensitive to frequencies outside the range of many native species. Thus, an acoustic array could be designed such that it primarily affects bighead and Silver Carps and has less effect on non-target species.  The Invasive Carp working group recommends development of redundant barrier systems within the Chicago Sanitary and Ship Canal, including acoustic bubble curtains (Conover et al 2007).

In addition to the Chicago Sanitary and Ship Canal, the Great Lakes Regional Collaboration Aquatic Invasives Species Strategy Team Action Plan (USEPA 2005) identifies the Ohio canals and waterways system as priority interbasin connections that must be addressed to prevent the spread of Invasive Carps into the Great Lakes. Particular attention should be directed to the Ohio and Erie and Miami and Erie canals. The potential for spread of Invasive Carps from the Ohio River Basin into the Great Lakes via these routes is significant (Conover et al 2007).  The Great Lakes and Mississippi River Interbasin Study (USACE 2014) includes site-specific analysis for 18 of these other aquatic pathways for dispersal between the two basins and makes specific recommendations for each.

High pressure sodium lights (1,000 watts) have been used to attract and hold fish to slow water areas located near a powerhouse spillway.   Mercury lights have also been used as attractants for species-specific applications. Attractants may be used in combination to congregate fish that are avoiding other behavioral barriers or deterrents.  The strobe light has been extensively evaluated as a fish deterrent in both laboratory and field situations and has been used in conjunction with other behavioral devices to increase the level of fish diversion. Combinations with bubble curtains may enhance the  effectiveness of both, as the light can be projected onto the bubble sheet. Strobe lights can repel fish by producing an avoidance response.

Increasing the commercial and recreational harvest of Invasive Carps in the Mississippi River basin is an important component of the strategy for preventing the spread of these fishes to the Great Lakes.  Reducing population size, particularly in waterways near the Great Lakes, reduces propagule pressure.  The effects of Invasive Carps on native ecosystems are likely to be proportional to their abundance, thus reducing population size can also be an important component of efforts to minimize impact (Conover et al 2007).

The report in Fuller et al. (1999) from Bay County, Florida was actually a Bighead Carp (UF 98162).

Voucher specimens: Illinois (SIUC 17716, 23043, 23046, 24415; INHS 88425); Louisiana (NLU 65811, 66858, 66859).