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Foreign Nonindigenous Carps and Minnows (Cyprinidae) in the United States - A Guide to their Identification, Distribution, and Biology

By Pamela J. Schofield, James D. Williams, Leo G. Nico, Pam Fuller, and Matthew R. Thomas



       The family Cyprinidae (carps and minnows) is naturally distributed throughout most of the world except Australia and South America (plate 1). It is the largest family of freshwater fishes with about 2,010 species in 210 genera. Most cyprinids (about 155 genera and 1,060 species) are found in Eurasia (Nelson, 1994). About 300 species in 50 genera are native to North America (Canada, Mexico, United States; Nelson and others, 2004). The high diversity and expansive distribution of cyprinids have contributed to a long history of human use (of both cultured and wild-caught stocks) as food, bait, sport, forage, and ornamentals. A few species are used to control nuisance plants or other problem organisms (for example, disease-carrying snails).

       Cyprinids exhibit considerable variation in morphology, diet, and habitat use. The largest member of the family is the Giant Barb (Catlocarpio siamensis) of southeastern Asia, which may reach almost 3 m total length (TL) (Smith, 1945). In contrast, the miniature translucent species Danionella translucida from Myanmar (formerly Burma) attains only 12 mm standard length (SL), and is the smallest freshwater fish known (Roberts, 1986; Britz, 2003). Trophic diversity is also great, and the family includes planktivores (Bighead Carp, Hypophthalmichthys nobilis, and Silver Carp, Hypophthalmichthys molitrix), herbivores (Grass Carp, Ctenopharyngodon idella), piscivores (Colorado Pikeminnow, Ptychocheilus lucius), and omnivores (Common Carp, Cyprinus carpio). Many species have specialized diets, which are reflected in particular morphological adaptations (for example, the Black  Carp, Mylopharyngodon piceus, has pharyngeal teeth modified for crushing mollusks). Cyprinids live in a wide variety of habitats from stagnant ponds and small streams to large lakes and swift-flowing rivers.

       This guide was designed to benefit biologists and others interested in nonindigenous fishes by providing information for the identification of foreign cyprinids introduced into the U.S. In addition to providing identification keys and descriptions of distinguishing characteristics, we give details concerning distribution and environmental biology. This guide is a valuable resource for several reasons:

       (1) The number and variety of foreign fishes introduced into the U.S. has increased dramatically in the past few decades;

       (2) Several introduced cyprinids have caused, or have the potential to cause, environmental or economic harm; and

       (3) Some foreign cyprinids superficially resemble native cyprinids or other (native or introduced) fishes.  Consequently, introduced cyprinids may be misidentified or may not even be recognized as non-natives.

       Correct identification is critical in documenting the occurrence and dispersal of foreign species and in rapidly responding to the appearance of new foreign species. As learned during recent decades, a swift response is critical to eliminate and/or control harmful non-native fishes before they become widespread and abundant.

Plate 1. Native distribution of the Family Cyprinidae (in dark blue).

Plate 1. Native distribution of the Family Cyprinidae (in dark blue).

Foreign Cyprinids Treated in this Guide

       Eleven foreign nonindigenous cyprinids are included in this guide. Eight species have established breeding populations in the U.S. (Goldfish, Grass Carp, Common Carp, Silver Carp, Bighead Carp, Bitterling, Rudd, and Tench). Three species are not known with certainty to have reproducing populations (Black Carp, Crucian Carp, Ide). There is mounting evidence suggesting that Black Carp is either already established in the wild or that it will become established in the near future (Nico and others, 2005). The Crucian Carp was reported to be thriving in Chicago, Illinois, in the early 1900s (Meek and Hildebrand, 1910); however, it is no longer believed to exist there (Smith 1979). Although there are currently no documented reproducing populations in the U.S., there is a possibility that Crucian Carp is established but has been misidentified or confused with the morphologically similar Goldfish. Also, recent interest in culturing Crucian Carp for the live food fish trade raises the possibility of future introductions. Ide was included based on records of multiple introductions and one established population in Connecticut that was eradicated (Fuller and others, 1999). However, Ide may still be present, but undetected, in other drainages.

       Although we include detailed information and accounts for a few foreign cyprinid species not yet established in the U.S. (for example, Crucian Carp), this guide does not provide information for the identification of several other foreign cyprinids that have been found or reported in the U.S. These excluded species include ones that either never became established, or those that only temporarily formed reproducing populations and then subsequently disappeared. The Zebra Danio (Brachydanio rerio) is one example; it was reported to have a single reproducing population in a spring system in New Mexico (Sublette and others, 1990; Fuller and others, 1999), but is no longer extant (Nelson and others, 2004). The Zebra Danio was also reported in the open waters of California, Connecticut, and Florida (Fuller and others, 1999), as the result of escapes from fish farms or aquarium releases; however, no evidence of reproduction has been reported. Two ornamental species from Asia, the Blackspot Barb (Puntius filamentosus) and the Green Barb (Puntius semifasciolatus), had reproducing populations in Nu’uanu Reservoir No. 2 in Oahu, Hawaii. However, neither species has been seen since the drought of 1984 when the reservoir almost dried (Yamamoto and Tagawa 2000). A few other foreign cyprinids have been reported in open waters of the U.S., but are not known to maintain reproducing populations appendix A).

What is a Cyprinid?

       Cyprinids are not always easy to distinguish from other fishes. Basic cyprinid characteristics include no teeth on the jaws, a single dorsal fin, pelvic fins in an abdominal position, pectoral fins low on the side, and no adipose fin. The first full-length ray of the dorsal and anal fins is hardened into a spine-like structure in some foreign cyprinids (for example, the Common Carp, Crucian Carp, and Goldfish). Native North American cyprinids lack a spine-like ray, except certain genera endemic to the desert southwest (such as the Plagopterus, Meda, and Lepidomeda). Cyprinids have 19 principal caudal-fin rays (17 branched, 2 unbranched). The scales are cycloid and the lateral-line system is typically well developed. The head typically has no scales. Cyprinids do not have jaw teeth; instead, they use a pharyngeal apparatus to process food. This apparatus consists of a modified thick fifth pharyngeal arch that bears teeth and a chewing (masticatory) pad that is located on roof of the pharynx. The size and shape of the arch and teeth are closely tied to the diet of the species. The lips usually are thin, but in some cyprinids they are enlarged and sucker-like or even lobed. Most cyprinids lack barbels on the lips but they are present in a few genera, including a few native to North America. A swim (gas) bladder is always present and usually two chambered. Cyprinids and several related groups have a Weberian apparatus comprised of modified anterior vertebrae that connect the swim bladder to the inner ear. The Weberian apparatus transmits sound vibrations to auditory receptors in the brain and is thought to give cyprinids a keen sense of hearing. Most cyprinids have a typical minnow-shaped body form, but some are elongated, some compressed, and others robust. Many cyprinids are sexually dimorphic. For example, males of many species become brightly colored during the breeding season, and may temporarily develop nuptial (breeding) tubercles. Females may also develop breeding tubercles, but these are generally less well developed than those of the males. Proliferation and distribution of tubercles on the body varies between sexes and among species, often appearing on parts of the head, body, and fins (Etnier and Starnes, 1993; Jenkins and Burkhead, 1994; Nico and others, 2005).

Introduced Versus Native Cyprinids

       Because the family is so diverse, there is no single set of characteristics that distinguishes all introduced cyprinids from native North American cyprinids. Nevertheless, a few introduced cyprinids are distinctive and can easily be identified as non-natives. Among these are Common Carp, Goldfish, and Crucian Carp, which have a stout, spine-like ray in the front part of their dorsal and anal fins. Only a few North American species native to the desert southwest possess similar spine-like rays. In addition, some foreign cyprinids have relatively long dorsal fins with 13 or more branched rays. In contrast, almost all native cyprinids have short dorsal fins, with 11 or fewer (usually 7-9) branched dorsal-fin rays. Moreover, most foreign cyprinids are medium-or large-sized fishes, reaching over a meter long. In contrast, the majority of native cyprinids are fairly small and few species exceed 15 cm TL. One notable exception is the native (and critically endangered) Colorado Pikeminnow, a species historically known to reach almost 2 m long and weigh 40 kg (Minckley 1973).

Other Fishes

       Only a few other non-cyprinid fishes native to North America are superficially similar to foreign cyprinids. Some members of the sucker family (Catostomidae), such as buffalos (Ictiobus) and carpsuckers (Carpiodes), resemble the Common Carp, Goldfish, and Crucian Carp. Suckers share some cyprinid characteristics, including pharyngeal teeth, cycloid scales, and nuptial tubercles on breeding males. Suckers also lack jaw teeth and adipose fins. However, suckers can be distinguished from cyprinids by the number of principle caudal-fin rays (18 in suckers, 19 in cyprinids). Barbels are present in some cyprinids, but are always absent in suckers. Additionally, suckers lack spine-like dorsal-and analfin rays that are characteristics of the Common Carp, Goldfish, and Crucian Carp. The pharyngeal teeth of suckers are typically long and fine, are arranged in a single comb-like row along the arch, and number 10 or more per side. In cyprinids, the pharyngeal teeth are often (but not always) stout or molar-like, and number less than 10 per side. The mouth of most suckers opens ventrally, and the lips are generally thick and fleshy (Jenkins and Burkhead, 1994).

Reporting Discovery of Nonindigenous Species

       The collection of any nonindigenous cyprinid should be reported to state game and fish agencies and to the Nonindigenous Aquatic Species database (http://nas.er.usgs.gov; appendix C). For those wishing to report a new record, be aware that positive identification is of the utmost importance. Consequently, we urge that the specimen be retained if possible or at least well documented with photographs. It is especially helpful for photographic images to be well-focused and to include shots of the entire fish from various angles as well as close-up views of the head and fins.



       Scientific names and authorities were taken from “A Catalog of the Species of Fishes” (Eschmeyer and others, 1998, and amendments) available through the California Academy of Science (online at: http://www.calacademy.org/research/ichthyology/catalog/fishcatsearch.html). Common names of fishes follow Nelson and others (2004).

Meristic and Morphometric Measures

       Methods of recording meristic (count) and morphometric (measurement) data were standardized by Hubbs and Lagler (1958) and reviewed in Strauss and Bond (1990). Additionally, several publications that provide information on fishes of particular regions or states list useful details regarding identification, including standard formats for expressing meristics and morphometrics (for example, Page and Burr, 1991; Etnier and Starnes, 1993; Jenkins and Burkhead, 1994; Ross, 2001; Boschung and Mayden, 2004). Meristic characteristics are variable within and among the cyprinids treated in this guide. Thus, meristic data from a number of sources are included for each species, including data from both native and introduced populations when available (appendix B). These data were reproduced as given by the original authors. Although most of the original reports follow the standards of Hubbs and Lagler (1958) and Strauss and Bond (1990), some did not (especially those in foreign publications) and, therefore, inconsistencies may exist. For detailed information on methods of data reporting, the cited authority should be consulted.

       Fin-ray counts, comprised of unbranched and branched rays, are often useful in the diagnosis of fish species. When reporting fin-ray counts, the first set of Roman numbers represents the number of soft unbranched rays. The second set (in parentheses) represents the number of soft branched rays. For example, a fin with three unbranched rays and eight branched rays would be represented as: iii (8). The spine-like ray present in the dorsal and anal fins of some foreign cyprinids (for example, Common Carp, Goldfish, Crucian Carp) and a few native fishes is counted as an unbranched ray. Because the unbranched rays of the dorsal and anal fins are often difficult to discern, most North American fish books report only a single number representing a single unbranched ray plus all the branched rays. The last two branched rays usually share a common basal skeletal element and are counted as one.

       Counts of gill rakers are based on the first (outer) gill arch and normally performed using the outer arch on the left side of fish. Gill rakers are often aligned in two rows on each gill arch, an outer row of longer rakers and an inner row of short stubs. Only rakers on the outer row are counted, including the smallest in the outer-row series at top and bottom of the arch (Calliet and others, 1986). Some authors give either upper and lower limb counts, or inner and outer (anterior and posterior) counts, and those data are listed in this guide.

       Length of fishes is conventionally given in either standard (SL) or total length (TL). Standard length is the straight-line distance from the tip of the snout to the posterior end of the vertebral column. The end of the vertebral column can be located by laterally flexing the caudal fin at its base. The crease in the flesh at the base of the caudal fin marks the end of the vertebral column (hypural plate) and the beginning of the caudal fin. Total length is the straight-line distance from the tip of the snout to the posterior tip of the longest caudal-fin rays. North Americans typically measure total length (TL) with the caudal fins compressed dorso-ventrally, resulting in a measure slightly longer than if the caudal fins are left in their natural position (Anderson and Neumann, 1996). Unfortunately, many authors do not report the method used to determine total length.

Pharyngeal Teeth

       Cyprinids do not have jaw teeth. Instead, they rely on their pharyngeal teeth and masticatory pads to crush or process their food. The number, size, and shape of the pharyngeal teeth are generally species specific. In cyprinids, the fifth branchial arches are located on the floor of the posterior pharynx, anterior to its junction with the esophagus. To clearly view the pharyngeal teeth, it is often necessary to extract the pharyngeal arch. This can be accomplished by removing the operculum, gills, and other surrounding tissues. Alternately, the gills and operculum can be folded forward to expose the pharyngeal teeth. Much care must be exercised to extract an arch without damaging the teeth and resulting in an incorrect tooth count. In this guide, dorsal-view illustrations of pharyngeal teeth are given in each species account. The number of pharyngeal teeth is represented by a standardized formula. For example, a count of 0,4-4,0 denotes one inner row of four teeth on each arch. Alternately, a count of 1,1,3-3,1,1 denotes three rows on each side, with three teeth in the innermost row and two outer rows with one tooth each. By convention, pharyngeal teeth are read from the outside of the left to the outside of the right. Some species show dramatic changes with growth, and young individuals sometimes have pharyngeal teeth quite different from adults. The pharyngeal teeth illustrated in this guide are those of adult fish.

Museum Specimens

       Preserved specimens referenced in this study are from the following sources: AUM (Auburn University Museum Fish Collection, Auburn, Alabama); SIUC (Southern Illinois University at Carbondale, Fluid Vertebrate Collection, Carbondale, Illinois); TU (Tulane University Museum of Natural History Fish Collection, New Orleans, Louisiana); UF (Florida Museum of Natural History, Department of Ichthyology, Gainesville, Florida).


       The U.S. distribution of each foreign cyprinid species is given on a map plate in the corresponding species account. Distribution information is presented by drainage and by state, each coverage indicating different levels of information.

       Distribution by drainage. - U.S. Geological Survey Hydrologic Unit Codes (HUCs) were used as a base to build maps. This is a nationwide system that delineates watersheds based on surface hydrologic regions (for more information, see: http://nas.er.usgs.gov/hucs.asp). Maps in this guide use a combination of 6-digit and 8-digit HUCs. The maps distinguish between HUCs where there is evidence of natural reproduction in the wild (coded as “reproducing”) and those where the species has been reported, but without evidence of reproduction (coded as “reported”). The distinction is important. Once a nonindigenous fish establishes a reproducing population, it may persist indefinitely, becoming a permanent addition to the fauna.

           (1) Reproducing species. - A species is coded as “reproducing” within a HUC if a naturally reproducing population is present. These drainages are color-coded red on the maps. Usually, evidence for reproduction is based on presence of adult fish as well as reports that eggs, larvae, or small juveniles have been collected within the HUC. Sometimes reproduction is inferred by persistence of a species in a particular water body over an extended period of time. Persistent reproduction is usually habitat dependent. Some of the species in this guide are capable of reproducing in still waters, such as lakes and ponds (Common Carp, Goldfish, and Tench). Other species, such as the Chinese carp (Grass Carp, Silver Carp, Bighead Carp, and Black Carp), naturally spawn in rivers.

           (2) Reported species. - HUCs are coded as “reported” if the species has been collected, stocked, or observed in one or more parts of a drainage, but no evidence exists that the species is successfully reproducing. These drainages are color-coded pink on the maps. Failed historical introductions (that are known to us) are included in this category.

           (3) Eradicated species. - Eradication is the complete elimination of all individuals in a given population. Many eradication attempts fail, as aquatic organisms are especially difficult to completely remove from a system despite vigorous and costly efforts to eliminate them. Years of monitoring are usually required to assure no survivors persist. In some cases, the source of introduction is never identified, consequently individuals continue to repopulate the area. In general, the only successful eradication projects are ones directed at fish confined in small, relatively closed systems, such as a pond or smaller, somewhat isolated reservoirs or drainages (see account on Ide, Leuciscus idus). In larger and unconfined water systems (rivers, canal networks, estuary complexes) the possibility of successful eradication is extremely low (or impossible) and the cost of such projects high. There is little hope of completely eradicating some widely-ranging nuisance fishes, especially with the techniques currently available.

       Eradication attempts are often poorly documented (typically in the gray or unpublished literature), making it difficult to evaluate their success. Additionally, reports of eradications from selected sites do not always include information on whether the species continued to survive in other parts of the drainage. The confusion and shortage of information complicate the task of tracking and accurately mapping changes in the distribution of introduced populations. For example, Dill and Cordone (1997) reported that a reproducing population of Tench was eliminated from a small farm pond in the Trinity River drainage in California, but did not comment on whether the species was known to exist or was reproducing elsewhere in that drainage. A later report (Moyle 2002) indicated there were no recent records for Tench in the Trinity River. Thus, it is unclear whether the local eradication effort reported by Dill and Cordone (1997) removed the only reproducing population of Tench or whether other reproducing populations of Tench throughout the drainage simply died out over time.

       Distribution by state. - In addition to providing the distribution of each species by drainage, we also include distribution information for all 50 states. States color coded green represent those from which the species has been recorded at least once from natural waters within state borders. In some instances, the record of occurrence may be nonspecific, with no information about the precise drainage or location within the state where the species was reported, released, or captured. In such cases, the entire state is shaded green but no drainage is shaded. In contrast to the HUC units described above, the state colors do not provide information concerning the population status of the species (that is, reproducing versus non-reproducing). If a foreign cyprinid is known only from one site in a drainage, then the entire drainage is color coded (either red or pink). Many HUC units cross state boundaries. For additional information on state-by-state occurrence of foreign cyprinids, see appendix A.

       Water bodies included in the coverage. - Distributions delineated on the maps pertain only to records from the wild, including habitats that are natural (rivers, lakes) or artificial (canals, farm ponds, reservoirs). Maps do not include records based on captive indoor settings or outdoor ponds and tanks at aquaculture facilities, as the focus of the database is to document wild-caught non-native fishes. However, it has become increasingly evident that documentation of the location of aquaculture facilities as well as the fish stocks cultured there is important due to the likelihood of escape from outdoor ponds to nearby open systems. Unfortunately, this data is often difficult to obtain.

       Source of map distribution data. - Data used to create these maps were primarily derived from the Nonindigenous Aquatic Species (NAS) database at the U.S. Geological Survey and from the closely related publication on nonindigenous fishes by Fuller and others (1999). The database is built from a variety of sources, including scientific literature, published and unpublished reports, and museum specimens. The database also relies on information from personal communications with scientists and others, as these sources are often the most recent (or only) documentation of a species’ occurrence in a particular area. We have attempted to make the maps in this guide as up-to-date as possible. Nevertheless, we may not be informed of all occurrences of nonindigenous cyprinids. Moreover, the distribution of nonnative fishes frequently changes over time, with many species expanding their ranges. Consequently, readers are reminded that distributions of introduced species are constantly in flux, so those who are attempting to identify a fish should not rule out a particular species simply because the map shows that it has not previously been reported from the area.

       Most of the distribution data provided in this guide can be found by querying the NAS database (http://nas.er.usgs.gov/queries). Summary collection information, including date, can be obtained by clicking on the dip-net icon. Detailed information about each collection can be found by clicking on the specimen ID. The database is constantly updated as new records are added and earlier records are reviewed and corrected. Individuals who have relevant information concerning the collection of nonindigenous cyprinids are asked to report their findings to state game and fish agencies and to the NAS database (appendix C).

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