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COSEWIC assessment and status report on the American Eel in Canada

Species information

Name and Classification
ClassActinopterygii
OrderAnguilliformes
FamilyAnguillidae
Genus Anguilla
Scientific Name[1]   Anguilla rostrata (LeSueur,1817)
Common Names1    
English  American Eel
FrenchAnguille d’Amérique
Other 
English2    Atlantic eel, common eel, freshwater eel, silver eel, yellow-bellied eel, green eel, black eel, bronze eel, elver, whip, and easgann
French  anguille argentée, anguille jaune, anguillette, and civelle
Mi’kmaq3   kat

      

Members of the genus Anguilla are termed freshwater eels, although some species (including the American eel) are able to complete their life cycle in salt water (Tsukamoto et al. 1998; Arai et al. 2004;Lamson et al. submitted). The American eel is the only North American species of the genus.

Morphological Description

The American eel has an elongated and serpentine body (Figure 1). Its single continuous dorsal fin extends posteriorly from a point about one third of the body length behind the head, around the tail to the vent. The pectoral fin is supported by 7 to 9 radialia (up to 11 in young specimens); the mouth is terminal; the lower jaw slightly longer than the upper; the teeth are small and arranged in several rows on the jaws and palate; a tongue is present; the lips are thick; the lateral line and the palatopterygoid arch are well developed; the gill openings are not confluent; and, the frontal bones are paired (Tesch 1977).

Tesch (1977) described three morphological features which persist through all stages from larvae to maturing eels: the total number of vertebrae (mean 107.2), the number of myomeres (mean 108.2; evaluated at 106.84 by Kleckner and McCleave 1985), and the distance between the origin of the dorsal fin to the anus (mean 9.1% of total length). Other morphological characteristics can only be used comparatively if the individuals are at the same stage of development (e.g. leptocephalus, glass eel, elver, yellow eel, silver eel). Details for each life stage are presented in the Biology section.

Figure 1.  The American eel (from www.mnr.gov.on.ca).


Figure 1.  The American eel (from www.mnr.gov.on.ca).

Genetic Description

All freshwater eels belong to the genus Anguilla. Anguillid eels of the North Atlantic Ocean have been divided into two species based on morphological characters (Ege 1939; Tesch 1977) and molecular genetics (Avise et al. 1986; Aoyama et al. 2001; Wirth and Bernatchez 2003). The American eel inhabits continental waters on the western side of the Atlantic Ocean, while the European eel (Anguilla anguilla) is found in continental waters on the eastern side of the Atlantic. Both species also occupy the Sargasso Sea in the southern North Atlantic and Iceland in the northern North Atlantic. Hybrids between American and European eels have been identified in Iceland (Avise et al. 1990).  Since these species are close relatives, some information from European eel studies has been applied to the American eel for the purpose of this status report.

Panmixia refers to a breeding system in which all members of a species mate randomly as a single breeding population. In panmictic species, genetic structure shows no geographical heterogeneity. Wirth and Bernatchez (2001) found mild geographic variation in the genetic structure of the European eel, which they interpreted as evidence against panmixia for that species. However, Dannewitz et al. (2005) argued that the reported genetic variation is due to temporal shifts and does not constitute evidence against panmixia. Microsatellite and mitochondrial DNA analysis of American eel samples from Gulf of St. Lawrence, the Atlantic coast of Canada, and the eastern seaboard of the U.S. indicate panmixia (Avise et al. 1986, Wirth and Bernatchez 2003) (Table 1).  In this paper the American eel is considered to be panmictic, although final confirmation of this status requires genetic sampling from all parts of the species’ range, including the St. Lawrence River and Lake Ontario.

Because of the American eel's panmictic nature, the word "population" refers to all members of the species.  Eels from any subset of the species' range are referred to as a component.

Table 1.  Pairwise sample differentiation estimates based on allelic variance at seven microsatellite loci in 21 North Atlantic eel samples (from Wirth and Bernatchez 2003). Bold indicates significant Өestimates following Bonferroni corrections (k =210, α =0.05/320 = 0.00024). Samples include eels from two Canadian sites in the Gulf of St. Lawrence: Trinité River and Long Pond, Prince Edward Island.

Table 1.  Pairwise sample differentiation estimates based on allelic variance at seven microsatellite loci in 21 North Atlantic eel samples (from Wirth and Bernatchez 2003). Bold indicates significant Өestimates following Bonferroni corrections (k =210, α =0.05/320 = 0.00024). Samples include eels from two Canadian sites in the Gulf of St. Lawrence: Trinité River and Long Pond, Prince Edward Island.

American eels spawn in the Sargasso Sea(Schmidt 1922). Leptocephali larvae are dispersed widely by ocean currents, including the Florida Current, the Gulf Stream and the North Atlantic Current, to western shores of the Atlantic Ocean. Because American eels are considered to belong to a single panmictic breeding population, they must be managed as a single stock (Castonguay et al. 1994a; Haro et al.2000; Casselman 2003).

 

Designatable Units

Sub-speciation, geographic heterogeneity, range disjunction, or biogeographic distinction have not been demonstrated in the American eel population, and so the consideration of assessment below the species level is not an option.  Yet, the panmictic nature of American eel life history implies that factors affecting any life stage, in any geographic area of the range, and in any array of habitats, have the potential to affect the abundance of all life stages of the species throughout the range.  Regional factors can affect eels as they disperse towards continental coastlines and during the long growth phase, such that eel components may be in difficulty in some parts of the range while maintaining strong numbers elsewhere.

Geographic trends are evident in American eels in the sizes and trajectories of components, types and magnitude of threats, and contribution to recruitment. In addition, migration in medium and large rivers may be density-dependent, e.g. the size of a component in a given geographic area is dependent on the densities of eels downstream.  Therefore, although panmixis precludes assignment of status below the species level, consideration of the status of the species can be aided by developing the discussion in terms of the applicable geographic areas or ecozones.  The American eel occupies fivefreshwater ecological areas (FEAs) in Canada, as defined by COSEWIC. These FEAs are: 1) Great Lakes-Western (Upper) St. Lawrence (Ontario and western and central Quebec); 2) Eastern (Lower) St. Lawrence (eastern Quebec); 3) Maritimes (New Brunswick, Nova Scotia, Prince Edward Island, and the central and southern parts of Quebec's Gaspé Peninsula); 4) Atlantic Islands (Newfoundland); and 5) Eastern Arctic (Labrador). Contrary to the COSEWIC ecological areas system, Anticosti Island is assigned to FEA2 rather than to FEA4. In this report, the part of FEA3 that drains into the Atlantic Ocean and the Bay of Fundy is referred to as Scotia-Fundy.


[1]Nelson et al. 2004.

2Scott and Crossman (1973), Scott and Scott (1988).

3Prosper (2001).