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COSEWIC assessment and status report on the American Eel in Canada

Biology

Life Cycle, Migratory Behaviour, Growth and Reproduction

The life history of the American eel encompasses oceanic, coastal, estuarine and freshwater environments. Spawning and hatching take place in the Sargasso Sea(Schmidt 1922). Larvae are transported to coastal waters by the Gulf Stream system formed by the Florida Current, the Gulf Stream and the North Atlantic Current. Some arriving juvenile eels migrate up rivers to become resident yellow eels of freshwater habitats; whereas others remain in brackish or salt waters, and still others show inter-habitat movements (Jessop et al. 2002; Cairns et al. 2004; Thibault et al. 2005; Lamson et al. submitted). After a number of years (typically 8 to 23; this report) in growth habitats, eels mature into silver eels that migrate back to the spawning grounds. Spawning occurs once; therefore, the American eel is a semelparous species (Helfman et al. 1987) and each mortality within continental waters is a pre-spawning mortality. The terminology of eel life history differentiates stages according to migration patterns and morphological characteristics. Life stages are detailed below.

 

Egg

The egg probably hatches within a week of deposition in the Sargasso Sea. McCleave et al. (1987) suggested that hatching peaks in February and may continue until April. Wang and Tzeng (2000) proposed, on the basis of otolith back-calculations, that hatching occurs from March to October and peaks in August. However, Cieri and McCleave (2000) argued that these back-calculated spawning dates do not match collection evidence and may be explained by resorption.

 

Leptocephalus

1


The leptocephalus is the larval form. Leptocephali are transparent willow leaf-like, laterally compressed larvae (Figure 4) that are passively transported west and north to coastal waters on the eastern coast of North America, by the surface currents of the Gulf Stream system (Schmidt 1922; Tesch 1977; Kleckner and McCleave 1982). This stochastic larval distribution is completed in 7 to 12 months (Kleckner and McCleave 1985; Wang and Tzeng 2000). Vertical distribution is usually restricted to the upper 350 m of the ocean (Kleckner and McCleave 1982; Castonguay and McCleave 1987). Growth has been evaluated at about 0.21 to 0.38 mm per day (Kleckner and McCleave 1985; Castonguay 1987; Tesch 1998; Wang and Tzeng 2000).

Figure 4.  Leptocephalus, larval stage [from kraskoff@mbari.organd P. Parks (OSF)].

 

Glass Eel

As they enter the continental shelf, leptocephali metamorphose into glass eels (Figure 5), which have the typical elongate and serpentine eel shape (McCleave et al. 1987). The term glass eel refers to all developmental stages from the end of metamorphosis of the leptocephalus to pigmentation (Tesch 1977). Metamorphosis occurs when leptocephali are about 55 to 65 mm long (Kleckner and McCleave 1985). Mean age at this metamorphosis has been evaluated at 200 days and estuarine arrival

2

Figure 5.  Glass eels, unpigmented elvers, post-larval stage (from MRNF, Secteur Faune Québec).

at 255 days; giving 55 days between glass eel metamorphosis and estuarine arrival (Wang and Tzeng 2000). During the elver run in the East River (Chester, Nova Scotia), the degree of elver pigmentation increased progressively over the run, and glass eels were rarely found after the end of May (Jessop 2003a). On the north shore of the Gulf of St. Lawrence, in the Petite Trinité River, glass eels occurred in the second half of June and were rare compared to elvers (Dutil et al. 1989).

Young eels use selective tidal stream transport to move up estuaries (Kleckner and McCleave 1982). As they enter coastal waters, the animals essentially transform from a pelagic oceanic organism to a benthic continental organism.

 

Elver

Glass eels become progressively pigmented as they approach the shore; these eels are termed elvers. The melanic pigmentation process (Bertin 1951; Élie et al. 1982; Grellier et al. 1991) occurs when the young eels are in coastal waters. At this phase of the life cycle, the eel is still sexually undifferentiated. The elver stage lasts about three to twelve months. Elvers that enter fresh water may spend much of this period migrating upstream (Haro and Krueger 1991; Jessop 1998a). Elver influx is linked to increased temperature and reduced flow early in the migration season, and to tidal cycle influence later on (Tesch 1977; Kleckner and McCleave 1982; Martin 1995; Jessop 2003b).

Elver length and arrival date increase from south to north along the Atlantic coast of North America (Vladykov 1966; Haro and Krueger 1988). In Atlantic coastal Nova Scotia, elver migration peaks between late April-early May and late June, although small numbers of elvers may continue to enter rivers until mid-August (Jessop 1998a). Total length averaged 60.14 ± 0.17 mm (50.4-70.5 mm) in 2000 on the East River, Chester, Nova Scotia (Jessop 2003c). On the Murray River (Prince Edward Island), elvers were caught between the end of June and the end of August (Cairns et al., submitted). In the Petite Trinité River (north shore of the Gulf of St. Lawrence), most individuals were already pigmented (elvers) in early July but arrived until the end of July (Dutil et al. 1989) and averaged 62.4 mm (59-69 mm).

 

Yellow Eel

The yellow stage is the growth phase of the species. The belly colour varies from yellowish to greenish or olive-brown, with the back remaining dark (Scott and Crossman 1973; Tesch 1977). The skin is thick and tough and may secrete copious amounts of mucous, which acts as a protective cover. Unlike the well-developed scales of most fishes, eel scales are rudimentary and embedded deeply within the skin.

Sexual differentiation occurs during the yellow stage and appears to be strongly influenced by environmental conditions (Krueger and Oliveira 1997; Oliveira 1997; Krueger and Oliveira 1999). Krueger and Oliveira (1999) suggested that density was the primary environmental factor influencing the sex ratio of eels in a river, with high densities promoting the production of males. From life history traits of four rivers of Maine, Oliveira and McCleave (2000) evaluated that sexual differentiation was completed by 270 mm total length.

In most Canadian waters more than 95% of sexually differentiated eels are female (Gray and Andrews 1970; Dolan and Power 1977; Dutil et al. 1985; Jessop 1987; Fournier and Caron 2005). In particular, eels in the upper St. Lawrence and Lake Ontario are virtually all female. Males appear to be more common in the Scotia-Fundy region than elsewhere in Canada. In the Saint John River, males were 7.4% of a sample of 970 eels (Ingraham 1999). In stream portions of the East River (Chester, Nova Scotia), more than 55% of yellow eels sampled were classified as males (Jessop et al., in press). Eels which had been captured as elvers in the Bay of Fundyand stocked in a lake on the south shore of the St. Lawrence River (FEA2) contained 27.2% males after four years of growth (Verreault et al., submitted). Yellow eels may continue to migrate upstream for many years. Female juvenile eels passing the eel ladder at the Moses-Saunders dam near Cornwall (Ontario) in the 1970s and 1980s ranged in age from 3 to 7 years, averaging 5 years (Casselman et al. 1997). Eels ascending this ladder in recent years are much older, ranging in age from 10 to 14 years, averaging 12 years (J.M. Casselman, pers. obs.). These eels are headed for Lake Ontario, the largest single growth habitat for the American eel within its distribution range.

In the Sud-Ouest River (south shore of the St. Lawrence River) and the Petite Trinité River (northwestern shore of the Gulf of St. Lawrence), upstream migrations occurred between June and August (Dutil et al. 1989; Verreault 2002; Fournier and Caron 2005). At Chambly, Beauharnois and Moses-Saunders dams, migration generally peaks in July and August (Casselman et al. 1997; Casselman 2003, Verdon et al. 2003; Bernard and Desrochers 2005).  A fall peak has also been observed at Moses-Saunders in 2004 and 2005 (J.M. Casselman, pers. obs.).  In components that have ready access to brackish or salt waters, eels are reported to move from fresh water to estuaries in the spring, and move back to fresh water in the fall (Medcof 1969; F. Caron, MNRF, Secteur Faune Quebec, pers. obs.).

Home ranges occupied by individual yellow eels vary with habitat type (stream, lake, tidal, creek, marsh, estuary) and movement patterns (freshwater residency, salt water residency, inter-habitat shifting). Home range was estimated to be relatively small (up to 2 ha) in estuarine habitats such as salt marshes (Ford and Mercer 1986) and tidal streams (Bozeman et al. 1985; Dutil et al. 1988), but in Lake Champlain, LaBar and Facey (1983) reported home ranges up to 65 ha.

Growth patterns are affected by habitat type. Eels utilizing brackish and salt waters grow much more rapidly than those in fresh waters (Jessop et al. 2002; Cairns et al. 2004; Jessop et al. 2004). Variation exists among freshwater habitats as well. Annual growth increments of eels translocated in a watershed originally without eels (see Stocking section) were much lower in rivers (40 mm per year) than in a lake (112 mm per year; Verreault et al., submitted).

In Canadian waters, American eels hibernate in mud during the winter. Wintering habitats include fresh waters, brackish estuaries, and bays with full strength salt water (Smith and Saunders 1955; D.K. Cairns, DFO, pers. obs.). Eels in estuaries often concentrate in mud that has freshwater upwelling, but they may also be found in bottoms that have no freshwater influx (D.K. Cairns, DFO, pers. obs.). Eels are reported to enter torpor (complete inactivity) at temperatures below 5°C (Walsh et al. 1983), and at less than about 2°C based on aquaculture experiments to minimize weight loss (B.M. Jessop, DFO, pers. comm.). Smith and Saunders (1955) caught two eels in a stream trap in Prince Edward Island in January and February. Eels speared through the ice in a Prince Edward Island estuary in January had stomachs bulging with fresh and undigested silversides (Menidia menidia), suggesting recent feeding activity (D.K. Cairns, DFO, pers. obs.). These observations suggest that eels are occasionally active during the normal period of hibernation.

 

Silver Eel

As the maturation process proceeds, the yellow eel metamorphoses into a silver eel. The silvering metamorphosis results in morphological and physiological modifications that prepare the animal to migrate back to the Sargasso Sea. The eel acquires a greyish colour with a whitish or cream coloration ventrally (Gray and Andrews 1971; Scott and Crossman 1973; Tesch 1977). The digestive tract degenerates (Pankhurst and Sorensen 1984; Durif 2003), the pectoral fins enlarge to improve swimming capacity (Pankhurst 1982a; McGrath et al. 2003; Durif 2003), eye diameter expands and visual pigments in the retina adapt to the oceanic environment (Vladykov 1966; Pankhurst 1982b; McGrath et al. 2003), the integument thickens (Tesch 1977; Pankhurst and Lythgoe 1982), percentage of somatic lipids increases to supply energy for migrating and spawning (Larsson et al. 1990; Tremblay 2004), gonadosomatic index (Verreault 2002; McGrath et al. 2003; Tremblay 2004) and oocyte diameter increase (Couillard et al. 1997), gonadotropin hormone (GTH-II) production increases (Durif et al. 2005), and osmoregulatory physiology changes (Dutil et al. 1987).

Distance travelled to reach the Sargasso Sea varies substantially over the geographic range of the American eel. A silver eel from the most distant growth habitat, western Lake Ontario, must migrate more than 4,500 km to reach the spawning grounds, whereas a maturing eel from the closest Canadian growth habitat, southern Nova Scotia, migrates about 2,000 km. Disparities in migration distance generate disparities in the timing of the onset of the migration, probably related to synchronous arrival in the Sargasso Sea, permitting spawning between February (peak) and April (Kleckner et al. 1983; Kleckner and McCleave 1985; Helfman et al. 1987). Maturing eels begin to descend the Richelieu River in May (Dumont et al. 1998).  Eels from Lake Ontario begin outmigrating in mid- to late June (McGrath et al. 2003).  Silver eels from Nova Scotia, which is much closer to the spawning grounds, outmigrate until November (Jessop 1987). Downstream migration occurs primarily at night.

There are large variations in reproductive characteristics of female silver eels across the species’ range (Nilo and Fortin 2001). Because the species is panmictic (Avise et al. 1986; Wirth and Bernatchez 2003), variations cannot be explained genetically. Eels from northern components show slower growth and greater length, weight, and age at migration (Hurley 1972; Facey and LaBar 1981; Helfman et al. 1987). Being less variable within a given component, size, rather than age, appears to be the main cue triggering maturation and migration (Helfman et al. 1987; Oliveira 1999; Verreault 2002; Tremblay 2004). Pre-spawning female eels from the St. Lawrence River and its tributaries are generally much larger (means 837-1043 mm; this report) than those from other freshwater rearing sites in Canada (means 650-694 mm; this report) (Table 2).

Table 2.  Migration periods, mean length and age of female silver eels exiting Canadian freshwater systems.

Site

FEAA

Migration period

n
Length (mm)
Age

Reference
Upper St. Lawrence River
St. Francis and St. Lawrence lakes,
Moses-Saunders Vicinity of Iroquois dam
1June–October200
53
30
915
976
1,001
20

21
Casselman 2003
McGrath et al. 2003
Tremblay 2004
Richelieu River1June-October4941,019 Dumont et al. 1998
St. Lawrence estuary1September-October474
4,529
30
840
853
837


20
Couillard et al. 1997
Verreault et al. 2003
Tremblay 2004
Sud-Ouest River
(South shore of the St. Lawrence)
2August-November107
30
1,026
1,043
21
23
Verreault 2002
Tremblay 2004
Rivière aux Pins2 100600 Couillard et al. 1997
Petite Trinité River
(North shore of the St. Lawrence)
2August-October424
30
650
679
17
20
Fournier and Caron 2005
Tremblay 2004
Long Pond
(Prince Edward Island)
3August-October3069320Tremblay 2004
Margaree River, Nova Scotia3September319642 D.K. Cairns, DFO, unpubl.
Topsail and Indian ponds, Salmon River, Burnt
Berry Brook, Topsail Barachois (Newfoundland)
4August-October9269412Gray and Andrews 1971
LaHave River
(Nova Scotia)
3August-November34661119Jessop 1987

The overall mean of mean age at spawning migration of female silver eels leaving Canadian freshwater sites is 19.3 years (SD 3.0, range 12-23, Table 2). Age of silver eels from brackish and saltwater habitat is poorly known because such eels are difficult to sample as they leave for the spawning grounds. Silver eels leaving Long Pond, a freshwater pond on Prince Edward Island, had a mean length of 693 mm (Table 2). If eels from the same geographic area have similar sizes at silvering, the age of silvering of eels reared in salt water can be estimated from growth trajectories of resident salt water eels. Eels from salt water bays a few km from Long Pond, whose exclusive use of salt water habitat was confirmed by strontium-calcium analysis, showed an annual growth of 55.8 mm/year (Lamson submitted). Given this growth rate these salt water residents would reach 693 mm at about age 7. Eel growth rate increases sharply with salinity (Lamson submitted), so the phenomenon of eels silvering much younger in marine than fresh water is probably widespread and general.

Eel ages are conventionally scored as being 0 during the year of continental arrival. Hence generation time is calculated as age at silvering + 2, to account for the time of migration from and to the spawning grounds. Because American eels are a long-lived species, population indicators of any continental stage other than glass eels/elvers typically include numerous year-classes.

American eel fecundity increases with body size (Wenner and Musick 1974; Barbin and McCleave 1997; Tremblay 2004). The large body size of female silver eels from the St. Lawrence River system (Table 2) means that these eels are much more fecund than those from elsewhere in Canada. In five components within the St. Lawrence River and Gulf, Tremblay (2004) found that absolute fecundity ranges from 3.4 to 22 million eggs for body lengths ranging from 53.2 to 111.0 cm and body weight ranging from 260 to 3,340 g. Large-bodied eels average about 6.5 million oocytes / kg while small-bodied eels have more than 10 million oocytes / kg.

Because of their large size, high fecundity, and exclusively female composition, silver eels exiting the upper St. Lawrence and Lake Ontario have the highest potential egg production per exiting migrant of any American eel component.

Male silver eels are more common in components south of the St. Lawrence River and Gulf, including the Scotia-Fundy region and the east coast of the United States. According to Oliveira et al. (2001), the proportion of male silver eels seems to be inversely related to the amount of lacustrine habitat. In a small Nova Scotian river (East River, Chester), Jessop et al. (2002) reported that 56.5% of silver eels (n=62) were males. In the Annaquatucket River (Rhode Island), sex ratios of silver eels ranged from 77% males in 1977 to 94% males in 1991 (Krueger and Oliveira 1997). Since males become silver eels at much smaller sizes than females, sex of silver eels can usually be evaluated by size alone. Winn et al. (1975) used a length threshold of 400 mm to identify females. Male silver eels from the Annaquatucket River have a mean length of 337.3 ± 0.4 mm (n = 2,998) and a mean age of 10.9 ± 0.1 years (n = 853) (Oliveira 1999). Compared to female silver eels, males have a more constrained size and age at migration. Unlike females which have large body sizes to increase fecundity, males appear to migrate back to the Sargasso Sea at the minimum size necessary to survive the spawning migration (Helfman et al. 1987).

 

Nutrition

Leptocephalus

Little is known about the food habits of leptocephali. Recent studies on other eel species (Otake et al. 1993; Mochioka and Iwamizu 1996) suggest that leptocephali do not feed on zooplankton but rather consume detrital particles such as marine snow and fecal pellets or particles such as discarded houses of larvacean tunicates.

Glass Eel and Elver

Based on laboratory experiments on European glass eels, Lecomte-Finiger (1983) reported that they were morphologically and physiologically unable to feed. However, Tesch (1977) found that elvers at a later stage of pigmentation, stage VIA4 (Élie et al. 1982), were feeding. Stomach examination of elvers caught during their upstream migration in the Petite Trinité River on the north shore of the Gulf of St. Lawrence revealed that elvers fed primarily on insect larvae (Dutil et al. 1989).

Yellow Eel

The yellow eel is essentially a nocturnal benthic omnivore. Prey includes fishes, molluscs, crustaceans, insect larvae, surface-dwelling insects, worms and plants. The eel prefers small prey animals which can easily be attacked (Tesch 1977). Food type varies with body size (Ogden 1970, cited in Tesch 1977). Stomachs of eels less than 40 cm and captured in streams contained mainly aquatic insect larvae, whereas larger eels fed predominantly on fishes and crayfishes. In Lake Champlain, food sources were mainly fish (38%), decapods (30%) and insects (10%). Insect abundance decreased in larger eels (Facey and LaBar 1981). The eel diet adapts to seasonal changes and the immediate environment. Feeding activity decreases or stops during the winter, and food intake ceases as eels physiologically prepare for the spawning migration.