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COSEWIC assessment and status report on the American Eel in Canada

Habitat

Habitat Requirements

The American eel uses a very broad diversity of habitats (Helfman et al. 1987). During their oceanic migrations, eels occupy salt water and, in their continental phase, they use all salinity zones. During the continental phase, marine habitat use is limited to shallow protected waters. Survival is affected by environmental conditions in any habitat (oceanic, estuarine, freshwater) occupied during any life cycle phase, and by anthropogenic factors such as hydro-dams, habitat modification and fisheries.

Growing eels are primarily benthic, utilizing substrate (rock, sand, mud) and bottom debris such as snags and submerged vegetation for protection and cover (Scott and Crossman 1973; Tesch 1977).

Eel densities typically diminish with distance from the sea in medium and large rivers (Smith and Saunders 1955; Gray and Andrews 1971; Smogor et al. 1995). This pattern, however, may be altered by natural or artificial obstacles. In a component of European eels, White and Knights (1997) reported that barriers to upstream migration had a greater effect on eel densities than distance from the ocean. Ability to overcome obstacles is size-dependent. Small eels (less than 10 cm long) are able to creep up damp vertical barriers (Legault 1988), but larger eels are generally unable to bypass large waterfalls and dams (McCleave 1980; Barbin and Krueger 1994). Hence, larger eels attempting to move upstream require unobstructed passage (Moriarty 1987).

Reduced survival of maturing eels in their seaward migration has been associated with passage through hydroelectric turbines (Desrochers 1995; Normandeau Associates and Skalski 2000), fisheries (Castonguay et al. 1994a; Caron et al. 2003; Verreault and Dumont 2003), and with obstructions which produce free falls of more than 13 m (Larinier and Travade 1999).

Continental-phase American eels are highly plastic in their habitat use. In streams, eels generally do not show consistent preferences for habitat type, cover, substrate, water temperature, and density of predators (Hawkins 1995; Smogor et al. 1995), but there is some association between eel densities and diversity of depth-velocity regimes (Wiley et al. 2004). In Prince Edward Island, eels are abundant in freshwater ponds formed by dams but are rare in most freshwater streams (Cairns et al. submitted).

Some continental-phase eels are predominantly sedentary but others are predominantly migrant (Feunteun et al. 2003). Since otoliths are essentially calcium carbonate in an organic proteinaceous matrix, Casselman (1982) analyzed strontium/calcium ratios in eel otoliths to document migratory stages. Recent investigations using otolith microchemistry (Jessop et al. 2002; Cairns et al. 2004; Thibault et al. 2005; Lamson et al. submitted) report three main movement patterns: salt water residency, freshwater residency, and inter-habitat shifting. In the St. Jean River on the Gaspé Peninsula, some freshwater resident eels perform very short intrusions into brackish or salt water (Daverat et al., in press). Inter-habitat shifting is more frequent in systems where dams do not hinder movements (Jessop et al. 2002; Cairns et al. 2004). Catadromy is no longer seen as obligate for eels, but rather is a facultative life history option (Tsukamoto et al. 1998; Jessop et al. 2002; Morrison et al. 2003; Arai et al. 2004; Lamson et al. submitted). Seasonal local movements associated with wintering may also involve habitat needs in terms of water temperature, oxygen concentration and water quality, but winter habitat requirements are poorly known (Tesch 1977; Feunteun et al. 2003).

Eels spawn in the Sargasso Sea (Schmidt 1922), east of the Bahamas and south-west of Bermuda (25°N; 60°W; McCleave et al. 1987), but habitat requirements for spawning and incubating are poorly understood. Kleckner and McCleave (1988) related the northern limit of spawning by Atlantic eels (Anguilla spp.) in the Sargasso Sea to thermal fronts and surface water masses, with spawning taking place south of east-west thermal fronts that separate southern Sargasso Sea surface water from mixed Subtropical Convergence Zone water to the north.

 

Habitat Trends

Freshwater habitat deterioration, migratory barriers generating habitat loss and fragmentation for upstream migrants, and turbine mortality for downstream migrants, are among the most frequently cited factors proposed to explain the declines of the American eel (Castonguay et al. 1994a; Haro et al. 2000; Verreault et al. 2004). The effects of dams are presented in the Limiting Factors and Threats section, under Habitat modifications and dams. In general, habitat factors have shown relative stability over the past several decades, and changes in these factors do not correspond to the timing of changes in eel populations (Castonguay et al. 1994a).

 

Protection/Ownership

In Canada, American eels occur primarily in publicly owned waters.  The species’ habitat, including ocean habitat used during migrations, receives protection against alteration and destruction by the Canadian Fisheries Act, the Canadian Environmental Protection Act, and numerous provincial acts including the Ontario Environmental Protection Act, the Ontario Water Resources Act, and the Quebec Environmental Quality Act.  Habitat that lies within National Parks, Provincial Parks, National Wildlife Areas, and Marine Protected Areas may be subject to additional protections through the National Parks Act, the Loi sur les Parcs in Quebec, the Ontario Provincial Parks Act in Ontario and the Canada Wildlife Act.  However, rules governing parks and conservation areas do not necessarily prohibit exploitation and do not automatically protect eels from other threats (see Limiting Factors and Threats section).